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Recovery Strategy for Paxton Lake, Enos Lake, and Vananda Creek Stickleback Species Pairs (Gasterosteus spp.) in Canada [Proposed]
- Description of the Species
- Description of Needs of the Species
- Habitat Trends
- Habitat Protection
- Critical Habitat
1. Description of the Species
1.1 General Biology
Threespine stickleback (Gasterosteus aculeatus) are small (usually 35-55 mm) fish that are abundant in coastal marine and freshwater throughout the northern hemisphere. In general, G. aculeatus has a laterally compressed body and individuals in most populations are well-armoured with retractable pelvic and dorsal spines, and calcified lateral plates. Freshwater populations are variable in extent of armour but usually have less than the marine form. Body color varies from silvery to mottled green and brown. Sexually mature males develop bright red throats during the breeding season, although in a few freshwater populations males turn completely black instead (McPhail 1969; Reimchen 1989).
Marine stickleback are phenotypically similar throughout their range, whereas freshwater stickleback are ecologically, behaviourally and morphologically variable. Several lakes on islands in the Strait of Georgia are especially noteworthy because they each contain two distinct, reproductively-isolated species (McPhail 1984, 1992, 1993, 1994; Schluter and McPhail 1992; Figure 1). We refer to the two stickleback species in each lake as a "species pair."
In each case, one of the species (referred to as "limnetic") primarily exploits plankton, and has morphological traits such as a fusiform body, narrow mouth and many, long gill rakers. These traits are considered adaptations to a zooplankton-consuming lifestyle (Magnuson and Heitz 1971; Kliewer 1970; Sanderson et al. 1991; Schluter and McPhail 1992, 1993). The other species (referred to as "benthic") mainly eats benthic invertebrates in the littoral zone, and has a robust body form, wide gape and few, short gill rakers, traits considered to be advantageous in benthic feeding (Schluter and McPhail 1992, 1993). The pattern of morphological and ecological divergence is similar in each of the lakes (Schluter and McPhail 1992), such that limnetics all look alike, as do all benthics. Despite similar appearance, phylogenies based on molecular genetics strongly indicate that the pairs are independently derived (Taylor and McPhail 2000). Thus, benthics from different lakes should be considered separate species, and the same for limnetics. There are thus at least eight separate species within the species pair complex--two in each of the four watersheds.
Limnetics mature early, usually after one year, and rarely live beyond two years. There is considerable sexual dimorphism: reproducing limnetic males tend to be bigger on average than gravid females. Large male size enables greater nest protection and territory defence (Rowland 1989). Typical fecundity is low, about 30-40 eggs per clutch for a gravid limnetic female, but they produce several clutches per season, usually in close succession if food availability is high.
Males are the sole providers of parental care. In the spring, they acquire territories in the littoral region where they build nests and mate (sometimes with many females). Limnetics prefer non-vegetated, open nesting locations (McPhail 1994; Hatfield and Schluter 1996). They often nest in less than 1 m of water on submerged logs, in shallow bays with gravel or rocky substrates, and on firm muddy substrate. Preferred spawning habitat is not uniformly distributed in the littoral zone, so nesting males are often clumped in their distribution. Despite reproducing at the same time of year limnetics and benthics rarely interbreed (McPhail 1992).
Benthics differ from limnetics in a number of ways: they generally mature later, live longer and reproduce less often than limnetics. There is little or no sexual dimorphism. Fecundity is typically higher than for limnetics, about 150-250 eggs for a gravid benthic. Females usually produce only one or two clutches per season, regardless of food availability. Benthics prefer densely vegetated nesting locations, usually among beds of Chara (Hatfield and Schluter 1996), and their nests are highly concealed. They tend to nest in water of greater depth than limnetics, though usually less than 2 m depth. As with limnetics, preferred spawning habitat for benthics is not uniformly distributed, so nesting benthics are clumped in their distribution. Benthics are similar to limnetics in most aspects of parental care and early development.
As adults, limnetic and benthic stickleback eat quite different foods. Limnetics feed primarily in the surface waters away from the lake margins. There they hunt in loose schools for copepods, Daphnia and insect larvae. Males will often forage for benthos when nesting in the littoral zone. As young juveniles, limnetics feed at the lake edges among reeds and submerged plants where they can seek cover from predators.
