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Recovery Strategy for the Nooksack Dace (Rhinichthys cataractae) in Canada

2. Threats

2.1 Identification of the threats to the survival of the species

The prospects for recovery of a species at risk depend upon its vulnerability to the threats facing it, their severity and ubiquity across the range. In the following sections we summarize detailed analyses of each of these factors, taken from(Pearson 2004a, b). 

Eight factors (Table 1) are considered threats based on knowledge of species biology and habitat conditions across the Canadian range. All are proximate, in that they act directly upon the fish or their habitats. The vulnerability of Nooksack dace to each threat, and the severity of each threat in each watershed are rated and summarized graphically in Table 2. The ratings are based on analyses of a suite of factors that cause, exacerbate, or mitigate threats (Figure 3), and are briefly summarized in the text. A summary by watershed is presented in Table 3.  For details of assessment methods and rationales for ratings see Pearson (2004a; 2004b).

Table 1: Potential threats to Nooksack dace in Canadain descending order of concern
ThreatManagement Concern
1.Physical Destruction of HabitatDrainage, dyking, channelization and infilling of water bodies destroying habitat.
2.Seasonal Lack of WaterLow flows in late summer eliminate habitat, reducing fitness or survival.
3.Sediment DepositionDeposited sediment degrading habitat.
4.Riffle Loss to Beaver PondsBeaver ponds flooding riffle habitat.
5.Habitat FragmentationPermanent or temporary barriers preventing or inhibiting fish from traversing some stream reaches. This restricts access to usable habitats and/or alters metapopulation dynamics to increase extinction risk.
6.ToxicityToxic discharges from point or non-point sources significantly reducing survival or fitness.
7.HypoxiaEpisodes of extreme hypoxia causing acute mortality or reduced fitness.
8.Increased PredationIntroduced predators consuming individuals or reducing their fitness by inducing behavioural changes.

 

Table 2: Summary of threats assessment for Nooksack dace
(see text for basis of assessment)
ThreatVulnerability of Nooksack DaceSeverity Across Range
Physical Destruction of Habitatmajor concernmajor concern
Seasonal Lack of Watermajor concernmajor concern
Sediment Depositionmajor concernmajor concern
Riffle Loss to Beaver Pondsmajor concernmoderate concern
Habitat Fragmentationmoderate concernmajor concern
Toxicitymoderate concernmoderate concern
Hypoxiaminor concernmoderate concern
Increased Predationminor concernmajor concern

 

Table 3: Assessment of threat severity in each of the four watersheds from which Nooksack dace are known in Canada. Background data and details of assessment methods for Bertrand, Pepin and Fishtrap Creeks are provided by Pearson (2004a). The Brunette River population was discovered in 2004 and a threats analysis has not been completed for it.
ThreatBertrand CreekPepin BrookFishtrap CreekBrunette River
Hypoxiamoderate concernmajor concernmoderate concernunknown
Physical Destruction of Habitatmoderate concernmajor concernmajor concernunknown
Habitat Fragmentationmajor concernmoderate concernmoderate concernunknown
Toxicitymoderate concernminor concernmajor concernunknown
Sediment Depositionmoderate concernmajor concernmoderate concernunknown
Seasonal Lack of watermajor concernminor concernmoderate concernunknown
Increased Predationmoderate concernmoderate concernminor concernminor concernunknown
Riffle Loss to Beaver Pondsminor concernmajor concernminor concernunknown

Threat 1: Physical Destruction of Habitat

Description

Channelization, dredging and infilling directly destroying or degrading stream habitats.

Vulnerability (major concern)

The riffle habitats required by Nooksack dace are the ‘high spots’ in a stream, and tend to be targeted for removal or alteration in drainage projects. Channelization and drainage work also typically eliminates the shallow marginal pools preferred by young-of-the-year.

Severity (major concern)

Approximately 77% of pre-settlement wetland areas in the Fraser Valley have been drained or infilled (Boyle et al. 1997). Fifteen percent of the area’s streams no longer exist, having been paved over or piped (Fisheries and Oceans Canada 1998). A large, but unknown, proportion of those that remain have been channelized and/or repeatedly dredged in agricultural drainage or urban development projects. It is difficult to overstate the historical extent of fish habitat loss to these activities. Both permitted and un-permitted dredging of ditches and stream channels for flood control and agricultural drainage still occur annually in all watersheds included in this strategy. In recent years, Fishtrap Creek has been most affected. The lower 5 km of the mainstem were dredged by the City of Abbotsford in 1990-1991 (Pearson 2004a), eliminating riffle from what was previously a densely populated reach (J.D. McPhail, UBC pers. comm.).

