Westslope cutthroat trout COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

Cutthroat trout are found in a wide range of habitats in Canada. Their relatively small size at maturity makes them particularly able to utilize smaller streams compared with other salmonids. Westslope cutthroat trout inhabit large rivers and lakes in BC, as well as many small mountain streams. In Alberta, genetically pure native populations are now largely restricted to the upper reaches of mainstem rivers and the headwaters of a few major tributaries. Stocked or apparently hybridized populations are more common, but are still largely restricted to headwater areas (Mayhood 2000).  While the scope and nature of variation between the two designatable units for ecological and life history traits is not known, the subspecies as a whole seems to thrive in streams with abundant pool habitat and cover. As with other salmonids, four main types of habitat are required to complete its life cycle:

1.  Spawning – Small, low-gradient streams with cold well-oxygenated water and clean unsilted gravels; spawning often occurs in the tailouts of deep pools at moderate to high-flow events, which are often of short duration (Brown and McKay 1995b; Schmetterling 2001). Proximity to cover is important for spawners; while residing in spawning tributaries, spawners are located almost exclusively in habitat units formed by large woody debris (LWD), boulders, or bedrock.  This instream structure creates the necessary pool habitat to catch and retain spawning gravels as well as providing cover from predation.  High mortality often results when suitable cover is lacking (Behnke 1992; Brown and Mackay 1995b). Shoal spawning has been confirmed (e.g., Carl and Stelfox 1989; Stelfox, pers. comm. 2006), but does not appear to be common.

2.  Rearing– Small streams (first to third order) which remain permanently wetted during low flows and have a diversity of cover are required juvenile rearing habitat (McIntyre and Rieman 1995). Young-of-the-year fry migrate to low energy lateral habitat (i.e., shallow riffle or backwater habitat) with protective cover and low water velocities (some populations may rear in lakes). Larger juveniles move into pools where they establish social dominance based on size. Parr require large territories and the availability of pool habitat often limits their productivity even in productive streams (e.g., Schmetterling 2001).

3.  history type involved (See BIOLOGY). The resident component of populations may remain in the natal stream their entire lives. Migratory forms will undergo a niche shift and leave small natal streams for larger systems or mainstem habitat where the potential for increased growth may be higher. For fluvial (riverine) forms, slow pools formed by boulders or LWD with fast adjacent water and plenty of cover (undercut banks, riparian vegetation, instream structure) are required. Adfluvial adults (migrating between lakes and rivers) will spend summer months feeding in lakes and reservoirs with temperatures less than 16°C (McIntyre and Rieman 1995).

4.  Overwintering – Overwintering habitat suitability appears to be largely determined by groundwater influx and the absence of anchor ice (e.g., Brown and Mackay 1995a). During winter months, fluvial adults will congregate in slow deep pools sheltered from high flows. Juveniles often utilize cover provided by boulders and other large instream structures, or in off-channel habitat such as sloughs or beaver ponds. Adfluvial fish will often overwinter in lakes.

 

Essential habitat parameters

The wide range of environmental conditions encountered by WCT might suggest some manner of flexibility in habitat utilization. However, it is apparent that populations have very strict habitat requirements during various life history stages and generally only do well in intact lotic environments requiring cold clean water and varied forms of cover (i.e., undercut banks, pool-riffle habitat, and riparian vegetation) to maintain their numbers.

Temperature

Stream temperature is likely an important habitat parameter affecting cold-water salmonids like WCT. Water temperature influences a host of biological processes including growth rate, swimming ability, as well as the capacity to ward off disease and capture food (Reiser and Bjornn 1979). Cutthroat trout are sensitive to changes in water temperature and are not usually found in waters where maximum stream temperature repeatedly exceeds 22°C (Behnke and Zarn 1976). Exposure to temperatures as high as 28-30°C quickly leads to loss of equilibrium, swimming difficulty, and ultimately death (Heath 1963). Preferred temperatures likely range from 9-12°C. Spawning generally occurs from 6-17°C (Hunter 1973). Optimum stream temperature for incubation of eggs is ~ 10-11°C and ~15°C for juvenile rearing (Merriman 1935; Snyder and Turner 1960). Their preference for cooler water temperatures appears to make WCT a superior competitor at higher elevation stream reaches (Griffith 1988, Fausch 1989, Paul and Post 2001). The current distribution of WCT populations in many headwater areas supports the idea of a “temperature/ elevation refugia” for WCT where populations are most able to resist invasion by non-native species (e.g., Paul and Post 2001)