Benthics on the other hand forage along the shallow margins of the lake for larger prey such as snails, clams, dragonfly nymphs, amphipods, and chironomids. These invertebrates are found among a variety of substrates including plants, rocks or mud. Benthics likely eat similar food types throughout their life, but gradually shift to larger prey as they grow.
Very little is known about diets during the initial life stages of the two species.
Stickleback species pairs were known to exist in four watersheds in the Georgia Basin, British Columbia: two watersheds on Texada Island, and one each on Lasqueti Island and Vancouver Island (Figure 2). Evidence indicates that the species pairs evolved independently in each watershed, meaning that this is a multi-species complex rather than two species spread over multiple watersheds (Taylor and McPhail 1999). Within the last decade the species pair on Lasqueti Island has been declared extinct (Hatfield 2001a), and the species pair in Enos Lake has collapsed into a single hybrid swarm (Kraak et al. 2001; D. Schluter and E. Taylor unpublished data). The present global range is therefore restricted to two watersheds on northern Texada Island – the Paxton Lake watershed, and the Vananda Creek watershed (with three lakes, Balkwill, Priest and Emily).
Together these losses represent a 50% reduction in the pool of distinct species pairs, and a 33% reduction in the total number of lakes with species pairs. It is possible that undiscovered species pairs currently exist elsewhere in BC, but this is unlikely given the thoroughness of past surveys (McPhail 1993). It is also possible that other pairs existed in the region but went extinct before being discovered.
Prior to the collapse of the Enos Lake stickleback species pair, a population of Enos Lake limnetics was established by adding wild fish in 1988 and 1999 (under permit from the Fisheries Branch of the British Columbia Ministry of Environment, Lands and Parks) to a pond in Murdo-Frazer Park in North Vancouver. A viable population was confirmed in spring 2002 (D. Schluter, unpublished data).
A captive breeding program is underway at UBC in an attempt to recover "pure" limnetics and benthics from the population in Murdo-Frazer Park and the hybrid swarm in Enos Lake. The ultimate success of this program is uncertain.
Figure 2. Map of southwestern British Columbia with historic distribution of stickleback species pairs indicated by red stars (upper: Texada Island, middle: Lasqueti Island, lower: Vancouver Island). These three locations are the only known sites to have had species pairs. Within the last decade the species pair on Lasqueti Island has been declared extinct, and the species pair in Enos Lake has collapsed into a single hybrid swarm.
Prior to their collapse, McPhail (1989) suggested that population sizes were on the order of 100,000 for each of the species in Enos Lake. Since then Matthews et al. (2000) estimated population sizes at 22,000 limnetics and 37,000 benthics. These estimates are likely confounded by species identification problems due to substantial hybridization between limnetics and benthics. Since most of the species pairs' lakes are similar in size, McPhail's (1989) abundance estimate for Enos Lake may be sufficiently accurate for other lakes.
There has been no systematic monitoring of abundance in any of the lakes, so population trends are unknown (Hatfield 2001a, b; Hatfield and Ptolemy 2001). However, stickleback species pairs from Paxton and Priest Lakes have been intensively studied by zoologists at UBC for the last two decades (e.g., Schluter and McPhail 1992; McPhail 1994; Taylor and McPhail 1999). Throughout this time both species have remained abundant and easy to trap in large numbers in Gee traps (Hatfield 2001b; Hatfield and Ptolemy 2001).
1.4 Ecological Role
Stickleback species pairs occupy an intermediate trophic level (Reimchen 1992). Limnetics utilize pelagic areas of the lake for feeding and are able to influence the density of plankton in this zone. Benthics feed primarily in the littoral zone and likely influence the density of littoral invertebrates upon which they feed. Juvenile stickleback are prey for several species of carnivorous benthic invertebrates, and older stickleback are preyed upon by coastal cutthroat trout (Oncorhynchus clarkii clarkii) and by piscivorous birds (e.g., herons (Ardea herodias), kingfishers (Megaceryle alcyon) and loons (Gavia immer)).
1.5 Importance to People
The significance of the stickleback species pairs is primarily aesthetic and scientific. Stickleback species pairs are widely regarded as a scientific treasure; they are as valuable to science as cichlid fish species in the Great Lakes of Africa, and Darwin's finches in the Galapagos Islands. In large part this is because they are among the youngest species on earth. The evolution of a new species is thought to usually take on the order of millions of years, but scientists believe the species pairs have evolved since the end of the last glaciation, a mere 13,000 years ago (McPhail 1994; Schluter and McPhail 1992). The speed with which these distinct fish species evolved has intrigued and excited scientists around the world. They are a remarkable research subject that will help us understand the biological and physical processes that give rise to the tremendous diversity of organisms we see around us. Newspapers, magazines and scientific journals have published the story of the discovery of these species, and have followed the results of ongoing scientific studies.
