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Recovery Strategy for White Sturgeon (Acipenser transmontanus) in Canada [Proposed]

3. Description of Needs of the Species

To complete their long lived life cycle, white sturgeon require suitable habitats, an abundant food base, and appropriate flows and water conditions. These needs are discussed separately in the following sections.

3.1 Physical Habitat

White sturgeon inhabit large rivers where they are often associated with slow, deep mainstem channels interspersed with zones of swift and turbulent water; extensive floodplains with sloughs and side channels; and, a snowmelt-driven hydrograph with prolonged spring floods (Coutant 2004). Most habitat use studies are recent and have been conducted on regulated rivers, particularly on the upper Columbia and Kootenay rivers. The few studies completed on the Fraser River, which is the only unregulated system in the species’ range, indicate that habitat use there may be quite different. Care must therefore be exercised when extrapolating observations and conclusions regarding sturgeon habitat and related behaviours from regulated systems, where in some cases white sturgeon are not presently successful in utilizing habitat for their life functions.

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3.1.1 Spawning and Incubation Habitat

White sturgeon naturally spawn during the spring freshet. There has been a considerable amount of work done to characterize white sturgeon spawning habitats, but much of the information has come from regulated rivers (e.g., Parsley and Beckman 1994; Parsley et al. 1993; Paragamian et al. 2001; Golder Associates Ltd. 2005c). These studies indicate strict requirements for deep, swift water and coarse substrates. Mean water column velocities typically range from 0.5 to 2.5 m/sec-1 (Parsely et al. 1993).

Spawning has occurred in the Kootenai River in an area characterized by large mobile sand deposits, but this area is believed to have extremely poor egg survival, since collected eggs were coated in sand (Duke et al. 1999; Kock et al. 2006) and only one wild sturgeon larvae has ever been collected (hatched on an egg collection mat; Vaughn L. Paragamian, Idaho Dept. of Fish and Game personal communication) despite significant collection effort. Spawning was observed in 2004 in the Nechako River over substrates dominated by gravel and fines, but these conditions appear to be one of the causes of ongoing recruitment failure experienced by this population (McAdam et al. 2005).

Evidence from the unregulated lower Fraser River, where there is successful recruitment, indicates that white sturgeon use large side channels for spawning (Perrin et al. 2003) as well as more turbulent areas downstream of the Fraser canyon (RL&L 2000a). Physical characteristics of the side channels included gravel, cobble and sand substrates, and mostly laminar flows with near-bed velocities averaging 1.7 m/sec-1 (Perrin et al. 2003). Boulder and cobble substrates predominate at the mainstem study site (Perrin et al. 2003). All sites were within a portion of the lower Fraser that is unconfined and largely unaffected by floodplain development (Appendix B).  Coutant (2004) noted that successful spawning is most often associated with turbulent or turbid river sections areas upstream of floodplains.

There appears to be a natural mechanism by which spawning is initiated when temperature exceeds a rough threshold. While other factors may have a secondary influence on spawn timing (e.g. flow and photoperiod – see Liebe et al. 2004) temperature appears to have a dominant effect. Threshold temperatures of 14ºC and 13ºC have been reported for the Columbia River at Waneta and the Nechako River respectively. Spawning in the Kootenai River occurs at a lower temperature (8.5-12ºC; Paragamian et al. 2001) as does spawning at the Revelstoke spawning site (10-11ºC; Golder Associates Ltd. 2009). In these two latter cases it is challenging to identify such thresholds due to historic anthropogenic changes. For example, it is possible that the fish have a biological threshold that is not reached by the current thermal regime (e.g. Revelstoke spawning site) or are uniquely adapted to a cooler thermal regime (suggested for the Kootenai population). Maximum temperatures may also be a concern, particularly at the Waneta spawning site where spawning has occurred above the 20ºC level where Wang et al. (1985) indicates that abnormal development may occur.

Incubation success is thought to be at its greatest when discharges are high and steady ( UCWSRI 2002). High velocities in egg deposition areas may exclude some predators and provide high turbidity, which may limit predator efficiency (Gadomski and Parsley 2005a). Substrate condition may also influence larval survival (Gessner et al. 2005; McAdam 2011). Recent research has indicated a preference of early yolk sac larvae for gravel sizes between 12 and 22 mm (Bennett et al. 2007; McAdam 2011).

3.1.2 Yolk Sac Larvae Habitat

Hiding in interstitial habitats is the predominant behaviour during this phase, and substrate quality affects survival both directly and indirectly (McAdam 2011, 2012; Boucher 2012; Crossman and Hildebrand 2012). Hiding immediately after hatching in the vicinity of the spawning location, and continuously until the initiation of exogenous feeding, should lead to the greatest survival under good habitat conditions. When substrate conditions are not optimal (e.g. embedded with fine substrates), drift may occur prior to hiding, or yolk sac larvae may be displaced. However, such movements are not considered optimal strategies for survival (McAdam 2011) due to the possibility for increased mortality. Similar to spawning and incubation phases, preferred substrates vary in grain size, but provide interstitial spaces suitable for yolk sac larvae hiding (for example, the interstitial spaces created by ¾” to 1 ½” gravel; McAdam 2011). Under natural conditions preferred substrate conditions will likely include a mixture of particle sizes ranging from gravel to large cobble.

Optimal temperatures for this phase are between 14ºC and 18ºC (Wang et al. 1985). Similar to eggs, lower temperatures delay development (Tiley 2006; Boucher 2012) and higher temperatures lead to increased deformities and mortality (Wang et al. 1985; Boucher 2012). Due to the interstitial location of this life phase, optimal water velocities (e.g. near bed velocities) would be primarily determined by their ability to maintain interstitial water quality (likely site dependant) and exclude some fish predation (e.g. velocity greater than 0.8 m/sec-1 has been suggested at the Waneta spawning site). This life phase likely prefers very low to no velocity within interstitial habitats (McAdam unpubl.).

White sturgeon early life history phases are particularly sensitive to contaminant effects and an LC50 (the concentration required to kill 50% of tested individuals) for copper (Cu) was identified at 22 ųg/mL (Vardy et al. 2013). Due to their residence within interstitial habitats, the ambient levels of contaminants may not be accurately reflected by measurements within the water column.

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3.1.3 Feeding Larvae Habitat

Habitat use by feeding larvae is quite uncertain, in part due to the rarity of this life stage in populations undergoing recruitment failure. Nocturnal drift at the beginning of this period allows dispersal to subsequent feeding habitats, however, characterisation of this process is limited by small larval sizes and sampling challenges in larger rivers. Laboratory studies suggest that feeding larvae forage over the open bottom and use less cover with increased age (Brannon et al. 1985). Important habitat attributes may include the presence of fluvial habitat downstream of spawning sites, and low velocity feeding habitats such as side channels or floodplains. The presence of white sturgeon in some areas where low velocity lateral habitats are limited (e.g. the middle Fraser River) also suggests the possibility that both feeding larvae and later stages can utilize mainstem habitat in some cases (e.g. the river bottom or margins). 

Water temperature preferences of feeding larvae were characterized by Wang et al. (1985) and vary from 14ºC to 18ºC. Similar to yolk sac larvae evaluations of contaminant effects suggest white sturgeon are sensitive relative to other species (Bennett and Farrell 1998; Vardy et al. 2013; Little et al. 2012). Some contaminants have the potential for sub-lethal effects such as growth limitations (e.g. Didecyldimethylammonium chloride (DDAC); Teh et al. 2003); however, there are no confirmed occurrences of ambient contaminant levels exceeding tolerance limits. Between 19 to 30 dph toxicology studies regarding the effects of Cu, cadmium (Cd) and zinc (Zn) exposure show increased mortality (based on LC50s), at concentrations of 4.1-9.9 ųg/L, 21.4 ųg/L and 340 ųg/L, respectively (Vardy et al. 2011; Little et al. 2012). Little et al. (2012) also identified sub-lethal effects at concentrations less than 4.1 ųg/L. Notably Vardy et al. (2013) show that feeding larvae were more sensitive to Cu (LC50= 10ųg/mL at 15 dph) than yolk sac larvae. Vardy et al. (2011) suggest that ambient water quality criteria should provide sufficient protection for the metals they tested, however Little et al. (2012) suggest that some water quality thresholds may not be sufficiently conservative.

The presence of fin deformities from hatchery progeny in the Columbia River (and not other hatcheries) suggests a possible non-lethal effect of contaminants; however, this has not been confirmed.

3.1.4 Juvenile Habitat

Juvenile (less than 2 years) habitat for white sturgeon varies considerably with stage of development. In general, information is limited about natural juvenile habitat use for white sturgeon populations in B.C., with most information coming from laboratory studies or studies in other river systems. Parsley et al. (1993) defined physical habitat for juvenile white sturgeon in the lower Columbia River as 2 to 58 m depth, 0.1 to 1.2 m/sec-1 mean column velocity, and near-substrate velocity of 0.1 to 0.8 m/sec-1. While the study was conducted downstream of McNary Dam, and the upper end of this depth range rarely exists in natural rivers, their observations nevertheless suggest that juvenile white sturgeon may be found at a range of depths, but that they prefer slow to moderate water velocities.

Observations and traditional ecological knowledge regarding a number of locations within the Canadian range (e.g., Bennett et al. 2005; Failing et al. 2003) shows that juveniles are often associated with the lower reaches or confluences of tributaries, large backwaters, side channels and sloughs. Sampling of side channel and slough habitat on the Kootenay has, however, shown little use of such habitats in comparison to the mainstem (Neufeld and Spence 2002). Extensive use of deep, low velocity mainstem habitats also occurs (RL&L Environmental Services 2000a; Golder Associates Ltd. 2003b; Neufeld and Spence 2004b), especially as fish grow larger. Substrates at collection sites have varied from finer particles through to boulder and hard clay (Parsley et al. 1993; Young and Scarnecchia 2005). Feeding juveniles showed a slight preference for sand substrates, but occupied other substrates if food was present (Brannon et al. 1985). In the Kootenay system, and perhaps in other systems, there is use of lake habitat by juveniles.