Current Velocity/ Stream Flow

While cutthroat occupy a wide range of habitats, they generally inhabit smaller streams with lower energy discharges. Spawning occurs at water depths of 20-50 cm and mean water velocities from 0.3-0.4 m/sec (Liknes 1984, Shepard et al. 1984). Young-of-the-year fry inhabit lateral habitat with flows ~ 0.06 m/s and depths over 3 cm (Bozek and Rahel 1991). Platts (1974) found that WCT densities peaked at a channel gradient of about 10%, which was higher than that for peak densities of bull trout (Salvelinus confluentus), brook trout (Salvelinus fontinalis), or RBT.  Changes to natural flow regimes and inadequate base flows have a significant impact on stream-dwelling salmonids (e.g., Spence et al. 1996). Eggs and alevins are sensitive to the infiltration of fine sediments into spawning gravels. In laboratory studies, embryo survival was less than 50% when the concentration of fine sediments exceeded 20% (Shepard et al. 1984). Adequate riffle coverage and flow velocities are required to maintain levels of habitat diversity, insect production and delivery to parr in pools. Low base flows can lead to substantial losses of marginal rearing habitat, elevated stream temperatures and may inhibit normal patterns of migration when populations become isolated to pockets of water (e.g., Slaney et al. 1996; Rosenau and Angelo 2003). Westslope cutthroat trout appear to have evolved to move with the rising limb and peak of the hydrograph, allowing them to negotiate seasonal barriers within streams where increased flows may be necessary to gain access (see BIOLOGY – Movement/ Dispersal).

Riparian and Instream Cover

Riparian cover and varied instream structure are essential elements of WCT habitat, contributing greatly to stream complexity and to the creation of areas of refuge. Riparian vegetation (e.g., alders, salmonberry, willow, poplar, etc.) serves to stabilize stream banks, reduce predation, and keep stream temperatures low by reducing solar insolation (reviewed by Reeves et al. 1997; Rosenfeld 2001). As well, the riparian input of terrestrial insects is often a significant food source for WCT during summer months (Behnke 1992). Undercut banks, root wads and boulders are also important in partitioning habitat and as areas of refuge. Bedrock outcroppings are perhaps of more importance in areas where trees are smaller, and debris jams are less frequent. The abundance of larger juveniles in streams is often limited by the availability of pools and large woody debris (e.g., Schmetterling 2001).  Processes such as riparian logging and the removal of large woody debris are known to adversely affect pool habitat, and lead to the loss of stream complexity, bank instability, sedimentation and the infilling of pools. They reduce egg-to-fry survival, the availability of rearing habitat and future production of aquatic invertebrates (reviewed by Reeves et al. 1997; Rosenfeld 2001).

 

Habitat trends in Canada

The native range of WCT is limited to the western provinces of British Columbia and Alberta, where economies are largely driven by land use and resource extraction. All available data suggest significant habitat loss and degradation throughout the range of both subspecies in Canada over the last 100 years. The largest losses have occurred as a result of resource extraction and associated road construction. Habitat loss and alteration due to water impoundment for hydroelectric projects and agricultural irrigation has also been implicated in several declines. Protected areas do exist within the range of WCT in Canada, but they are often small and do not necessarily encompass all the habitats required by the various life history forms within an area (particularly migratory forms). It is apparent that in the absence of more rigorous protection, required habitat will continue to be degraded and populations increasingly fragmented.

British Columbia

The major habitat threats to WCT habitat in BC include logging, mining and urbanization.  Logging is by far the dominant resource industry in BC; forest products accounted for more than half ($15 billion, or 52%) of the province's total exports in 1999 (BC Stats, 2001). The loss of forest cover is known to adversely affect fish populations by changing temperature and hydrological regimes within streams.For many years, poor and outdated harvest practices have contributed greatly to habitat loss in Canada, and until recently, the numerous small streams and tributaries associated with these forests often received little formal protection. They are often still improperly culverted or logged to the streambanks (See BIOLOGY – Movements/ Dispersal). Urbanization and local development have adversely affected some populations. The East Kootenay region, for example, is home to ~ 65,000 people, most of whom live in the Cranbrook-Kimberly area.  The City of Cranbrook has grown extensively around Joseph Creek (St. Mary’s River drainage). According to traditional First Nations knowledge, the creek used to be a very important spawning area for WCT (Prince and Morris 2002). Extensive habitat degradation and alteration (e.g., impassable culverts, storm-drain runoff, siltation) and extremely low flows during summer months have severely impacted juvenile rearing in the system (Bill Westover, BC Ministry of Water, Land, and Air Protection, personal communication, 2003) Currently, this creek does not appear to support WCT but does support non-native brook trout.

In terms of mining activity, there are currently eleven operating mines in the East Kootenay region of BC. Six of these are industrial mineral mines, and five are coal mines. Impacts include the construction of rock drains on creeks (typically the infilling of valley bottoms and related habitat destruction), chemical loading (e.g., selenium) and stream diversion. The most detrimental impact of the mining industry on freshwater habitat is water contamination.  Rainbow trout collected downstream from a coal mine end-pit lake in Alberta had higher concentrations of selenium in muscle and gonad tissues than control fish. These elevated selenium levels increased the overall mortality to the swim-up stage and increased the incidence of spinal deformities and edema in fry (Holm et al. 2003). Accompanying these primary industries has been an increase in road density that promotes further habitat fragmentation, degradation, and the opening of new access points for harvest and non-native introductions (e.g., Reeves et al. 1997).