2. Description of Needs of the Species
A healthy lake ecosystem in which limnetic and benthic stickleback are expected to thrive has abundant littoral and pelagic habitat, with secondary productivity of these two major habitat types sufficient to support populations in the 10s of thousands for each species, and a simple fish community comprised of only stickleback and cutthroat trout. Specific environmental features required to maintain viable population levels and sustain reproductive isolation of limnetic and benthic species include: general water quality parameters (oxygen, temperature, pH, and pollutants), specific water quality parameters (light transmission and nutrients), littoral habitat (extent and productivity), and macrophytes. Lake levels and extent of macrophytes should be maintained within the natural range for species pair lakes.
2.1 Physical Habitat Requirements
Knowledge of habitat requirements comes mainly from observations in Paxton and Enos lakes, and is assumed to be representative of other species pairs. Habitat requirements vary throughout the year for each life stage. In general, stickleback species pairs spawn in littoral areas in the spring, rear in littoral and pelagic areas in spring and summer, and overwinter in deep water habitats during the fall and winter. The species' life history timing is presented in Table 1; detailed descriptions of habitat use are presented below.
Stickleback species pairs spawn in the shallow littoral area of lakes (McPhail 1994). Males construct nests, which they guard and defend, until fry are about a week old. The nests and contents remain vulnerable to predators of different kinds (Foster 1994). Benthics build their nests under cover of macrophytes or other structures; limnetics tend to spawn in open habitats (McPhail 1994; Hatfield and Schluter 1996).
The extent of available spawning habitat may conceivably limit populations in some lakes where shallow littoral areas are uncommon. Although spawning habitat may limit limnetic or benthic abundance when spawning populations are very large, the total area of littoral habitat available for spawning appears to be extensive in each species pair lake, at least under present conditions (Hatfield 2001a; Hatfield and Ptolemy 2001).
A more important issue is the potential for changes in the quality of littoral habitat to affect reproductive isolation of the two species. Homogeneous littoral habitats may preclude the ability of limnetics and benthics to exercise preferences for specific microhabitats (Hatfield and Schluter 1996; Boughman 2001). For example, loss of macrophyte beds may lead to limnetics and benthics nesting in close proximity, possibly increasing the likelihood of hybridization between the two species (Hatfield and Schluter 1996). Females may be less able to differentiate between males of different species if nesting habitat preferences cannot be exercised. Species pair lakes naturally have abundant macrophytes, presumably facilitating assortative mating through expression of differences in male nesting habitat selection.
Juvenile rearing habitat
Immediately after leaving the protection of paternal care, both limnetic and benthic fry utilize the littoral zone, where there is abundant food and cover from predators. Macrophyte beds constitute both a source of food (benthic invertebrates associated with the lake bottom and macrophyte surfaces) and refuge from predation. The extent of habitat partitioning by benthic and limnetic fry within macrophyte beds is unknown, but it appears that both species use this general habitat type. As individuals grow, habitat partitioning likely increases, and eventually limnetics move offshore to feed in pelagic areas (Schluter 1995). The timing of movement into the pelagic region by limnetic juveniles is likely dictated by a combination of relative growth rates and predation risk in littoral and pelagic habitats (Schluter 2003), which may vary among lakes and among years. Benthic juveniles rear only in littoral areas.
Availability of suitable rearing habitat for juveniles may limit benthic and limnetic stickleback adult population size, although it is unclear when this is the case. Species pair lakes (with the recent exception of Enos Lake) have abundant macrophytes, but the extent of suitable beds may differentially affect survival of juvenile benthic and limnetic stickleback. Altering the relative abundance of benthic and planktonic prey may alter the selective environment for stickleback (Schluter and McPhail 1993; Schluter 1994, 1995, 2003; Vamosi et al. 2000). For example, loss of suitable rearing habitat for benthics may increase the relative fitness of hybrids or limnetics at the expense of the benthic species, possibly facilitating hybridization and species collapse.
Adult rearing habitat
Adult limnetics (with the exception of nesting males) feed on zooplankton in the pelagic zone of the lake, whereas adult benthics feed on benthic invertebrates in the littoral zone (Schluter 1995). Productive littoral and pelagic habitats are required for the persistence of stickleback species pairs.