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3.1.5 Sub-Adult and Mature Adult Habitat

Late juvenile (over 2 years) and mature adult habitat use is variable, depending on time of year and function, such as spawning, feeding, overwintering, and movements to and from these key habitats (RL&L Environmental Services 2000a; Neufeld 2005). In general, white sturgeon adults are found in deep areas, adjacent to heavy flows, defined by deposits of sand and fine gravels with backwater and eddy flow characteristics (RL&L Environmental Services 1994a, 2000a). Adults in the upper Fraser River may be widely dispersed including use of tributaries, and may require long migrations to reach feeding and spawning habitats (Yarmish and Toth 2002). Most studies of adult habitat use have focussed on the physical features of spawning habitat. Considerably less attention has been given to other adult habitat requirements including overwintering, feeding, holding habitats, or migration habitats. Large lakes and rivers, where available, are extensively used at all times of the year (e.g., RL&L Environmental Services 1999b; Golder Associates Ltd. 2006c).


During this period, which is typically July to September, the movements of white sturgeon in most populations are volitional and tend to be more localized than in the spring to early summer or fall. In the Columbia (RL&L Environmental Services 1994a; Brannon and Setter 1992) and Kootenay rivers (Apperson and Anders 1991), white sturgeon were reported to use shallower depths during the spring to summer period and exhibited frequent, short distance forays between shallow and deep-water areas to feed.

High-use areas in the upper Columbia River are generally depositional areas where food items settle out.  In Kootenay Lake, adults undertake an annual migration from the south end of the lake to the outlet of the Duncan River at the north end, where large numbers of spawning kokanee provide an excellent food source (RL&L Environmental Services 1999b). In the upper Fraser, the pattern of habitat use in summer is linked to sturgeon spawning, but potentially also feeding on spawning cyprinids and the end of the period is clearly linked to the upstream migration of spawning salmon, especially sockeye.


Reduced activity is generally observed during winter months (e.g., RL&L Environmental Services Ltd. 2000a; Nelson et al. 2004). Individuals in all populations tend to utilize deeper, lower-velocity areas during this period. Large lakes and rivers are extensively used, where available (e.g., RL&L Environmental Services 1999b; Golder Associates Ltd. 2006c).

Migration Movements

Migration is defined in RL&L Environmental Services (2000a) as sustained, unidirectional movements, either upstream or downstream but not both, likely for feeding, spawning or overwintering. Migration patterns are being studied in many populations, but are not well understood for any of the B.C. populations. However, white sturgeon movements are volitional and may be attributed to spawning and feeding. For example, migrations have been observed during the spring/summer period and have been associated with staging, and spawning, as well as feeding activities in response to spring invertebrate hatches and spawning of other fish species. Fall movements may be associated with feeding opportunities (e.g., kokanee spawning near creek confluences). White sturgeon do not generally reproduce annually, especially females, so male and female spawning migration patterns may vary between years. The proximity between overwintering, spawning, and feeding areas is a key determinant of the extent of white sturgeon movements; however, further analysis on sturgeon movement is required.

Connectivity among habitats is necessary, since fish must be able to move freely between feeding, holding and spawning areas to complete their life cycle. At present, connectivity is maintained throughout much of the species’ range, with the exception of some habitat for the transboundary component of the Columbia River population. Connectivity is acknowledged as key for conservation planning of this species.

For the ALR component in the Columbia River, connectivity is required for the segment upstream from the Highway 1 Bridge to Big Eddy and from Big Eddy to the spawning site at the Columbia River adjacent to Revelstoke Golf Course. Connectivity is identified specifically for those locations because flow releases from REV Dam are required to ensure their connectivity. Since the addition of unit 5 turbine the minimum flow at REV Dam has gone from 8.5cm to 142cm increasing total wetted riverbed area by 37%; monitoring is ongoing to determine improvements in habitat connectivity as well as hydraulic properties within incubation and early rearing habitats (Jamie Crossman, B.C. Hydro, personal communication). For the transboundary component in the Columbia River, connectivity is also an issue. Based on preliminary genetic data, it is believed that HLK currently divides a formerly contiguous population. The limited ability to alter present connectivity levels at HLK is acknowledged; however, connectivity was likely critical prior to dam construction. At a minimum the present levels of connectivity within the transboundary reach should be maintained.

3.2 Diets and Food

An adequate prey source is an important habitat feature for all white sturgeon populations. White sturgeon feeding behaviour is specialized for dark, benthic habitats where prey are often located through direct contact, using highly sensitive taste receptors on barbels near the mouth (Brannon et al. 1985). Juvenile white sturgeon are primarily benthic feeders, feeding on a range of invertebrate and fish species. Diet varies throughout the year and with location depending on availability. Juveniles reportedly eat a variety of aquatic insects, isopods, mysids, clams, snails, small fishes, and fish eggs (Scott and Crossman 1973; McCabe et al. 1993). In the Upper Columbia River, Mysis relicta, a non-native pelagic crustacean, is the most common prey item of 1 – 2 year old juveniles (Golder Associates Ltd. 2006a). Adults feed predominantly on fish, particularly migratory salmonids where available, although crayfish and chironomids are also consumed (Scott and Crossman 1973; Partridge 1980).

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3.3 Water Conditions

The Columbia, Kootenay, Nechako and Fraser are large rivers, and are the receiving waters for a wide variety of point and non-point source pollutant discharges over a broad geographic range. These include point source discharges from pulp mills and smelters, industrial plants, treated and untreated municipal and private sewage, and various other industrial, agricultural and urban discharges, as well as non-point sources of pollution from agriculture, forestry, and urban areas.

Aquatic species are at risk when water conditions degrade beyond specific thresholds for oxygen, temperature, pH, or pollutants. The current provincial water quality guidelines provide general direction for the protection of aquatic life and may be sufficient for describing overall requirements for white sturgeon until further studies are completed. Additional research is needed to determine if certain aspects of these water quality guidelines require lower tolerance limits for different white sturgeon life stages. Specific concerns include treated and untreated municipal and private sewage, and various other industrial and urban discharges, and non-point sources of pollution from agriculture, forestry, and urban areas. Benthic sediment contamination (e.g., metals, organochlorine compounds) in and downstream of urban and industrialized areas may also provide a pathway for uptake and accumulation of harmful contaminants by white sturgeon (Fairchild et al. 2012).

Sturgeon are susceptible in particular to chemicals that can result in bioaccumulation. Current water quality guidelines should provide adequate protection; however, significant point or non-point discharges could result in impacts to habitat and/or individuals. Toxicology studies regarding the effects of Cu, Cd and Zn exposure have shown increased mortality at concentrations of 1.5 ųg/L, 5.5 ųg/L and 112 ųg/L (Vardy et al. 2011) and U.S. studies have shown behavioural effects at lower concentrations (Little et al. 2012).

3.3.1 Water Temperature

One water quality variable for which there is a better understanding is the interaction between temperature and spawn timing. There appears to be a natural mechanism by which spawning is initiated when temperature exceeds a rough threshold. While other factors may have a secondary influence on spawn timing (e.g., flow and photoperiod – see Liebe et al. 2004) temperature appears to have a dominant effect. Threshold temperatures of 14ºC and 13ºC have been reported for the Columbia River at Waneta and the Nechako River respectively. Spawning in the Kootenai River occurs at a lower temperature (8.5-12ºC; Paragamian et al. 2001) as does spawning at the Revelstoke site (10-11ºC; Golder Associates Ltd. 2009). In these two latter cases it is challenging to identify such thresholds due to historic anthropogenic changes. For example, it is possible that the fish have a biological threshold that is not reached by the current thermal regime (e.g., Revelstoke spawning site) or are uniquely adapted to a cooler thermal regime (suggested for the Kootenai population). Maximum temperatures may also be a concern, particularly at the Waneta spawning site where spawning has occurred at temperatures above 20ºC, a level that Wang et al. (1985) observed can lead to abnormal development. Unfortunately, temperature requirements, and limitations, for other life stages are not as well understood at this time.

For all rivers where white sturgeon are undergoing recruitment failure it is important to note that thermal regime alterations are not considered to be the primary cause of recruitment failure (e.g. Revelstoke – Gregory and Long 2008, McAdam 2012). The Nechako River is perhaps the best example, since thermal regime would have been affected by flow regulation for 15 years prior to the initiation of recruitment failure (1967). Ongoing monitoring under the current thermal regime indicates that both past and existing thermal regimes were/are not a primary cause of recruitment failure. However, they may have secondary effects including a delay in the timing of spawning and duration of embryo development. In addition, with warmer winters and cooler spring summer temperatures fish are metabolically somewhat more active in the winter and have less growth potential in the summer (Jamie Crossman, B.C. Hydro, personal communication).

3.4 Ecological Role and Limiting Factors

White sturgeon are both predator and prey. During early life stages, white sturgeon are preyed on by other fishes and wildlife and thereby form part of the food base of those species. As they increase in size, and with the help of their sharp scutes, sturgeon avoid predation by all species except humans and large marine mammals, such as pinnipeds (Stansell et al. 2010). The extent to which white sturgeon play a role in limiting abundance of prey species is not known. White sturgeon are known more as an opportunistic feeder than a voracious predator, but Scott and Crossman (1973) indicated that even one year old sturgeon take live fish as prey, and white sturgeon are known to be more piscivorous than any other North American sturgeon species. Abundance of fish populations that are highly-aggregated in space and/or time, such as spawning anadromous species, or spawning congregations of cyprinids, for example, would be heavily preyed on and could be affected by white sturgeon.

The intrinsic biological factors most limiting to white sturgeon population growth are very low early survival and delayed maturation. Gross et al. (2002) used a form of population modeling called “elasticity analysis” to assess the sensitivity of sturgeon population growth to changes in age and life-stage specific survival and fecundity. The analysis indicated that population growth is most sensitive to changes in early survival rates. Changes in survival of older fish and changes in fecundity had considerably less effect on population growth. The authors stress that since white sturgeon survival during the first year is extremely low, the overall opportunity for affecting population growth is strongest in that age class because of its exceptional potential for increased survival rates. Older fish already have fairly high survival rates, so there is less potential to effect higher population growth through increases in survival of these age classes.
However, even small changes in early juvenile survival, when compounded over many years, can have substantial effects on the number of fish reaching maturity.