Figure 3: Factors known or suspected to drive or trigger threats to Nooksack dace (from Pearson 2004b)

Figure 3: Factors known or suspected to drive or trigger threats to Nooksack dace

Threat 2: Seasonal Lack of Water

Description

During late summer, when rainfall is sparse, Fraser Valley stream flows are maintained almost solely by groundwater. Stream hydrographs vary widely depending on surface soil permeability and water use. Watersheds with large unconfined aquifers maintain steady flows of cold water throughout this critical period, while surface flows may cease completely in watersheds with impermeable surface soils. Unfortunately the late summer low-flow period coincides with peak demand for water withdrawal from wells and streams for irrigation and domestic use. Common land use changes in the Fraser Valley also tend to exacerbate problems with water availability. Gravel mining reduces the size of the aquifer contributing to baseflow, urban development increases the area of impermeable surfaces (reducing infiltration to the aquifer), and agricultural drainage lowers water tables, further reducing flows.

Vulnerability (major concern)

Nooksack dace are highly vulnerable to lack of water. Adults inhabit riffles and young-of-the-year school in nearby shallow pools (McPhail 1997). These habitats are the first to be affected by lack of water. Adults also spawn in riffles, but during spring and early summer when water is more plentiful.

Severity (major concern)

Low surface flows have reduced the availability of suitable habitat in Bertrand and Fishtrap creeks for several weeks during very dry years (Pearson, pers. obs.). Nooksack dace are especially vulnerable to further wetland drainage, increases in impermeable surfaces and/or water withdrawal. Extensive gravel mining is underway in two watersheds and will reduce baseflow in these systems by an unknown amount in future.

Threat 3: Sediment Deposition

Description

Sediment deposition is controlled by the balance between the rate of sediment delivery to the channel and capacity of the stream to mobilize and carry it downstream. Sediment delivery may be increased by direct discharges, storm drain runoff, or bank erosion accelerated by lack of riparian vegetation and/or increased peak flows (Waters 1995). All of these sources are likely to increase with urban, agricultural and mining development in a watershed.

Vulnerability (major concern)

Adult dace spawn, forage and rest in the crevasses between and under coarse riffle substrate (McPhail 1997). Sedimentation clogs these spaces and inhibits the flow of oxygenated water through the substrate. It is less likely to be a problem for young-of-the-year dace, which inhabit the water column in shallow pools (McPhail 1997).

Severity (major concern)

Significant sediment deposition occurs in portions of all watersheds (Pearson 2004a).

Threat 4: Riffle Loss to Beaver Ponds

Description

Beaver ponds have been shown to influence fish populations both positively and negatively (Hanson & Campbell 1963; Keast & Fox 1990; Lavkulich et al. 1999; Schlosser 1995). The impacts of riffle loss through ponding have received scant attention, but may be significant for species like Nooksack dace, which depend on these habitats.

Vulnerability (major concern)

Nooksack dace are riffle specialists. The proportion of riffle habitat a reach contains is the best predictor of their presence, and dace are absent from long sections of continuous deep pool, like beaver ponds, even when riffles are present (Pearson 2004a).

Severity (moderate concern)

Riffle loss to beaver ponding is a major concern in at least one watershed, Pepin Brook. In 1999, beavers had impounded 47% of its 6.4 km mainstem. By 2001 an additional 690 m of channel was impounded, eliminating 10% of the 938 m of riffle recorded in the 1999 survey (Pearson 2004a). Impounded area did not change in two other watersheds monitored over the same period (Bertrand and Fishtrap creeks) as higher winter flows washed out dams regularly and narrower riparian forest strips probably limit the food supply of beavers (Pearson 2004a).

Threat 5: Habitat Fragmentation

Description

Physical barriers such as perched culverts, beaver dams, and agricultural weirs commonly prevent movement between habitats for all or part of the year in Fraser Valley streams. In addition, any of the other threats discussed may fragment habitat by preventing or curtailing movement of fish through affected reaches. On a larger scale, connections between watersheds during floods were undoubtedly more common prior to the extensive dyking and drainage works of the past century.

Vulnerability (moderate concern)

Most Nooksack dace have very small home ranges, covering less than 50 m of channel, although a few individuals appear to venture further (Pearson 2004a). The distribution of populations is also very clumped within streams. In combination, these data suggest that each watershed is inhabited by loosely connected subpopulations. Most barriers and habitat fragmentation in Nooksack dace watersheds date from 50 to 130 years ago, and surviving populations have shown some resilience (Pearson 2004a). The effects of less movement between subpopulations and reduced ability to colonize new habitat, however, may occur over longer time frames. The extent and importance of this to the long-term persistence of individual subpopulations and to recolonization following local extinctions of subpopulations is unclear.

Severity (major concern)

The extensive destruction of aquatic habitat that has occurred within the Fraser Valley over the past 150 years (see Physical Destruction of Habitat above) has fragmented habitat badly. Within watersheds, physical barriers and degraded habitat have likely affected movement patterns between subpopulations. Bertrand, Pepin and Fishtrap Creeks are all tributaries of the Nooksack River, but are isolated from one another by poor habitat conditions in the Washington State portion of their watersheds (McPhail 1997). Fish and habitat distributions within the Brunette system have yet to be surveyed.