Abundance of benthic and limnetic stickleback is likely determined by many factors, but total area and secondary productivity of littoral and pelagic zones is assumed to have a strong effect. Habitat alterations that result in changes to body size (e.g., through lower growth rates or reduced size at maturity) may result in poorer mate discrimination and increased hybridization between limnetics and benthics, since body size is a key factor determining mate selection (Nagel and Schluter 1998).
By late summer individuals begin moving to deeper water habitats where they overwinter. Little is known about habitat requirements of limnetics and benthics during this stage, except that trapping and seining consistently indicate use of deeper water by early fall and through the winter.
The stickleback species pairs appear to have endured in the presence of only one other fish species, coastal cutthroat trout (Vamosi 2003). Maintaining a simple ecological community is necessary if the species pairs are to be retained, as underscored by the rapid extinction of the Hadley Lake species pair following introduction of brown bullhead (Ameiurus nebulosus; Hatfield 2001b).
High productivity of the two major habitats (in comparison with most other coastal lakes) is believed necessary for persistence of the species pairs. Furthermore, secondary production should be relatively similar in the two habitats (though the absolute and relative prey productivities necessary to maintain species pairs are unknown). Stickleback abundance is likely strongly correlated with available secondary production, and lower production will likely increase risk of extinction. Extinction risk is also believed to increase if the relative productivity of each habitat type is altered beyond its normal range. If productivity in one habitat declines substantially one or both species may be affected by increased hybridization (Figure 3).
Specific environmental features required to maintain safe population levels and promote reproductive isolation of limnetic and benthic species are identified below.
Figure 3. Types of benthic and pelagic productivities and the likely consequences for stickleback species pairs. Available secondary production (depicted by area of each circle) must be high and roughly equivalent in the two major habitats to avoid extinction (scenario A). As production declines in one or more habitats, extinction (shown as a red X) becomes more likely for one or both species (scenarios B to D).
2.2 Pelagic and Littoral Habitats
Stickleback species pairs will be at risk when water quality degrades beyond certain levels for oxygen, temperature, pH, or pollutants. As a group, sticklebacks are tolerant of a fairly large range of water quality conditions. The current provincial water quality standards for the protection of aquatic life are appropriate guidelines for basic parameters of water quality in lakes with stickleback species pairs (see http://srmwww.gov.bc.ca/risc/pubs/aquatic/interp/index.htm). However, some aspects of water quality in species pair lakes need to be maintained in a much narrower range than that required for short-term individual survival, as described below.
A significant issue for stickleback species pairs is how changes in water quality may affect barriers to reproductive isolation (Boughman 2001). In particular, there is concern that increases in turbidity that alter transmission of different wavelengths of light may interfere with behavioural mechanisms that influence mate recognition and choice (cf. Seehausen et al. 1997). Differences in breeding colouration between benthics and limnetics are key breeding cues used in mate discrimination (Boughman 2001). Changes in concentration of suspended solids, dissolved organic carbon (e.g., tannins), or other aspects of water chemistry that affect light transmission may disrupt mate recognition.
Factors affecting light transmission therefore need to be maintained within the natural range of conditions present in species pair lakes. Changes in water quality that alter light transmission properties outside of this range may lead to impaired mate discrimination and result in a higher frequency of hybridization, and possibly species collapse (cf. Seehausen et al. 1997).
Production of phytoplankton and benthic algae form the base of the aquatic food chain, and are driven by nutrient availability in the water column, which is itself determined by geology of the watershed. Solitary stickleback populations exist across a broad range of lake productivities in British Columbia (Lavin and McPhail 1985, 1986, 1987). In contrast, stickleback species pairs are found only in lakes with relatively high productivity, typically with calcareous bedrock present in the watershed (McPhail 1994; Schluter unpublished data). The evolution of stickleback species pairs is believed to have been possible only under specific levels of benthic and pelagic invertebrate production (see Figure 3), which facilitated exclusive adaptations to either a pelagic (zooplankton) or littoral (benthic invertebrate) food resource. Changes to nutrient status that alter the relative productivity of zooplankton and benthos could therefore alter the adaptive environment for stickleback species pairs (Schluter 1995; Vamosi et al. 2000). Altered nutrient status may lead to demographic collapse, or hybridization between the two species by altering the fitness of limnetics, benthics, or hybrids.