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3.5 Recruitment Failure

In the Columbia, Kootenay and Nechako rivers the cause of decline is primarily ongoing recruitment failure associated with changes in habitat, flows, and the ecological community. In each of the three rivers, regular spawning occurs, but viable offspring do not recruit to the juvenile stage in sufficient numbers to sustain the population. In addition, spawning habitat is limited and may be degraded such that it is impacting egg or larval survival and unknowns remain about the requirements of early juvenile habitat. These are key factors for addressing recruitment failure. This pattern has been well documented and there have been numerous general studies, but detailed research into the specific causes of recruitment failure is relatively recent. The research is complicated by the possibility of multiple causal factors that may differ among populations.

There are clearly many natural and anthropogenic factors influencing abundance and distribution of all life stages of white sturgeon (see Section 3: Description of Needs of the Species and Section 4: Threats). Recruitment failure is strongest in dam-affected rivers, yet reasonable recruitment rates occur in the lower Columbia River, which is also affected by multiple mainstem dams, so it is more complicated than simply the presence of dams. Gregory and Long (2008) examined recruitment failure hypotheses for the upper Columbia River population, defined key hypotheses, and identified potential programs for assessment and recovery, but there was surprisingly little agreement among experts on the primary drivers of recruitment failure in that system. More recently, greater agreement seems to be emerging, based on new research in the laboratory and in the field, across multiple basins.

The general pattern of recruitment failure points to an interaction of geomorphology, substrate conditions, flow and fish behaviour: adults spawn successfully (i.e., viable gametes are produced and eggs are fertilized), but spawning occurs in areas in which substrates are inappropriate for egg incubation and survival and development of yolk sac larvae. In the Kootenay River, adults spawn in a restricted area where high levels of sand substrate lead to essentially zero survival (e.g., Paragamian et al. 2001, Paragamian et al. 2002, Paragamian et al. 2005). For the Nechako River population, substrate conditions at the spawning site diminish both egg and yolk sac larvae survival (McAdam et al. 2005, McAdam 2011, McAdam 2012). Additional layers of complexity seem to be part of the recruitment failure pattern, including high levels of fidelity (see Forsythe et al. 2012 for related work on lake sturgeon) for sites that have suitable water velocities and depths across a range of flows (Paragamian et al. 2009, McDonald et al. 2010), little or no direct preference for substrate type and condition, and larval behaviour, growth and survival that are highly dependent on substrate conditions (McAdam 2011, Boucher 2012, McAdam 2012). Further limitations likely exist, such as water temperature (Boucher 2012), predation (Gadomski and Parsley 2005a, b) and other factors (Gregory and Long 2008), which may interact with the primary drivers of recruitment failure. Understanding the causal factors related to recruitment failure is further complicated by biological characteristics of white sturgeon including very low early life stage survival and late maturation; efforts to resolve recruitment failure through habitat restoration must take these considerations into account.

4. Threats

Adequate assessment of threats to white sturgeon in Canada not only requires identification of those threats, but also understanding the relative importance of different threats. It is also necessary to assess the supporting evidence for each threat and the populations to which they apply. Threats differ among the white sturgeon populations and are analysed and discussed in detail elsewhere: threats to lower, middle, and upper Fraser River populations are discussed in Hatfield et al. (2004); threats to Nechako River white sturgeon are presented in Nechako White Sturgeon Recovery Initiative (2004); threats to the Kootenay River population are found in U.S. Fish and Wildlife Service (1999); and, threats to Columbia River white sturgeon can be found in UCWSRI 2002), Gregory and Long (2008) and McAdam (McAdam 2012).

The following discussion includes primary threats that historically may have caused a population decline (e.g., demonstrated by recruitment failure) and may have ongoing effects as well as threats that may presently cause declines or limit recovery. The risk assigned to each threat for white sturgeon include ratings of negligible, low, moderate, high, and unknown, as defined in Table 3.

Table 3. Definition for levels of relative risk to the viability for the white sturgeon population and its recovery.

Negligible: Threat has no detectable effect on the population or does not occur at this time.

Low: Threat produces measurable habitat, behavioural and/or physiological effects but does not affect population viability and recovery potential.

Moderate: Threat reduces habitat suitability, produces chronic behavioural effects and/or promotes physiological changes, which reduce population viability and recovery potential.

High: Threat results in habitat destruction and/or lethal effects that produce a severe, continuous and near-term effect on population viability and recovery potential.

Unknown: Available information is insufficient to gauge the degree to which the threat may affect the population viability and recovery potential.

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Threats to each white sturgeon population are summarized in Table 4 and discussed briefly in the following sections. Threats are plausible mechanisms, caused by human activities, which influence abundance, distribution, and health of white sturgeon. Much of this information is based on expert opinion as knowledge of many of these threats is limited. Different expert groups have assessed threats for each population as part of watershed-based conservation planning processes, and Technical Working Group (TWG) members for each population were involved in these threat assessments. Detailed hypotheses and population level impacts will be addressed as further information on the specific nature of impacts is developed.

The threats are grouped as abiotic or biotic, and ordered within each of these groups by threat severity. Severities were assigned based on related laboratory or field evidence, correlations between historic ecosystem changes and observed changes in population structure, or the strength of underlying components of the logic pathway for each hypothesis (Table 4).  Not all threats are equally valid for each population, and there are differing degrees of support for each stressor and its ability to influence the abundance, distribution, and health of each population of white sturgeon. Also, there may not be one single threat that caused population declines, but several threats may have cumulatively affected white sturgeon.

Threat rankings, presented in Table 4, are not directly comparable across populations. Table 4 also introduces the term “dam-affected system” to describe the state of the river basin. Dam-affected system is a generic way of describing the Columbia, Kootenay, and Nechako river systems, which are regulated by dams. Other anthropogenic factors are also present in these watersheds, particularly the Columbia and Kootenay rivers. River regulation is not necessarily the sole cause of recruitment failure; however, recruitment failure consistently occurs in systems with significant flow regulation.

Overall, the primary human activities that threaten white sturgeon in the wild are direct habitat loss, river regulation, harvest of prey/food, introduction of invasive non-native fish species, direct and indirect harvest, and release of pollutants (Table 4). However, addressing conservation needs for sturgeon will require more than simply limiting or prohibiting these activities. For example, at this time it is not feasible to remove large dams or flood control dikes to reclaim lost habitats. It may be necessary to understand the underlying mechanisms that control sturgeon abundance and distribution and use this information to develop acceptable strategies for protecting and restoring sturgeon populations. Further information about each threat presented in Table 4 is provided below. The National Recovery Team has also prioritized research and management activities needed to meet population and distribution objectives across all watersheds and these priorities are presented in Section 7.5 (Research and Management Activities Needed to Meet Population and Distribution Objectives).

Table 4. Table 4 describes historic and ongoing threats to white sturgeon and their habitats. Definitions for the levels of relative risk are provided in Table 3 and include the following categories: negligible, low, moderate (mod), high, and unknown. These are not comparable across populations. The table has two main columns: Potential Threat to Species and Habitat, and Level of Relative Risk. Under the “Potential Threat to Species and habitat” header are two sub-categories: Stressor and Activity. Under the “Level of Relative Risk” header are six sub-categories as follows: Lower Fraser, Mid Fraser, Upper Fraser, Nechako, Columbia and Kootenay; these last three sub-categories are indicated as Dam Affected Systems. Below the column headings there are seven abiotic and five biotic threats, each with its own dedicated row. As the table is read from left to right each threat is described and ranked as low, moderate or high impact for each population respectively.

Table 4. Summary of historic and ongoing  threats to white sturgeon and their habitats. Stressors are listed by type (abiotic or biotic), and the affected populations are indicated. To some extent these threats include natural limitations such as habitat or productivity. Definitions for the levels of relative risk are provided in Table 3 and include the following categories: negligible, low, moderate (mod), high, and unknown. These are not comparable across populations. Accessible version of Table 4.
Potential Threat to Species or HabitatLevel of Relative Risk
 Dam Affected System
StressorActivityLower FraserMid FraserUpper FraserNechakoColumbiaKootenay
Loss of habitat quality and quantity1Habitat changes are associated with flow regulation (e.g. affecting geomorphology, depth, velocity, substrate), as well as gravel and sand extraction; upland, foreshore, floodplain and estuary use and development, including bank protection, dyking and infilling, and other in-channel works.highmodmodhighhighhigh
Habitat fragmentationHabitat fragmentation occurs where there are impassable dams and/or dykes (e.g., Kootenay River) and through inadequate flows or water level changes.lowlowlowlowhighlow-mod
Altered hydrograph componentsAltered hydrograph components may be related to flow regulation, flow diversion, and anthropogenic activities causing climate change.lowlowlowhighhighhigh
PollutionPollutant sources include industrial inputs (pulp mill effluents, various wastewater, and smelting effluents), municipal and domestic sanitary and storm sewage, non-point source urban runoff, point source agricultural discharges and chemical over-sprays, and non-point source agricultural runoff.modmodlowlowmodlow
Fishing and industrial effects (direct and indirect)Fishing effects are related to poaching (illegal retention), recreational catch-and-release fishery, scientific inquiry and monitoring, aboriginal and commercial net fisheries, and by-catch in the aboriginal and recreational fisheries. Industrial effects include interactions with industrial facilities or operations, including equipment at hydro-electric facilities (turbines, draft tubes, locks),highmodlowmodmodlow
Reduced turbidityReduced turbidity may be related to flow regulation and stream channelization, which can influence water claritylowlowlowlowmodlow
Altered thermal regimeThermal regimes are affected by flow regulation and anthropogenic activities causing climate change.lowlowlowlowmodmod
Effects of small population sizeAnthropogenic factors causing recruitment failure.lowmodhighhighlowhigh
Hatchery and aquaculture effects on health and populationThese effects may occur from conservation aquaculture and commercial aquaculture.lowmodmodmodlowlow
Reduced or altered food supply (including fishing of white sturgeon prey base)Food supply is affected by commercial, Aboriginal, and recreational fishing, upland, foreshore, floodplain and estuary development, dams (fragmentation and hydrograph changes) and anthropogenic activities causing climate change.modhighhighhighmodmod
Change in ecological community (predation/competition)Ecological community composition can be affected by flow regulation, species introductions and movements, fishing effects, habitat alteration, and anthropogenic activities causing climate change.modlowlowlowhighlow-mod
DiseaseDisease rates can be affected by aquaculture, thermal regime changes (e.g., anthropogenic activities causing climate change, river regulation), introduction of pathogens, and introduction of pollutant stressors.lowlowlowlowlowlow

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4.1 Abiotic Threats

4.1.1 Loss of Habitat Quality and Quantity

Description -- The large rivers occupied by white sturgeon have a variety of interlinked habitats, including main channel, tributary confluence, foreshore, seasonally-inundated, and tidal and estuarine areas. Both habitat quality and quantity have declined throughout the species’ range, particularly in the regulated systems and the lower Fraser. Changes to white sturgeon habitat or to the habitats of prey species are believed to be directly related to impacts on recruitment and overall carrying capacity.