Threat 6: Toxicity

Description

Toxic compounds enter Fraser Valley streams through urban storm runoff, contaminated groundwater (e.g. agricultural pesticides and herbicides), direct industrial discharges, sewage treatment plant effluents, aerial deposition, and accidental spills (Hall et al. 1991). Concentrations in the water column are widely variable over time because dilution varies with stream discharge and inputs are often pulsed (e.g. first flush of stormwater following a long dry spell, Hall et al. 1991). Some contaminants, particularly heavy metals, bind to sediments where they may be taken up and bioaccumulated by aquatic invertebrates and subsequently fish.

Vulnerability (moderate concern)

Data on threshold concentrations for lethal and sublethal effects of toxic compounds on Nooksack dace are lacking. As a bottom-dwelling species, they may be sensitive to contaminants bound to sediment as well as those in food items and the water column.

Severity (moderate concern)

Toxicity is likely to impact some Nooksack dace populations.  Large portions of the Fishtrap Creek, Bertrand Creek, and the Brunette River watersheds are urbanized, which generally causes elevated levels of copper, lead and zinc in stream sediments (Hall et al. 1991). Row crop agriculture with intensive pesticide/herbicide use is also common in the Fishtrap Creek watershed (Pearson 2004a). The range of compounds that could enter creeks from spraying, poor waste management, and accidental spills is enormous.

Threat 7: Hypoxia

Description

Hypoxia is ultimately caused by the cumulative effects of local and watershed-scale impacts.  Increased nutrients result in algal blooms and high densities of macrophytes that strip the water of oxygen at night. Decomposition of dead algae and vegetation exacerbates the problem and may severely depress daytime oxygen levels as well. Nutrients in Fraser Valley groundwater and streams are elevated, primarily a consequence of over-application of manure and fertilizers to agriculture lands (Lavkulich et al. 1999; Schreier et al. 2003), but also of urban stormwater runoff and septic systems (Lavkulich et al. 1999). Lack of shade from riparian vegetation permits water temperatures to rise. Warmer water has less capacity for dissolved oxygen and increases the metabolic demands of fish and other organisms. Reduced water movement impairs reoxygenation of water and may be caused by channelization, (Schreier et al. 2003), beaver ponds(Fox & Keast 1990; Schlosser & Kallemyn 2000), or low flows.

Vulnerability (minor concern)

Lethally low levels of hypoxia are unknown for Nooksack dace, but riffles are generally well-oxygenated habitats and species that are specialized to inhabit them are unlikely to be well adapted to hypoxia. Even moderate levels of chronic hypoxia may reduce growth, condition, and fecundity. In the absence of better information, the federal guideline for the protection of aquatic life (5mg.l-1, CCREM 1987) is a useful target.

Severity (moderate concern)

Hypoxia is a major concern in at least one stream, Pepin Brook, and a moderate concern in Bertrand and Fishtrap Creeks. Fish inhabiting riffles and shallow pools immediately below hypoxic reaches may be affected, although this comprises a small proportion of total habitat.

Threat 8: Increased Predation

Description

Increased predation is most likely to arise from the introduction of new species to Nooksack dace habitats. Such introductions are implicated in the extinction of numerous native fishes across North America (Gido & Brown 1999; Miller et al. 1989; Richter 1997).

Vulnerability (minor concern)

The impacts of introduced predators on Nooksack dace populations are unknown. Populations have coexisted with bullheads (Ameiurus nebulosis), bullfrogs (Rana catesbeiana), smallmouth bass (Micropterus dolomieu), or largemouth bass (M. salmoides) for at least ten years in these watersheds (Pearson, unpubl.). All of these species would undoubtedly prey upon Nooksack dace given the opportunity, but there is little habitat overlap. These predators thrive in warm water littoral zones (Corkran & Thoms 1996; Scott & Crossman 1973) and are very rarely found in riffles. Lack of water could, however, force Nooksack dace out of riffles and into pools where predation risk is likely to be much higher. The possibility of a new, effective predator being introduced to Nooksack dace habitat is also ever present.

Severity (major concern)

Introduced predators inhabit every stream known to contain Nooksack dace.

2.2 Summary of Threats Analysis

Nooksack dace populations appear to be most vulnerable to seasonal lack of water, habitat loss to drainage activities, sediment deposition, and riffle loss to beaver ponds. Habitat fragmentation is likely having some impacts in all watersheds and is considered a moderate concern. Introduced predators are widespread in the range but probably have minimal impacts on Nooksack dace because of lack of habitat overlap. Hypoxia and toxicity are significant threats in some sections of at least one watershed, but do not threaten the species throughout its range.