Increases in nutrients may alter the relative productivities of the benthic and pelagic zones, either by favouring production of unpalatable algae that cannot be consumed by zooplankton, or by phytoplankton blooms reducing macrophyte abundance through shading (Wetzel 2001). Land use practices that lead to lake eutrophication should be avoided, and nutrient levels (nitrogen, phosphorous, total alkalinity) need to be maintained within the natural range for lakes with stickleback species pairs.
Extent of littoral habitat
Persistence of benthic stickleback depends on littoral zone production sufficient to support a large population of benthic individuals. The physical extent of the littoral zone depends on both the shape of the lake basin and the amount of water in the basin. The bathymetric profile of a lake is geomorphically fixed and not readily amenable to human alteration. The amount of water in the basin is determined by climate, but also is subject to human influence through the construction of dams and the extraction of water, which will influence the quantity of habitat available and therefore total population size of stickleback.
Productivity of littoral areas is determined by physical and biological factors, including depth of the euphotic zone, presence of macrophytes, soil types, nutrient levels, area available for colonization by benthos, and interactions among species. Littoral production is confined to shallow areas along the lake margin, where light penetration is sufficient to support significant macrophyte and algal production. The compensation depth (the depth at which light level allows energy gain from photosynthesis to equal energy loss to respiration, the euphotic zone) is usually defined as the depth at which irradiance is 1% of surface irradiance (Wetzel 2001). In practical terms, the depth of the littoral zone is the maximum depth of rooted aquatic vegetation, which rarely exceeds 10m in most lakes, with the majority of photosynthetic production occurring in depths less than 3m.
Maintenance of the littoral zone is very important to the persistence of stickleback species pairs, and water level changes that are outside the natural range for the species pairs' lakes should be avoided. The relative extent of littoral habitat may affect reproductive isolation during nesting, growth and survival of juveniles of both species, adult abundance and individual size, as well as hybrid fitness. Variation in the extent of littoral habitat outside of the natural range will significantly increase the probability of species hybridization and collapse. Genetic evidence indicates that historic hybridization has been considerably higher in the Paxton Lake species than for the other species pairs (E. Taylor, unpublished data). This lake has had the greatest drawdowns from water abstraction, and there is an hypothesized link between the two.
Extent of macrophyte beds
As noted above, macrophyte beds are the primary nesting locations for benthics, key rearing habitats for juveniles of both species, and foraging habitat for adult benthics. Macrophytes are a key feature mediating mate recognition, because differential nest site selection with respect to macrophyte cover maintains some degree of spatial isolation between limnetic and benthic spawners (McPhail 1994; Hatfield and Schluter 1996). Macrophytes also contribute significantly to the production of benthic macroinvertebrates that support the benthic stickleback species. Macrophytes are therefore important in limiting hybridization and play a significant role in maintaining the balance of benthic and invertebrate production that is essential for maintenance of the two species.
A variety of specific threats can be described based on experience in other watersheds and prioritized based on professional judgement. The threats are summarized here and described in greater detail in Hatfield (2003). Immediate recovery efforts should focus on the issues identified below. By prioritizing threats there is no intent to imply that other threats are not significant or worthy of attention. Quantitative risk assessment is not currently possible when analyzing threats to stickleback species pairs due to absence of information on the effects of different threats on population vital rates (e.g. hybridization rates, growth, survival, reproductive success). A summary of population status and threats is presented in Table 2.
|Hadley Lake||Enos Lake||Paxton Lake||Vananda Cr. watershed|
|Current population status||Extinct||Hybrid swarm||Apparently Stable||Apparently Stable|
|Cutthroat trout present||assumed absent||No||rare||present|
|Introduced species||Brown bullhead||Signal crayfish||none||none|
|Forest harvest||assumed to be minor|
|Other land use||may be developed for residential use|
3.1 Exotic Species
The species pairs appear to depend critically on the maintenance of several ecological factors, including a simple fish community. Species pairs occur in lakes that naturally have only stickleback and cutthroat trout (Vamosi 2003).