Potential influences -- Habitat changes are associated with flow regulation (e.g., geomorphology, depth, velocity, substrate), as well as gravel and sand extraction, upland, foreshore, floodplain and estuary use and development, including bank protection, dyking and infilling, and other in-channel works.

Assessment and level of confidence -- This is a very broad category and it is difficult to tease out the effects of quality from quantity. White sturgeon abundance and distribution is believed to be related to the availability of suitable habitat. Habitat use by white sturgeon is best known in the regulated watersheds, and least known in the upper and middle Fraser River. Changes in habitat associated with flow regulation are clearly implicated in the recruitment failure of the Nechako, Columbia, and Kootenay river populations. However, less is known about how other physical habitat modifications might have affected, or might now constrain the abundance and distribution of white sturgeon.

Sturgeon habitat can be directly impacted by river regulation in two important ways: i) changes in abundance and distribution of hydraulically suitable habitat; and, ii) geomorphic changes to instream habitats. If the regulated regime is sufficiently different from the natural flow regime then there is a potential for reduced function or loss of key spawning, incubation and rearing areas. Predation impacts may also be exacerbated due to increased water clarity below dams. In regulated rivers, fine sediments may build up both in the river channel and within reservoirs, and fine sediments have been shown to directly reduce embryo survival (Kock et al. 2006). The effects of fine sediment accumulation at spawning sites has now been identified as a likely mechanism for all three dam-affected populations (McAdam et al. 2005, Paragamian et al. 2009, McAdam 2012). However, this mechanism is yet to be proven by successfully restoring recruitment.

Upland, foreshore, floodplain and estuary use and development, floodplain and estuary removal through dyking, and instream modifications have led to habitat changes in most of the systems and are implicated in overall reductions in carrying capacity. Many of the foreshore and instream impacts continue to occur, but most large scale floodplain and estuary changes occurred several decades ago.

In both the upper and middle Fraser River the primary limiting factor appears to be habitat carrying capacity. River gradient is higher and the confined canyon means habitat is spatially limited. Prey species populations like salmon have also been reduced from historic levels and overall biological productivity is lower.

4.1.2 Habitat Fragmentation

Description -- Dam and reservoir construction has had a large influence on the distribution of aquatic habitat within the natural range of white sturgeon (Figure 2 and Figure 3 ). Formerly continuous habitats were split into smaller portions by impassable dams. Usable habitat has become permanently alienated, and dams have subdivided continuous populations. Habitat fragmentation is most pronounced in the Columbia and Kootenay rivers, although other systems are also affected.

Potential influences -- Habitat fragmentation occurs where there are impassable dams and/or dykes (e.g., Kootenay River) and through inadequate flows or water level changes.

Assessment and level of confidence -- In the upper Columbia River, white sturgeon are fragmented by impassable dams at Mica, REV and HLK. Habitat above Mica (now Kinbasket Reservoir) is no longer accessible, but was used historically (Prince 2001). Anecdotal and other evidence indicates white sturgeon may still occur upstream of REV and Mica (Prince 2001). HLK divides white sturgeon in the ALR from the transboundary population component; although the lock at the dam is used at least on a limited basis for passage, it is unknown whether it is directly used by white sturgeon to access upstream areas. However, white sturgeon have been observed in draft tubes at HLK dam. In the Kootenay system, Duncan Dam has alienated habitat in the Duncan River and a small number of white sturgeon are known to reside in Duncan Reservoir, but there is no viable spawning habitat in that area. Multiple dams on the Kootenay River downstream of Kootenay Lake are impassable, but most have been integrated with natural water falls located near Bonnington (i.e., South Slocan, upper Bonnington and lower Bonnington dams) and the level of fragmentation has not increased with the addition of these facilities. However, Brilliant and Corra Linn Dams in the lower Kootenay River were not constructed on natural water falls. In the Kootenai River upstream of Kootenay Lake, Libby Dam is located upstream of Kootenai Falls (Idaho), the suspected historic upstream limit of white sturgeon.

In the Fraser watershed, Seton Dam precludes use of Seton Lake, which was used historically by white sturgeon. In contrast, the Kenney Dam on the Nechako River does not fragment historic white sturgeon habitat. In the lower Fraser River a significant amount of floodplain and estuary habitat that was available historically is now isolated from the mainstem by dykes and is currently unavailable for use by sturgeon.

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4.1.3 Altered Hydrograph Components

Description -- The life history of white sturgeon is closely linked to river hydrology. White sturgeon are endangered in the Columbia, Kootenay, and Nechako rivers where the flows are highly regulated; additionally the Nechako River is subject to substantial out of basin diversion. The precise mechanisms responsible for population decline and recruitment failure are still unproven, but river regulation is heavily implicated. This hypothesis is not meant to address geomorphic-related impacts, which are discussed under Loss of Habitat Quality and Quantity.

Potential influences -- Altered hydrograph components may be related to flow regulation, flow diversion, and anthropogenic activities causing climate change.

Assessment and level of confidence -- Sturgeon are adapted to the natural hydrograph, having persisted successfully in large rivers for millennia. This impact hypothesis is built around the idea that sturgeon require a natural hydrograph, with seasonal fluctuations in river flow, particularly a pronounced spring freshet with bed mobilizing flows. The importance of winter flows has been considered by the Nechako River White Sturgeon Recovery Initiative (NWSRI) and the Kootenay River White Sturgeon Recovery Team (KRWSRT), but has received less attention elsewhere. In some cases, there is attention paid to flow-related microhabitat conditions in the regulated systems such as immediately below REV, but in general more attention has been paid to effects of flow regulation on substrate conditions, turbidity, and geomorphic influences on habitat.It is important to note that hydrographs may also affect lake and reservoir levels, with potentially significant impacts on white sturgeon. For example, Kootenay Lake water elevations in combination with Libby Dam operations influence sediment transport and water depth in spawning habitat on the Kootenai River (upstream in Idaho). Ongoing work in Washington State (Lake Roosevelt) is assessing the relationship between decreased flows and access to feeding habitats (by drifting larvae).

This impact hypothesis is not believed to be relevant at present to white sturgeon in the lower, middle, and upper Fraser River, due to the absence of mainstem dams, though the increasing demands for hydroelectric power and anthropogenic activities causing climate change may affect the hydrograph in the future and thereby affect localized areas.

4.1.4 Pollution

Description -- The large rivers in which white sturgeon reside are the receiving waters for a wide variety of point and non-point source pollutant discharges, which are introduced over a very broad geographic scale, especially in the more urbanized or industrialized areas such as the densely populated Lower Mainland. This concern is important because white sturgeon are more sensitive to contaminants than other species (Bennett and Farrell 1998, Vardy et al. 2011). Some point and non-point discharges, like antisapstain chemicals (Bennett and Farrell 1998), industrial effluents (Bruno 2004) and metals (Vardy et al. 2011, Little et al. 2012, Vardy et al. 2013) are known to be acutely toxic to juvenile white sturgeon. Waterborne toxins and pollutants in sediments may both present risks (Vardy et al. 2011, Fairchild et al. 2012). This threat refers to inputs of both liquid and solid waste discharges to rivers.

Potential influences -- Pollutant sources include industrial inputs (pulp mill effluents, various wastewater, and smelting effluents), municipal and domestic sanitary and storm sewage, non-point source urban runoff, point source agricultural discharges and chemical over-sprays, and non-point source agricultural runoff.

Assessment and level of confidence -- Laboratory studies assessing pollution impacts to white sturgeon have not been confirmed or disproven by field studies to date (Bruno 2004). Additionally, despite the sensitivity of white sturgeon, current water quality criteria apparently provide sufficient protection against metals like Cu, Pb and Zn (Vardy et al. 2011, Vardy et al. 2013). Point source discharges include pulp mill effluent, municipal and private sewage, and other industrial and agricultural effluents. Non-point sources include industrial, urban and agricultural runoff. Because white sturgeon are long lived and large in size they are susceptible to not only the direct effects of pollutants, but also to bioaccumulation of toxins in tissue and organs. Sturgeon in the Fraser River system that consume marine-derived foods such as salmon and eulachon may be less susceptible to local pollutants that could bioaccumulate through prey items, but may be more susceptible to global marine-derived pollutants. White sturgeon that depend solely on locally-derived food sources such as benthic invertebrates and resident fish may be more susceptible to bioaccumulation of “local” pollutants. Contaminant loads likely differ within and among populations depending in part on proximity to pollutant sources.

4.1.5 Reduced Turbidity

Description -- Drift by white sturgeon larvae may expose them to predation, which may decrease when water is turbid (e.g., during freshet). Regulated systems tend to have reduced turbidity due to sediment settlement in reservoirs and the diminished erosive potential of lower peak flows. Since predators of sturgeon larvae are primarily visual hunters, increased water clarity in regulated systems may allow for a higher predation rate on early juveniles (i.e., less than 1 year old).

Potential influences -- Reduced turbidity may be related to flow regulation and stream channelization, which can influence water clarity.

Assessment and level of confidence -- Laboratory tests indicate that predation rates decreased when turbidities increased above 60 NTU, but showed no statistically discernible effects at lower turbidity levels (Gadomski and Parsley 2005a). Natural turbidity levels associated with successful sturgeon spawning and recruitment were 6 to 92 NTU in the lower Fraser with an average during the spawning period of 42 NTU (Perrin et al. 2000). In the Columbia River below HLK, turbidities have been observed to range from 1 to 3 NTU (Upper Columbia White Sturgeon Recovery Initiative 2002), whereas limited pre-regulation data are only slightly higher (Steve McAdam, B.C. Ministry of Environment, personal communication).