Alberta

Urbanization, water diversion, and agricultural practices have had obvious impacts on WCT habitat in Alberta. Cumulative impact assessments conducted on 98 fourth order or higher watersheds in the upper Oldman, Crowsnest, and Carbondale (Castle River drainage) basins found that approximately two-thirds of the watersheds are at moderate risk of degradation, which would result in further loss of WCT habitat. In addition, all but three of the remainder are at high risk of degradation from increased peak flows and surface erosion caused by extensive clearcutting and road development (Mayhood et al. 1997; reviewed in Mayhood 2000). Resource exploration has led to a dramatic increase in road density in Alberta, translating into an explosion of wilderness access points (e.g., roads, cut-lines). Increasingly, off-road vehicle traffic is leading to increased stream bank erosion and sedimentation, as well as increased angling pressure. For example, in the Ghost-Waiparous area, there are 189 km of designated trails, but on long weekends up to 2000 km of largely undesignated trails are actually being used by nearly 15,000 people (Alberta Wilderness Association 2002). Habitat degradation along the Bow River is severe; the city of Calgary is built around its banks and several major transportation thoroughfares run along much of its course.

The human population in the South Saskatchewan River basin is expected to grow to ~ 2 million by 2021 (from 1.3 million in 1996; Alberta Environment 2003a). Accompanying this population growth there will be a projected increase in domestic water demand of 29-66%. This is troubling considering that Alberta has no substantial ground water supply to draw upon; 97.5% of the water used in Alberta is from surface runoff (Alberta Environment 2003b). Presently, 41.5 % of the running waters of the Bow River valley watershed in Banff have been regulated, obstructed, or otherwise impounded (Schindler and Pacas 1996). There are four TransAlta hydroelectric plants on the Bow River mainstem alone (11 in total on the Kananaskis/Bow River system) and the health of the aquatic environment downstream on the Bow and Oldman rivers appears to be declining (Golder Associates Ltd. 2003). In 2001, the amount of water flowing down the Bow and Oldman rivers where they merge into the South Saskatchewan River (near Medicine Hat) hit a 31-year low. At least 70.4% of the median natural flow in the Oldman River (and 68.1% for the Bow River) is now allocated for industrial and domestic purposes (Environment Canada 2003). Irrigation licences account for about 75% of the total volume of South Saskatchewan River basin allocations (Alberta Environment 2003b).  Alteration of flow rates and regimes may be detrimental to the long-term well-being of WCT (see Limiting Factors).

While the major withdrawals occur in the lower parts of these systems and below existing WCT populations, it is likely that such withdrawals have contributed to the extirpation of populations in the Highwood, Bow, and Oldman rivers. Their disappearance came soon after the development of the dams and the stocking of rainbow trout into the reservoirs (Nelson 1965). The pattern is not unique. Dams have been a major factor in the decline of the Kananaskis, lower Spray and Cascade WCT populations.  While abundant in lower Kananaskis and Spray lakes before they were dammed, WCT are now virtually absent (Stelfox 1987a, b). Before dam construction in 1913, WCT were also notably present throughout the Kananaskis River system below Twin Falls (between the Upper and Lower Kananaskis lakes, in Lower Kananaskis Lake, and in the Kananaskis River). Today, they are virtually absent from Lower Kananaskis Lake, the Kananaskis River mainstem and the upper reaches of all but three of its small tributaries (Rocky, Evan-Thomas and Porcupine). Similarly, no WCT were found between the Ghost Dam on the Bow River and the Bearspaw Reservoir (RL & L Environmental 1998) or from the TransAlta Pocaterra Power plant to Pocaterra Creek (Kananaskis River drainage; Golder and Associates Ltd. 1999). Both areas historically supported WCT populations.

 

Habitat protection/ownership

All fish habitat in Canada is protected under provisions in the Fisheries Act.  In addition this species is found within Waterton and Banff National Parks as well as a number of federal reserves east of the Rockies, and in such cases are regulated pursuant to the National Parks Act.  The Department of Fisheries and Oceans (DFO), in partnership with provincial governmental agencies, has the legislative mandate to protect fisheries resources, fish habitat, and water quality in Canada. However, resource managers are often limited in their ability to avoid or mitigate developmental impacts where the land base is privately owned, and compliance with existing policies may appear equivocal (e.g., Harper and Quigley 2000; G3 Consulting, Ltd. 2000). 

Various park systems and protected areas do exist throughout the range of WCT in Canada (http://www.pdac.ca/pdac/advocacy/land-use/protected-areas.html).  Yet, the majority of their range remains subject to development and various types of resource extraction. Several higher land use planning processes have been undertaken. However, in the East Kootenay region of BC for example, less than 16% of the land base is formally protected; 9% is privately owned and the remaining 75% is subject to resource extraction, recreational use, and environmental stewardship (Owen 1994). In October 2002, the BC government implemented the Kootenay Boundary higher-level plan, which removes industry’s obligation to maintain mature forest cover in the region (Bergenske 2002). In Alberta, a relatively large proportion (28%) of the land base is privately owned; only 12.4% of the landbase is protected and resource extraction may be permitted in ecological reserves and provincial parks with government approval (Prospectors and Developers Association of Canada, 2003).

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