The Hadley Lake species pair quickly became extinct following the introduction of brown bullhead (Ameiurus nebulosus), which likely preyed upon or interfered with nesting stickleback, ultimately leading to complete recruitment failure (Hatfield 2001a). Bullhead were introduced to Hadley Lake in the early 1990s and stickleback were absent by 1995 (Hatfield 2001a). This highlights the vulnerability of the stickleback and the speed with which a species pair can be affected by an introduced species. The Enos Lake species pair has collapsed due to hybridization (Kraak et al. 2001; D. Schluter and E. Taylor unpublished data), and the recent appearance of the signal crayfish (Pacifastacus leniusculus) is implicated in the collapse. The mechanism by which the crayfish are affecting stickleback may be direct (e.g., predation or displacement from nesting habitat) or indirect (e.g., competition for food resources, increased turbidity, altered macrophyte distribution), but the ultimate effects (e.g., severe population declines) of crayfish introductions on stickleback populations have been observed elsewhere (Foster et al. 2003).
The threat of species introductions applies to a number of other species that are in nearby lakes and spreading throughout the region. These species include largemouth and smallmouth bass (Micropterus salmoides and M. dolomieu), pumpkinseed sunfish (Lepomis gibbosus), and yellow perch (Perca flavescens), which are spread by anglers and other members of the public. Potential threats also include the spread of amphibians like the bullfrog (Rana catesbeiana) and invasive aquatic vegetation such as Eurasian milfoil (Myriophyllum spicatum) and purple loosestrife (Lythrum salicaria). Although the stickleback species pairs have co-evolved with coastal cutthroat trout (McPhail 1994), we do not know the extent to which the pairs would be affected by introduction of other native salmonids, such as rainbow trout (Oncorhynchus mykiss), or indigenous non-salmonids such as sculpins (Cottus sp.). This suite of potential invaders could affect stickleback species pairs through a number of mechanisms including, predation of adults, juveniles, or nests; competition for pelagic or littoral food resources; introduction of disease; or alterations to water quality, which may affect mating decisions or food resources.
3.2 Water Management
Concerns with development activities relate to water quality and water quantity, both of which can alter the ecology of a lake to the detriment of a species pair. Water licences dictate the parameters for diversion and storage and therefore have direct bearing on lake levels. Existing licences are large relative to volume of some of the lakes and size of the catchments. For example, existing water licences on Paxton Lake allow annual diversions of more than twice the volume of the lake, yet inflows are low due to small catchment area and limited precipitation. Severe drawdowns have occurred in the past as part of mining operations (Larson 1976).
Depending on the timing and duration, lake level drawdown may cause loss of effective littoral zone available for foraging and nesting. Large drawdowns can shrink lake volume and depth to such an extent that pelagic habitat essentially disappears and littoral habitat is all that remains. Water level increases associated with damming of lake outlets may also alter the extent of littoral habitat, depending on the morphology of the basin. Large fluctuations have impacts on littoral productivity and pelagic volume and likely have a direct effect on stickleback, severely limiting both spawning and feeding habitats.
3.3 Land Use
There have been numerous land-based development activities in watersheds inhabited by species pairs: forestry, mining, road building, pipeline construction, and housing developments (Larson 1976; McPhail 1994). The main concerns from such activities include cumulative impacts on water quality leading to eutrophication, sedimentation, and habitat destruction or alteration. The greatest of these risks appears to be introduction of suspended sediments (i.e., increased turbidity), but at present, the risk is difficult to gauge.
Additional impacts may occur from other activities, including fishing, recreation, disease, climate change, and pollution. These threats are of concern to the Recovery Team, but are believed to present a lower risk to the species pairs than other threats noted above. It may be possible to effectively manage many of these threats through the development and implementation of good stewardship practices.
4. Habitat Trends
Trends in habitat quantity and quality vary among the species pairs' watersheds. Habitat trends can be assessed only qualitatively, since there has been no systematic monitoring in any of the lakes.
Existing water licences permit substantial water extraction from Paxton and Emily Lakes and moderate volumes from Priest Lake. Historical trends in water use cannot be determined directly because water use was not gauged on all these watersheds. Larson (1976) noted that water extractions from Paxton Lake caused severe drawdowns in the past. Nevertheless, industrial use of water has declined during the last 30 years due to a shift in mining activities from underground extraction of ores to open pit quarrying of limestone. The decline in water use has likely had a positive effect on stability and productivity of littoral and pelagic habitats. Land-based activities have the potential to negatively affect within-lake habitats. Mining and forest harvest have been most extensive in the Paxton Lake watershed. For the most part the influences of land use are difficult to quantify.
Existing water licences for Hadley Lake permit substantial water storage and extraction relative to lake volume and inflow. Land-based development in the watershed includes roads, housing, and perhaps some minor forest harvest. These activities have the potential to negatively affect within-lake habitats, but are assumed to be of relatively minor consequence at present. It is possible that introduced brown bullhead have modified the littoral habitat, but the extent and magnitude of any change is unknown.