This hypothesis has received the most attention in the Columbia River, and has some limited support from the Nechako and Kootenay Recovery Teams. The impact hypothesis is not considered to be currently relevant in any of the Fraser River populations.

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4.1.6 Fishing and Industrial Effects

Description -- White sturgeon (adults and late juveniles) are caught both intentionally and incidentally by fisheries that use a range of capture methods and gear, with effects varying by timing and gear type. White sturgeon also occasionally interact with industrial sites, including hydro-electric facilities, which can cause negative effects such as harm or mortality.

Potential influences -- Fishing effects are related to poaching (illegal retention), recreational catch-and-release fishery, scientific inquiry and monitoring, aboriginal and commercial net fisheries, and by-catch in the aboriginal and recreational fisheries. Industrial effects are related to interactions with hydro-electric facilities (e.g. turbines and draft tubes) and other industrial sites.

Assessment and level of confidence -- Late juveniles and adult white sturgeon have relatively high catchability, highlighting the potential for this impact mechanism. Multiple recaptures from many years of set line fishing for research purposes indicate this technique appears to cause little mortality, if set-lines are well-monitored. Information for size classes less than 140 cm also suggests that mortality due to catch-and-release fishing appears to be low (Robichaud et al. 2006). The same gear comparison study indicated that set salmon gillnet by-catch has a more notable mortality effect, than by-catch from drift gill nets or from catch-and-release angling. Effects on reproductive fitness from catch-and-release fisheries are unknown. The DFO Recovery Potential Assessment for White Sturgeon (Wood et al. 2007) indicated that the estimated level of by-catch in 2006 would not prevent recovery if natural recruitment were restored to historic levels . However, as juvenile populations increase from conservation aquaculture efforts, particularly in the Columbia, Kootenay and Nechako rivers, by-catch of these additional juveniles from angling or other capture methods is becoming more common.

Recent information indicates that incidental capture of juvenile white sturgeon might occasionally cause injury or mortality. Anecdotal information from provincial Conservation Officers and regional biologists suggest that juvenile (and potentially adult) by-catch in the Columbia River walleye (Stizostedion vitreum) and rainbow trout (Oncorhynchus mykiss) fishery (transboundary population component) is noteworthy, though it is unquantified at present. Recent autopsies of juvenile white sturgeon from the Columbia River suggest angling has caused mortality, but again, the frequency of deaths from angling is uncertain, and the impact on population viability is uncertain. Efforts to quantify mortality from recreational by-catch of juvenile white sturgeon will be necessary to determine the level of threat this poses to the Columbia River population. Further study of this issue as a whole is required to determine fishing effects on juveniles.

Directed harvest of white sturgeon is prohibited at present, not only for the SARA-listed populations, but the non-SARA-listed populations as well. The release of white sturgeon by-catch in recreational and commercial fisheries is mandatory and strictly regulated. The same is true for First Nations salmon gillnet fisheries, though in recent years First Nations have been known to retain white sturgeon caught as by-catch if they are found dead in their gillnets. Efforts to reduce or eliminate white sturgeon encounters in these FSC fisheries are underway. In the lower Fraser there is anecdotal information that illegal retention of sturgeon is increasing but the level of this impact is not well known. Studies of delayed mortality associated with net interception and angling in the lower Fraser River suggest that sturgeon captured in both set and drifted gill nets may be subjected to considerable levels of mortality (Robichaud et al. 2006). Mitigation of fishing impacts is presently being pursued on the Nechako River, as evidence indicates continuing pressure from set lines, gillnets, and rod and reel gear.

Ongoing industrial activities on the river are known to occasionally result in impacts to sturgeon and work is ongoing to mitigate those impacts. Several white sturgeon mortalities have been noted on the Kootenay and Columbia River that are thought to be associated with dams, likely through attempted upstream or downstream passage (Wood et al., 2007). More research is needed to understand the specific causal mechanism in some cases (Wood et al., 2007), but interaction with turbines, draft tubes, and locks may lead to negative effects. Several mortalities in and around Columbia River and Kootenay River dams and other industrial sites have been reported to DFO in the past several years, and mitigation efforts for industrial activities continue.

4.1.7 Altered Thermal Regime

Description -- White sturgeon metabolic rates are directly related to water temperature, so changes to temperature may affect multiple aspects of sturgeon biology. Temperature has also been implicated in terminating pre-spawning behaviour at certain locations (e.g., Paragamian and Kruse 2001). Increased winter temperatures along with higher flows may impact winter survival, although this has not been investigated yet.

Potential influences -- Thermal regimes are affected by flow regulation and anthropogenic activities causing climate change.

Assessment and level of confidence -- Increased winter temperature can increase energy demands during periods of low food abundance. Water temperature has been identified as an important spawning cue (Golder Associates Ltd. 2005a, Triton 2009), and altered temperatures during the spawning period have been associated with shifts in the timing of spawning (Tiley 2006). High temperatures have occasionally exceeded egg viability thresholds (e.g., at Waneta spawning site), and high temperatures were implicated as a potential cause in lower Fraser River adult white sturgeon die-offs in the mid-1990s (McAdam 1995). However, McAdam (2001) suggested that summer temperature changes below REV may not have been substantial, whereas in the fall, water temperatures below REV may actually be warmer than historic due to thermal retention. Further research on the effects of changes to the thermal regime on larval and juvenile stages is currently underway.

Water temperatures in the Nechako, Columbia and Kootenay rivers have been affected by river regulation. Thermal inertia in reservoirs and other large bodies of water can act to slow downstream river warming in the spring, alter peak summer temperatures (either up or down), and delay or reduce winter cooling (Hamblin and McAdam 2003). Scientific data also clearly indicate that the climate is changing and animal and plant distributions are responding to these changes (Parmesan and Yohe 2003). River temperatures have changed in response to global climate change and this trend is expected to continue (Morrison et al. 2002). Impacts from climate change may exacerbate other impacts to white sturgeon and their prey, including either competition or predation from invasive, non-native fish species. The direct and indirect response of white sturgeon populations to climate change is of concern. However, these changes are beyond the scope of this strategy.

For all rivers where white sturgeon are undergoing recruitment failure it is important to note that thermal regime alterations are not considered the primary cause of recruitment failure. The Nechako River is perhaps the best example, since thermal regime would have been affected by flow regulation for 15 years prior to the initiation of recruitment failure (1967). Ongoing monitoring under the current thermal regime also indicates that both past and existing thermal regimes were/are not a primary cause of recruitment failure. However, an altered thermal regime may nevertheless limit recovery of the species, with the best example being the effects of temperature at the Revelstoke spawning site (Tiley 2006).

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4.2 Biotic Threats

4.2.1 Effects of Small Population Size

Description -- Small populations face substantial risks to their long-term viability, even when habitat and food resources are not limiting. For example, small populations are more susceptible to random demographic and environmental variability, and mortality can begin to increase as the population declines below a specific abundance threshold (Allee effect), due to changes in predation or mating success. Genetic effects also become significant, leading to further reductions in survival through inbreeding depression and a loss of genetic variance that compromises adaptability to future conditions.

Potential influences -- Small population size may occur from historic influences (e.g., food supply, habitat availability, water quality, disease, etc.), or anthropogenic factors causing recruitment failure.

Assessment and level of confidence -- The general effects of small population size are well-known and well-supported in the scientific literature through empirical and theoretical studies. The extent to which specific risks apply to white sturgeon has not been well-studied, but several of the populations are below general guidelines for long-term viability, and some are below guidelines for short- to medium-term viability. The ratings in Table 4 are based on knowledge of current population size, and do not include consideration of past and potential current isolation between components of these populations.

4.2.2 Hatchery and Aquaculture Effects

Description -- Hatchery effects are well-known for salmon and other species with captive breeding programs. There are specific risks to naturally-reproducing white sturgeon from conservation and commercial aquaculture programs, including population effects, genetic effects, and disease transfer. There is also concern regarding imprinting behaviour and whether hatchery-reared fish will be able to find or select suitable spawning locations at maturity.

Potential influences -- These effects may occur from conservation aquaculture and commercial aquaculture.

Assessment and level of confidence -- Specific risks to white sturgeon include displacement of wild fish by those of hatchery origin, loss of genetic integrity of the wild population, accidental release of a large number of closely-related individuals, and wild population reduction from wild broodstock capture. Many of these effects are well-known from the scientific literature. The populations at greatest risk are those with proposed or ongoing supplementation programs, since migration among populations is likely very low. However, downstream or adjacent (e.g., Upper Fraser) populations could also be put at risk. The proposed or ongoing conservation fish culture programs have been designed around breeding plans (Kootenai Tribe of Idaho 2004, Nechako White Sturgeon Recovery Initiative 2005), so the risk of unintended effects is deemed to be low to moderate (Table 4), and is considered a net-benefit when compared to the unacceptably high risk of population extirpation in the absence of these programs. Current risks from commercial aquaculture are low, due to the low intensity of upland contained commercial operations and the absence of these operations from marine waters or the floodplain of sturgeon-bearing rivers. These risks could increase if sturgeon aquaculture intensifies or if rearing methods change, though commercial aquaculture is not permitted for the SARA-listed populations. Impacts related to this hypothesis have not been observed, and observation of such effects would likely increase the assessed threat ranking.

4.2.3 Reduced or Altered Food Supply

Description -- The elimination, reduction or alteration of white sturgeon prey base may have important effects on white sturgeon abundance and distribution.

Potential influences -- Food supply is affected by commercial, Aboriginal, and recreational fishing, upland, foreshore, floodplain and estuary development, dams (fragmentation and hydrograph changes) and anthropogenic activities causing climate change.