Recently issued water licences for Enos Lake permit substantial water storage and extraction and a new dam has been constructed at the lake outlet. Present and historic water use is unknown, as is its effect on stability and productivity of littoral and pelagic habitats. Changes in littoral habitat and nutrient status of the lake has likely occurred following raising of the lake level (Stockner et al. 2000). On-land activities are presently minimal, but will likely increase with the expansion of the Fairwinds development (a large residential / golf course development on the privately-owned land surrounding Enos Lake). The development's negative or positive influences on within-lake habitats are difficult to predict until the plans are assessed in detail. Littoral macrophyte abundance seems to have declined considerably; it is possible that crayfish, which were introduced, have modified the littoral habitat.
5. Habitat Protection
At present none of the lands surrounding species pairs' lakes are formally protected, although the Vananda Creek species pair is listed as an Identified Wildlife species under the Forest Practices and Range Act (Wood et al. 2003) and prescriptions for a Wildlife Habitat Area are under development. The intention of the Wildlife Habitat Area and associated prescriptions will be to protect the species pair from potential adverse effects of forest harvest, although the details of the Wildlife Habitat Area and associated prescriptions are as yet undetermined, so it is unclear how effective they will be. Lands surrounding the Texada Island lakes should be considered highest priority for stewardship and habitat conservation programs.
6. Critical Habitat
Critical habitat is defined in SARA as "habitat that is necessary for the survival or recovery of a listed wildlife species and that is identified as the species' critical habitat in the recovery strategy or in an action plan for the species." Stickleback species pairs utilize both the pelagic and littoral zones of the lakes they inhabit, for both reproduction and feeding (McPhail 1993, 1994). However, critical habitat has not yet been defined for any of the stickleback species pairs. Defining critical habitat is an important action required to meet recovery objectives, and to help place acceptable bounds on activities that impact the species.
6.1 Distinction Between Species Pairs and Solitary Populations
Solitary stickleback populations (i.e., those populations for which a single form inhabits a lake) occur in a wide range of habitat conditions and are fairly resilient to habitat change (Wooton 1976; Bell and Foster 1994). Stickleback species pairs, on the other hand, are highly restricted in their distribution and are believed to have considerably more stringent habitat requirements, and to be much more sensitive to habitat change (e.g., Bentzen and McPhail 1984; Schluter and McPhail 1992). All lake-dwelling stickleback populations require spawning habitat (typically the benthic nearshore littoral zone), juvenile rearing habitat (typically the littoral zone), and adult rearing habitat (typically the littoral and pelagic zones) -- habitat requirements that include most of the lake (Wooton 1976; Bell and Foster 1994). Critical habitat for species pairs should include these same aspects of habitat (i.e., spawning and rearing habitat), as well as the specific aspects of habitat that permit species to co-exist without collapsing into a hybrid swarm. In other words, loss of spawning and rearing habitat is of concern, but so too are small changes in turbidity (which may affect light transmission and therefore mate identification) or water chemistry (which may affect benthic or littoral productivity) because they may cause hybridization and species collapse. Specific ecological factors such as these do not usually form part of critical habitat definitions, but in the case of stickleback species pairs these components may be considered. The exact nature of these factors is still unknown, so additional research will be required to help define critical habitat for stickleback species pairs. Since delineating critical habitat will take considerable effort and time, efforts should focus first on those components of habitat that act as barriers to hybridization and that are the most likely to be affected by human activities.
6.2 Specific Critical Habitat Features for Stickleback Species Pairs
There are two key components of critical habitat for stickleback species pairs:
- Habitat features that control the abundance of limnetics and benthics, and
- Features of the environment that ensure proper mate recognition.
Critical habitat then should be a collection of environmental features whose alteration or loss will lead to reduction in abundance to an unviable population level, or breakdown of reproductive barriers sufficient to cause collapse into a hybrid swarm.
The general needs of species pairs are described in detail in Section 0. These needs will inform the definition of critical habitat for these species, but there is generally a requirement for more research to allow a more specific description of critical habitat and precise delineation in the field. Individual features that could be considered as part of critical habitat are presented below and defined to the extent possible, given current information. These descriptions will be updated as information becomes available.
Extent of littoral habitat
The importance of the littoral zone to persistence of stickleback species pairs indicates that a very substantial portion of the littoral zone could be identified as habitat with high potential productivity. In general, water level changes that are outside the natural range for lakes with stickleback species pairs should be avoided. The relative extent of littoral habitat may affect reproductive isolation during nesting, growth and survival of juveniles of both species, adult abundance and individual size, as well as hybrid fitness. Variation in the extent of littoral habitat outside of the natural range will increase the probability of species hybridization and collapse. Genetic evidence indicates that historic hybridization has been considerably higher in the Paxton Lake species than for the other species pairs (E. Taylor, unpublished data). This lake has had the greatest drawdowns from water abstraction.
Extent of macrophyte beds
Macrophyte beds warrant consideration as habitat with high potential productivity given their key role as rearing and spawning habitat and also in mediating processes that maintain reproductive isolation between limnetic and benthic species. The natural temporal range in distribution and abundance of macrophyte beds over time is not currently known. The specific extent of macrophyte loss that can be sustained before hybridization rates reach a level that causes the species to collapse into a hybrid swarm is also not known. We therefore recommend that macrophyte abundance and distribution be maintained within the natural range present in lakes with stickleback species pairs.
The pelagic zone is the prime rearing area for limnetics, and this habitat must be sufficient in area and quality to support a robust population of limnetics. The qualities of pelagic habitat that relate to this component of habitat with high potential productivity are captured under nutrient status, water quality, and littoral area (which is related to pelagic volume).
Little is known about overwintering habitat, except that the species pairs appear to utilize deeper portions of the lakes. Additional work would be required to include a description of overwintering habitat as part of a critical habitat definition.
The stickleback species pairs have evolved and endured in the presence of only one other fish species, coastal cutthroat trout (Vamosi 2003). This simple ecological community could be considered a component of critical habitat.
Basic water quality parameters
Water is essential for aquatic species, and aquatic species will be at risk when water quality degrades beyond specific thresholds for oxygen, temperature, pH, or pollutants. The current provincial water quality standards for the protection of aquatic life are appropriate guidelines for basic parameters of water quality in lakes with stickleback species pairs (see http://srmwww.gov.bc.ca/risc/pubs/aquatic/interp/index.htm). However, some aspects of water quality in species pair lakes must be maintained in a much narrower range than that required for short-term individual survival (see light transmission and nutrient status).
Aspects of water chemistry that affect light transmission could be a component of a critical habitat definition for stickleback species pairs, because changes may disrupt mate recognition and therefore reproductive isolation (Boughman 2001). At present, the relationships between various water quality factors and mating preferences are not sufficiently precise to allow their inclusion in a detailed definition of critical habitat. However, due to their importance we suggest in the interim that factors affecting light transmission be maintained within the natural range of conditions present in species pair lakes.
Nutrients and productivity
The evolution and maintenance of stickleback species pairs is believed to have been possible only under certain levels of benthic and pelagic invertebrate production, which facilitated exclusive adaptations to either a pelagic (zooplankton) or littoral (benthic invertebrate) food resource. Changes to nutrient status that alter the relative productivity of zooplankton and benthos could therefore alter the adaptive environment for stickleback species pairs (Schluter 1995; Vamosi et al. 2000), leading to demographic collapse, or excessive hybridization. The precise relation between nutrient status and maintenance of stickleback species pairs is not known at present, so it is suggested here that a critical habitat definition could include nutrient levels that are within the natural range for these lakes.
6.3 Schedule of Study Needs to Identify Critical Habitat
The following information is needed to identify the range of conditions typical of species pair lakes, and the aspects of habitat that need to be maintained to permit long-term persistence of species pairs.
- Identify and fill information gaps (life-history and habitat use) that prevent objective definition of critical habitat. (2006-2008)
- Determine acceptable minimum population levels for limnetics and benthics that will ensure species persistence. (2006-2008)
- Develop water quality guidelines for species pair lakes. (2006-2007)
- Map the present extent of littoral habitat and extent of macrophytes. (2006-2008)
- Determine crayfish effects on stickleback recruitment and critical habitat. (2006-2008)
- Define acceptable levels of water fluctuations/drawdowns that will permit species pair persistence, based on extent of littoral habitat at different water levels, historic data, and a comparison between conditions in species pair lakes and single-species lakes. (2006-2008)
- Develop acceptable ranges of invertebrate production in the benthos and pelagic that will permit species pair persistence, by comparison between species pair lakes and single-species lakes. (2006-2008)
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