Assessment and level of confidence -- Anadromous species including smelt, eulachon and salmon, are an important part of the food base for Fraser and Nechako white sturgeon, and anadromous salmon were formerly an important part of the prey base for upper Columbia white sturgeon. In Pacific Canadian waters, salmon are harvested in commercial, Aboriginal, and recreational fisheries, smelt are harvested in limited commercial and recreational fisheries, eulachon are harvested in Aboriginal fisheries. White sturgeon are acknowledged as predators of these species, but currently there are no explicit management actions in Canadian smelt, salmon or eulachon fisheries related to these species’ roles as a critical food source for white sturgeon. The construction of Grand Coulee Dam (Washington, U.S.) eliminated anadromous salmon runs to the upper Columbia River. Changing nutrient regimes may also cause diet shifts or impact prey species. For example, the introduction of Mysis relicta has provided an additional food source for juvenile white sturgeon in the Columbia River. Land use and development changes in some systems continue to have a large impact on foreshore, floodplain and estuarine habitats that provided substantial food inputs. Identification of critical habitat for each population in areas where sturgeon are known to feed may help in protecting these habitats not only for sturgeon, but for the prey species critical to their survival (see Section 8: Critical Habitat). The extent to which changes in food supply have affected or continue to influence white sturgeon abundance has not been quantified, but diminished food production has likely had some impact. The extent to which each of the white sturgeon populations are currently food limited is not known.

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4.2.4 Change in Ecological Community

Description -- Increased predator and/or competitor abundance may be an important threat to white sturgeon, especially in the Columbia, Kootenay and lower Fraser rivers. This could include shifts in native fauna and/or introduction of non-native species.

Potential influences -- Ecological community composition can be affected by flow regulation, species introductions and movements, fishing effects, habitat alteration, and anthropogenic activities causing climate change.

Assessment and level of confidence -- The ecological community has changed dramatically in most basins due to habitat alterations (e.g., hydroelectric dams, channelization) and non-native species introductions. Introduced species may be able to take advantage of habitat alterations and out-compete or prey on the native fish community. Non-native species may increase predation on white sturgeon eggs and larvae or out-compete early juveniles for food. Non-native species introductions include walleye and Mysis relicta into the Columbia River and increased native or non-native cyprinid and centrarchid species in the Nechako and Fraser river systems.

After deliberate introduction into the Columbia River system in the U.S., non-native walleye have dispersed into the upper Columbia River and some of its tributaries. Predation of early juveniles and eggs by walleye has been a suggested cause of recruitment failure of white sturgeon in the Columbia River. Other community changes in the Columbia River include the loss of anadromous fish populations, increases in the rainbow trout population, and introductions of warm-water species such as perch, pumpkinseed, crappie, bass, etc. Anecdotal information indicates that an increase in cyprinid species has been observed in the Nechako River and may be related to white sturgeon decline. Non-native fishes such as largemouth bass, black crappie and carp have spread and increased significantly over the last decade in the lower and mid-Fraser River through range extension and continued unauthorized introductions (Tovey et al. 2008; Erin Stoddard, B.C. Ministry of Forests, Lands and Natural Resource Operations, personal communication). The cyprinid population in the lower Fraser is also significant and appears to be changing.

The plausibility of this hypothesis was initially rated as high for the Nechako River (Korman and Walters 2001), but support for it has not been maintained by the Nechako Recovery Team, in part because the hypothesis does not explain the rapidity of white sturgeon decline. Predators such as walleye are a substantial concern in the lower Columbia River, but effects are mostly unstudied. If potential impacts are proven in any area, impact rankings would likely increase. If valid, this impact may be one of several causes of declines in early juvenile survival.

4.2.5 Disease

Description -- Several parasites and diseases of white sturgeon are known to be present in British Columbia, and additional pathogens may be introduced. For example, the white sturgeon papova-like virus was identified from one wild, subyearling Columbia River white sturgeon (Canadian Columbia River Inter-Tribal Fisheries Commission 2005). No obvious external signs were noted, but microscopic lesions were detected in the gill, liver, spleen and kidney (Canadian Columbia River Inter-Tribal Fisheries Commission 2005). Also, five parasites have been documented in the Columbia River including three trematodes (Nitzschia quadritestes, Tubulovesicula lindbergi, Cestrahelmins rivularis), a cestode (Amphilina bipunctata), and a nematode (Cystoopsis acipenseri) (Canadian Columbia River Inter-Tribal Fisheries Commission 2005). There was a recent occurrence at the Kootenay Trout Hatchery of the cnidarian parasite Polypodium hydriforme (Ron Ek, Kootenay Trout Hatchery, personal communication), which is known to infect the ovaries and eggs of several species of sturgeon and similar fishes (Raikova 2002). Diseased juvenile sturgeon are regularly encountered in the lower Fraser, but the causes of disease have not been diagnosed (Erin Stoddard, B.C. Ministry of Forests, Lands and Natural Resource Operations, personal communication).

Potential influences -- Disease rates can be affected by aquaculture, thermal regime changes (e.g., anthropogenic activities causing climate change, river regulation), introduction of pathogens, and introduction of pollutant stressors.

Assessment and level of confidence -- The persistence of white sturgeon demonstrates their ability to co-exist with naturally-occurring diseases and parasites. However, risks of disease outbreak would increase under stressful conditions (e.g., increased temperature, high pollutant concentration). Risks associated with introduced diseases and their vectors or mechanisms of spread are unknown. The risks of disease are generally not well-defined at this time. The movement of large numbers of fish between watersheds (e.g., release from hatcheries) is also a potential threat, but is amenable to control. Disease transfer may also occur from hatchery and aquaculture facilities. Introduced species may also be a vector for diseases, but the risks associated with this are unknown.

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5. Habitat Trends

White sturgeon habitat has declined in both quality and quantity throughout the species’ range in Canada. The primary influences on habitat are river regulation, discharge of pollutants, and foreshore, floodplain and estuarine development. The extent and magnitude of these influences vary among geographic locations. River regulation from hydroelectric developments has had a large influence on habitat in the Columbia, Kootenay and Nechako systems. The effects of hydroelectric dams and developments may include creation of migration barriers, changes to water quality (turbidity, nutrient status, Total Gas Pressure (TGP), etc.), streamflow patterns, water temperature, and physically suitable habitat. These factors can also alter the ecological communities that reside in these systems. Floodplain isolation and development has altered habitats in the lower Fraser and Kootenay rivers, and to a lesser extent in other systems. Estuary development has altered estuarine habitats that are used by lower Fraser River white sturgeon. Pollutant discharges and spills occur on all systems but are most notable in the highly populated areas of the lower Fraser River (where there is considerable urban, agricultural and industrial use and development) and on the upper Columbia and Kootenay rivers (with a long history of mining, smelting and pulp mill operations). Below is a discussion of general habitat trends, focusing on alterations that may impact white sturgeon for each geographic region.

5.1 Fraser River

Overall, numerous anthropogenic changes have occurred in this system, particularly in the lower Fraser River with its large urban population, but few linkages exist to demonstrate direct impacts from these changes to white sturgeon distribution, abundance or biology. In the middle and upper Fraser River there have been relatively few changes in physical habitat, although some impacts to water quality have occurred from industrial and municipal effluent discharges (e.g., pulp mills).

White sturgeon populations in the Fraser River are the only populations still experiencing relatively natural hydrographs. The Fraser River mainstem does not have any hydroelectric developments, thus these populations are not considered to be directly affected by dams. However, dams exist on tributary systems (e.g., Nechako, Bridge, Seton, Stave, Coquitlam, and Allouette rivers). While flow regulation is believed to have had only a minor influence on overall flows in the Fraser River (Hatfield and Long 2004), localized habitat changes may occur in the vicinity of some facilities (Erin Stoddard, B.C. Ministry of Forests, Lands and Natural Resource Operations, personal communication). Tributary impoundments can affect downstream food/prey availability, and since they are all salmon spawning streams they have the potential to affect salmon abundance. White sturgeon rely heavily on returning spawning salmon for food.

Historically, significant flooding occurred annually in the braided channels at the Fraser-Nechako confluence near Prince George. These flooded reaches were likely used by Nechako and upper Fraser River white sturgeon. Extensive seasonal flooding also occurred throughout the lower Fraser River, where vast braided reaches and enormous areas of connected swamp and marsh occurred (North and Teversham 1984, Perry 1984, Boyle et al. 1997). However,, most of the floodplain and estuary areas in the lower Fraser River were dyked and drained in the 1940s for agriculture and urban development (e.g., Sumas Lake), which affected white sturgeon habitat areas. There are currently over 600 km of dykes that isolate braided channels and floodplain areas in the lower Fraser River (B.C. Ministry of Water, Land and Air Protection 2002). Hardened bank protection, dredging, gravel mining, land reclamation, and channelization activities occurred historically and continues in the lower Fraser River (Lane and Rosenau 1995, RL&L Environmental Services Ltd. 2000a, Rosenau and Angelo 2000, Rosenau and Angelo 2005). In contrast, the limited floodplain areas in the middle Fraser River are primarily intact. Dyking and floodplain drainage has not been widespread in these reaches upstream of the town of Hope due to the natural scarcity of floodplain areas, and the river’s higher gradient and confined channels associated with the steeper topography.

With often steep, naturally confined channels and less abundant spawning salmon, habitat and prey/food availability in the upper and middle Fraser River is likely a limiting factor for sturgeon population size. Habitat availability in the lower Fraser River is less abundant compared to historic levels, due to human activities and development, though detailed information on habitat use by age-class and life history stage is limited. The extent of estuarine or marine habitat use, or habitat used for spawning remains unclear. However, it is likely that sturgeon productivity is increasingly limited by the availability of prey/food. For example, spawning eulachon and salmon returns have been significantly lower in recent years, and demands for human use of returning salmon have been increasing (e.g., Fisheries and Oceans Canada 2010a, b).

The Fraser River acts as the receiving waters for many point and non-point source pollutant discharges, especially since the Fraser River Basin supports most of the province’s economy and human population. Pollutants are introduced into the Fraser River over a broad geographic scale, though the majority are downstream of the Fraser-Nechako River confluence, and the highest levels of discharge occur in the Lower Mainland downstream of Mission (Fraser River White Sturgeon Working Group 2005).

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5.2 Nechako River

The Nechako River is affected by anthropogenic impacts and currently has the smallest SARA-listed white sturgeon population. The main impact in the watershed was the construction of the Kenney Dam in 1952, which created the Nechako Reservoir and diverted water into the Kemano River; downstream habitat is influenced by this dam. Subsequent water management in the Nechako River system has significantly altered its hydrology by decreasing overall annual flow and by altering its seasonality.  However, the thermal regime of the river is not substantially different than that observed pre-dam because summer water temperatures are actively managed primarily for migrating sockeye salmon; without this active management the thermal regime would likely show substantial impacts (Nechako White Sturgeon Recovery Initiative 2004).

Post-impoundment, the Nechako River’s channel has become more simplified. Side channels near Vanderhoof decreased in the 1960s due to the combined effect of flow regulation and a large upstream avulsion at Cheslatta Falls (Rood and Neill 1987, Hay and Company Consultants Inc. 2000). Fine sediments were deposited over the white sturgeon spawning reach near Vanderhoof and marked the initiation of recruitment failure (McAdam et al. 2005). Hydrodynamic modeling (Northwest Hydraulic Consultants 2006, 2008) has confirmed that hydraulic characteristics of the site have changed following regulation, and indicate that flow restoration may not be sufficient to restore the site to its previous state.

Water quality characteristics (e.g., temperature, turbidity, nutrients, pollutants) in the Nechako River are influenced primarily by river regulation from Nechako Reservoir, local weather effects and discharge of municipal effluent at Vanderhoof, but their effects on white sturgeon are unknown. Pulp mills, located at the mouth of the Nechako River, discharge into the Fraser River and are assumed to have had minimal influence on Nechako River white sturgeon.

5.3 Columbia River

The Columbia River is affected by several anthropogenic impacts, but the main influence on aquatic habitat has been the construction of large mainstem dams for flood control and electricity generation. Impacts from hydroelectric developments on upper Columbia River white sturgeon likely first occurred in 1941 with the completion of Grand Coulee Dam on the Columbia River downstream of the Canada-U.S. border (Figure 3). The construction of Grand Coulee Dam resulted in the loss of anadromous salmon returns to the upper Columbia River, which were likely a main source of prey for white sturgeon. Completion of the third powerhouse at Grand Coulee Dam in 1974 subsequently altered the operations of FDR. Massive drawdowns of Lake Roosevelt occurred in 1969 and 1974 related to the powerhouse construction activities, in both cases exposing Kettle Falls. In 1968, the construction of HLK isolated white sturgeon in the former Arrow Lakes system and created the ALR (Figure 3). Analysis of mitochondrial DNA indicates that white sturgeon within the ALR and fish located immediately downstream of HLK Dam are genetically similar, which suggests that they historically spawned in the same location (McAdam 2012). It is currently unclear whether this group (ALR and HLK fish combined) historically spawned upstream of HLK, but fin ray chemistry indicates that juveniles may have reared upstream (Clarke et al. 2011). The ecosystem above ALR was further fragmented by the construction of Mica Dam in 1973, which flooded over 250 km of the Columbia River mainstem, and replaced riverine habitat with a reservoir environment (Upper Columbia White Sturgeon Recovery Initiative 2002). The construction of REV in 1984 effectively eliminated the 130 km section of flowing river between Mica Dam and ALR and replaced it with reservoir habitat (Upper Columbia White Sturgeon Recovery Initiative 2002).

On the upper Columbia River, habitat diversity and contiguity was lost as a direct result of impoundment, and habitat suitability was affected by flow regulation (Upper Columbia White Sturgeon Recovery Initiative 2002). Regulation of flows on the Columbia River have resulted in a more uniform river channel, decreased the overall annual flow, and altered flow seasonality (Upper Columbia White Sturgeon Recovery Initiative 2002). These changes have reduced aquatic habitat diversity, altered flow conditions at known and potential spawning and nursery areas, and may have altered substrates in rearing habitats necessary for survival. Complex habitats were lost that may have provided important seasonal forage areas and refuges from high discharges, such as side channels and low-lying marshlands.

In addition to physical changes, dams on the Columbia River have caused a number of changes in water quality, including temperature, turbidity, total gas pressure and nutrient status. Upstream of REV, water temperatures are cooler in summer and warmer in fall and winter, in comparison to the pre-impoundment period (McAdam 2001, Tiley 2005). Downstream of HLK, average fall and winter temperatures are similar, but temperatures from May through September are approximately 2° to 3°C warmer than occurred historically (Hamblin and McAdam 2003). On the Pend d’Oreille River, a regulated tributary to the Columbia River located just upstream of the border, water temperatures may be warmer than they were historically; relative to the Columbia River they rise faster during the spawning season and become much warmer (e.g., 24˚C in 1998). However, pre-impoundment data are lacking for comparison. A wider range of water temperatures and more complex thermal environment occurs in FDR relative to historic conditions in the river.

Water quality has been influenced by a variety of industrial and municipal activities, but has improved in recent years through better waste discharge and management practices. Residual effects are still a concern, since historic and current industrial activity and residential development along the river have contributed metals and a myriad of organic compounds to water and sediments.

Turbidity in the Columbia River at Birchbank, downstream of the Kootenay River confluence, is generally below 3 NTU year round (Ministry of Environment data – EMS database; Golder Associates Ltd. 2006d). Anecdotal evidence indicates that turbidity was historically higher due to runoff from glacial systems; though historical data are limited (see Van Winkle 1914, McAdam 2012), they do not indicate that turbidity during the freshet period has declined substantially. Settlement within the historical Arrow Lakes may explain the presence of naturally reduced turbidity downstream in the Columbia River. Turbidity data on Pend d’Oreille River is limited, but indicates higher turbidity events during some peak freshets (e.g., 13 NTU in 1997) even after dams were installed.

Natural nutrient inputs into the upper Columbia River system have been reduced by the combined effects of the elimination of anadromous salmon runs and reservoir construction. Prior to the construction of Grand Coulee Dam, anadromous fish runs were likely an important food source for white sturgeon and a significant source of nutrients (e.g., nitrogen, phosphorus, and trace elements) for aquatic food webs. Reservoirs act as nutrient sinks and reduce downstream transport from the upper basin. These changes have likely reduced the carrying capacity of the system for many fish species, including white sturgeon. The ongoing ALR fertilization program is designed to offset some of this impact by improving overall pelagic production, with the specific aim to improve kokanee abundance (Schindler et al. 2006). The entrainment of Mysis relicta through HLK Dam has become an important downstream food source for juvenile sturgeon (Golder Associates Ltd. 2007).

Substantial changes have also occurred within the aquatic ecological community through the introduction of exotic species, which have flourished because of anthropogenic changes in the upper Columbia River. The pre-development fish community included large numbers of anadromous fishes including spring and summer Chinook salmon (Oncorhynchus tshawytscha), sockeye salmon (Oncorhynchus nerka), steelhead (Oncorhynchus mykiss), and possibly Pacific lamprey (Lampetra tridentata). The resident fish community included bull trout (Salvelinus confluentus) and burbot (Lota lota). The primary changes have been the elimination of anadromous species and an increase in introduced predatory species such as walleye.

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5.4 Kootenay River

There is a long history of development within the Kootenay River watershed that includes hydroelectric development, as well as channel stabilization and modification for flood control. Channel alteration and other factors apparently affected white sturgeon recruitment prior to the completion of Libby Dam in 1972; however, Libby Dam operations appear to have been the primary factor that led to complete recruitment failure (Duke et al. 1999).

Several dams occur in the Kootenay watershed, including five facilities on the lower Kootenay River (Corra Linn Dam, Upper and Lower Bonnington Dams, South Slocan Dam, and Brilliant Dam), Libby Dam on the Kootenai River, and Duncan Dam on the Duncan River, a tributary to Kootenay Lake (Figure 3). The construction and operation of these dams led to habitat alterations similar to those discussed for the Columbia River population. Upper and Lower Bonnington and South Slocan dams did not fragment the white sturgeon population because they were built on natural waterfalls that have existed for approximately 10,000 years (Northcote 1973).

Changes to aquatic habitat with the construction of Libby Dam in 1972 have been notable because of the significant flow alterations and their effect on spawning habitats (Duke et al. 1999). For example, average spring peak flows have been reduced by more than 50 percent, and winter flows have increased by 300 percent compared to pre-dam values (Paragamian et al. 2005). The naturally high spring flows, thought to be required by white sturgeon for reproduction, now rarely occur. Lower freshet flows at Libby Dam and lower spring maximum elevation of Kootenay Lake are thought to have contributed to white sturgeon decline by affecting depths at their spawning location and causing white sturgeon to spawn in locations with sub-optimal conditions (U.S. Fish and Wildlife Service 2006; Matt Neufeld, B.C. Ministry of Forests, Lands and Natural Resource Operations, personal communication). Habitat changes from construction and operation of Duncan Dam (e.g., nutrient retention, lack of access to prey habitats) have also been implicated in contributing to the decline of Kootenay River white sturgeon (Duke et al. 1999).

River regulation has altered water temperatures in the Kootenai River so that they are now typically warmer (by approximately 3˚C) during the winter and colder (by approximately 1 to 2˚C) during the summer compared to pre-Libby Dam conditions (Partridge 1983, Duke et al. 1999). Spring temperature conditions are also currently cooler compared to pre-dam conditions due to thermal stratification in the reservoir and constraints around elevation control for withdrawals (Brian Marotz, Montana Fish, Wildlife & Parks, personal communication).

Alterations to floodplain habitats in the Kootenay system are a contributing factor to white sturgeon decline (Duke et al. 1999). For example, side-channel and slough habitats were eliminated from Bonners Ferry (Idaho) to Creston (B.C.) due to dyking and bank stabilization in the early 1950s for flood protection (Figure 3; Constable 1957, Duke et al. 1999). These habitat changes resulted in reduced aquatic habitat diversity, altered access or flow conditions at potential spawning and nursery areas, and altered substrates in incubation and rearing habitats (Partridge 1983, Apperson and Anders 1991, Duke et al. 1999).

Kootenay Lake is an important habitat area for Kootenay River white sturgeon. Based on a review of limnological studies of Kootenay Lake, Daley et al. (1981) concluded that biological productivity decreased markedly since construction of Libby Dam. Reduced productivity is thought to have decreased prey abundance and food availability for some life stages of sturgeon downstream of Libby Dam, and possible reduction in the carrying capacity of Kootenai River and Kootenay Lake. The ongoing Kootenay Lake Nutrient Restoration Program has been successful in partially offsetting nutrient declines and increasing biological productivity in the lake (Binsted and Ashley 2006, Sebastian et al. 2010). Monitoring confirms that the density and abundance of kokanee is higher than before fertilization started (Sebastian et al. 2010).

Excessive nutrients were once a problem in the upper Kootenai River, prior to the construction and operation of Libby Dam. Waste water control and effluent recycling measures were initiated in the late 1960s and significant improvements in Kootenai River water quality were noted by 1977 (Duke et al. 1999). Fertilizer processing, sewage, lead-zinc mine, and vermiculite discharges have been eliminated, but many of these pollutants and contaminants persist, and are primarily bound in sediments (Duke et al. 1999). Ultimately, the effects of these pollutants on sturgeon reproduction and survival are unknown but are likely minimal, especially in the Canadian section.

6. Knowledge Gaps

There are a number of key knowledge gaps that apply to almost all white sturgeon populations, whereas others are specific only to certain populations. General knowledge gaps are discussed in this section and are separated from knowledge gaps specific to the task of identifying critical habitat (see Section 8: Critical Habitat). Studies to address the knowledge gaps identified here are provided in Appendix A. In some cases it will not be necessary to address data gaps separately for each population because there are opportunities to fill gaps in one area and to transpose that learning across the species’ range. Whereas impacts across populations are more likely to be similar in nature, it is possible that impact mechanisms may differ. Activities needed to meet population and distribution objectives for SARA-listed populations, including those addressing population-specific and general knowledge gaps, are provided in Section 7.5 (Research and Management Activities Needed to Meet Population and Distribution Objectives).

The four most significant data gaps across the entire species’ range are: i) causes of recruitment failure in dam-affected systems; ii) design and implementation details of conservation fish culture; iii) clarification of existing threats; and, iv) basic biological information requirements. These are discussed further below.

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6.1 Recruitment Failure

Chronic recruitment failure is the primary cause of population declines in dam-affected systems. In all the three rivers showing recruitment failure (i.e., Columbia, Kootenay and Nechako) regular spawning occurs, but viable offspring do not recruit to the juvenile stage in sufficient numbers to sustain the population. This pattern has been well described, but detailed research into the causes of recruitment failure is relatively recent. Such research is complicated by the possibility of multiple causal factors and likely variation in the strength of particular factors among populations.

Recent reviews completed for the Columbia River population examined a variety of recruitment failure hypotheses, defined key hypotheses, and identified potential programs for assessment and recovery (Gregory and Long 2008, McAdam 2012). Key hypotheses for the Columbia River population included: effects of hydrograph alterations on predation of yolk sac larvae and feeding larvae; effects of diminished substrate suitability on early life stage habitat availability; effects of changes in the fish community on early life stage predation rates; and, effects of altered substrate on decreasing prey availability for early life stages and early juveniles. Nearly all hypotheses apply to the first year of life, and principally the early life history stages of egg, yolk sac larvae and feeding larvae. Although evaluations based on expert elicitation indicated similar levels of support for multiple hypotheses (Gregory and Long 2008) a recent weight of evidence evaluation suggests substrate change may be the proximate cause of recruitment failure. Similar findings have also been achieved for the Nechako (McAdam et al. 2005) and Kootenay (Paragamian et al. 2009, McDonald et al. 2010). Research on the Kootenay River population has gathered a broad variety of evidence over multiple years of studies (e.g., Paragamian et al. 2001, Paragamian et al. 2002, Paragamian et al. 2005), the culmination of which suggests that adults are spawning in areas where high levels of sand substrate in the river lead to essentially zero survival to the yolk sac larvae stage. For the Nechako River population, the presence of a clear association between historic habitat changes and recruitment patterns (McAdam et al. 2005), in conjunction with subsequent laboratory and field studies, provides further evidence that substrate alterations have led to habitat changes that diminish both egg and yolk sac larvae survival (McAdam 2012). While both the Kootenay and Nechako River findings emphasize survival at either the egg or yolk sac larvae stage, evidence from spawning sites in Washington State indicates that survival during the initiation of exogenous feeding may also be limiting (Howell and McLellan 2011).
Studies on all three dam-affected populations represent important advances in understanding recruitment failure. However, significant information gaps will remain until experimental recruitment restoration can be successfully demonstrated. To that end, laboratory and in situ (i.e. in-river) studies are required to identify factors within these rivers currently limiting early life history survival and particularly how habitats might be effectively altered to restore recruitment. It is particularly important to emphasize the importance of in situ studies for these challenging early life stages, since it is critically important that the results can be demonstrated under natural conditions. A useful approach may be the sequential increase in the scale of field studies in order to build an understanding sufficient to confidently examine recruitment restoration methods at the whole-river scale. Recent experimental substrate restoration studies in the Nechako (McAdam 2012) and Columbia (Crossman and Hildebrand 2012) represent very important steps in the direction toward larger scale restoration.

The following knowledge gaps are particularly important.

  1. Improved understanding of survival rates at the egg, yolk sac larvae and feeding larvae stages.
  2. Manipulative experiments, and particularly in situ experiments, to understand factors affecting survival during early life history.
  3. Understanding of habitat changes that were likely associated with both historic recruitment failure and contemporary recruitment pulses.
  4. Additional experimental evaluation of habitat restoration measures (and their maintenance) at a scale sufficient to create detectable recruitment. This could be done with out-planted eggs/yolk sac larvae/feeding larvae, but ultimately must address habitat restoration in conjunction with wild spawning fish.
  5. Investigation of feasible means by which identified habitat restoration measures can be implemented and maintained in a manner that leads to sustained recruitment in conjunction with wild spawning.

6.2 Conservation Fish Culture (CFC)

Population supplementation is proposed as a temporary, but long term (potentially 40+ years) measure to prevent extirpation of impacted white sturgeon populations. The general techniques required to spawn and rear white sturgeon in a hatchery environment are well-understood and conservation aquaculture measures have been successfully used in the Nechako, Columbia and Kootenay rivers. However, there are still some knowledge gaps that should be investigated to maximize the effectiveness of this recovery technique, including:

  1. Investigate the role of environmental imprinting in the hatchery as it may relate to later life stages.
  2. Determine optimal size at release to maximize survival rates.
  3. Investigate and adapt CFC practices to include captive broodstock strategies should they become required.
  4. Update breeding plans as new genetic information becomes available.
  5. Investigate improvements in hatchery rearing practices.
  6. Update hatchery practices as technology and methods become available.
  7. Develop and implement best management practices for white sturgeon CFC.

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6.3 Broad Strategy to Address Threats

Many threats to white sturgeon are known and have been described in some detail (see Section 4: Threats), although in most cases it remains difficult to quantify risks or the efficacy of different mitigation measures. Clarifying threats would allow resources to be more effectively distributed to the most useful mitigation measures. Specific needs include:

  1. An assessment of direct and indirect effects from various targeted and non-targeted incidental recreational, commercial and Aboriginal fisheries. This work should build on results from previous studies (e.g., Robichaud et al. 2006), which indicated significant potential effects from direct and indirect catch. This could also include determination of the potential threat of the white sturgeon recreational kill fishery in northern Washington State marine waters.
  2. Assessment of whether ecological shifts have occurred to such an extent as to inhibit recovery efforts (e.g., multiple stable ecological states, introduced and invasive species).
  3. Effect of pollutants on wild sturgeon:
    1. Identify the major pollutants that affect each white sturgeon life stage;
    2. Develop an understanding of the lethal and sublethal effects of these pollutants on each white sturgeon life stage; and,
    3. Develop an understanding of bioaccumulation in sturgeon tissues and effects of pollution on sturgeon populations.
  4. Flow issues have been identified as a significant threat in the three dam-affected populations. However, it is unclear how factors such as flow, river stage, and temperature influence these populations. The dam-affected systems are regulated under Canadian and international regulations and agreements and there is no current plan to remove the dams, so it will be necessary to understand how to manage flows to address recruitment failure, habitat quantity and quality, predation and other threats to white sturgeon.
  5. An assessment and/or monitoring program for angling effects on juvenile white sturgeon and their survival is required for areas supplemented by conservation fish culture. This is currently most needed for the transboundary reach of the Columbia River.

6.4 Basic Biological Information Needs

There are numerous knowledge gaps in our understanding of the biology of white sturgeon. Such information is of key importance in helping to monitor populations, model population dynamics, and inform mitigation and recovery techniques. These gaps include:

1. Demography:

  • Population abundance and trends;
  • Population structure (size, age, sex ratios, genetic differences within and among populations);
  • Genetically effective population sizes; and,
  • Age-specific mortality rates and mortality sources.

2. Life history ecology:

  • Ecology of early life stages (particularly habitat use, survival rates, growth rates and recruitment);
  • Frequency of spawning;
  • Fidelity and frequency of spawning habitat use, and,
  • Influences on timing and location of spawning, such as spawning cues and habitat requirements.

3. Spawning Migration:

  • Spawning triggers in populations not affected by dams;
  • Gene flow between and within populations.

4. Feeding:

  • Food sources by life stage and population, an understanding of historic trends in these prey species, and a functional relationship between sturgeon indicators (e.g., survival, growth, reproduction, etc.) and food supply.

5. Habitat:

  • Abundance and distribution of available habitats;
  • Quality of available habitat;
  • Historic habitat availability (this information provides context for discussions regarding population targets and what can be achieved in terms of white sturgeon abundance);
  • Limiting habitats of major life stages (this relates to critical habitat definitions); and,
  • Carrying capacity of currently available habitats, and assessment of whether abundance targets can be met with this (i.e., is habitat restoration required?).

6. Monitoring Methods:

  • Development of good field indicators of sex, age, sexual maturity, and population identity;
  • Development of additional nuclear DNA markers to allow identification of parentage for hatchery-produced sturgeon (for Kootenay and Nechako populations only); and
  • Development of additional techniques for monitoring early life stages (e.g., stage specific survival, larval quality indicators).

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