Westslope cutthroat trout COSEWIC assessment and status report: chapter 6

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Biology

General

Cutthroat trout show a remarkable diversity in phenotypic traits and life history characteristics throughout their range. The scope of this diversity and its underlying biology, however, remains understudied and poorly understood relative to other salmonid species. It is evident that WCT inhabit smaller, less productive streams, preferring cooler water temperatures than other closely related species. Populations are generally small but show strongly developed natal philopatry and well-defined population structure (see previous sections). Populations appear sensitive to habitat perturbation and the introduction of non-native fishes. Habitat degradation may make populations especially susceptible to displacement and/ or hybridization with introduced species. As such, populations in degraded habitats are likely subject to declines, and their high degree of demographic independence suggests that losses are not likely to be offset by immigration from nearby sources over the short term.

Life history diversity

Cutthroat trout are arguably the most diverse salmonid species in North America and show extensive phenotypic variation in the size, colouration, and life history characteristics typical of populations (reviewed by Trotter 1987, Behnke 2002).  Much of this diversity is adaptive in nature and has evolved in response to local environmental conditions (sensu Taylor 1991).  Several different life history types are present throughout the range of WCT: fluvial and resident populations are common throughout the Canadian range (adfluvial perhaps less so); different strategies may often be present within the same population.  The relationships between these life history types and the amount of interaction between them is not clear, particularly with respect to shared resources and habitat utilization (much of the available information on particular life history types has been collected in the US).  Within an area, however, it is apparent that different life history types are more closely related to each other than to similar types from other areas (e.g., reviewed by Johnson et al. 1999).  Rather than having a common origin, the life history patterns within a particular area appear to evolve independently as part of a necessary ecological succession to minimize competition for resources.  Different life history components of a population may share certain habitats (e.g., the same overwintering or summer habitat) while exploiting different spawning habitat. The relative size differences between individuals with different life history strategies may provide some opportunity for spatial/ temporal isolation on spawning grounds: stream resident WCT seldom exceed 25 – 30 cm fork length, while fluvial and adfluvial fish can attain sizes of >30 cm FL and 0.9-1.4 kg in weight (Shepard et al. 1984; McIntyre and Rieiman 1995).

Reproduction

Cutthroat trout exhibit a mating system typical of other salmonids (reviewed by Fleming 1998). Spawners home to small natal streams where females compete for preferred spawning areas (usually in the tailouts of deep pools) and males compete for access to females (although alternate ‘sneaking’ strategies are employed by small stream resident males). Brown and Mackay (1995b) found fluvial WCT in the Ram River, Alberta to maintain territories of ~400 m in the natal creek. Within this area, females would dig several redds and males would attempt to mate with all females within its section. Sex ratios on the spawning grounds appear to vary considerably and may partly correspond with life history type. For example, Downs et al. (1997) found that the sex ratio favoured males in headwater resident populations (1.3:1). Published estimates for migratory populations reviewed by Downs et al. (1997) were generally lower, ranging between 0.2 and 0.9 males per female. The authors suggest that a proportion of males (which are likely more susceptible to angling due to their aggressive territorial behaviour) may be removed from larger systems prior to spawning. Headwater resident populations, which are generally less accessible, are less likely to receive the same type of angling pressure.

The age and size of individuals at sexual maturity similarly varies across populations and life history types. Downs et al. (1997) found that males in isolated headwater populations from Montana first reach maturity at age 2 and all were mature by 4 years of age. The youngest female found to be mature was 3 years old while most were mature by the age of 5 years. Length was found to be a better predictor of sexual maturity than age; males matured at 110-160 mm fork length (FL) and females at 150 – 180 mm FL.  Mean fecundity (±SD) was 227 eggs (±41.1) for fish 150 – 174 mm, 346 (±85.6) for 175 – 199 mm fish, and 459 (±150.8) for fish 200 mm and longer. Migratory fish which generally mature at a larger size have correspondingly higher fecundities. Large migratory females with a forklength of 350 mm may contain 1000-1500 eggs (Liknes and Graham 1988).

Spawning generally takes place between May and August in Canada (depending on location) and is likely stimulated by rising water temperatures (~ 5 – 6.5°C). Its timing often coincides with freshet conditions in many interior areas, making WCT prone to year-class failure where habitat degradation leads to increased levels of erosion and sedimentation near redds. Spawning may occur relatively quickly; while fluvial WCT in the Blackfoot River, Montana were found to occupy spawning tributaries from 4 to 63 days, they were found to spawn over a relatively short period (1-3 d) with spawners spending less time in smaller creeks (Downs et al. 1997).  Eggs generally incubate in the spawning gravels for 6-7 weeks, depending on water temperature. Eggs in the Flathead River drainage (just south of the BC/ Montana border) required ~ 310 temperature units (degree days) for full development. Once hatched, alevins remained in the substrate until their yolk sac was absorbed (a further 100-150 temperature units; Shepard et al. 1984). Fry are ~ 20 mm when they emerge from the streambed in early July to late August and quickly migrate to low energy lateral habitats.

Cutthroat trout are iteroparous and some fish may reproduce every year or every alternate year but post-mating mortality may be high, especially for males. There appear to be very few repeat spawners (0.7 – 2.9%; Schmetterling 2001) although higher values have been reported elsewhere (Shepard et al. 1984; McIntyre and Rieman 1995). As female fecundity is known to increase with size, (Giger 1972; Downs et al. 1997), the importance of maintaining these repeat spawners is particularly relevant for small populations subject to habitat degradation. Not only do larger females produce more eggs, but the eggs are larger and produce larger alevins, increasing their chances for survival. Unfortunately, while size restrictions and other harvest regulations are often in place, large females are highly prized by sport fisherman and may be subject to high harvesting pressure or to by-catch in other fisheries.

Survival

Survival rates for cutthroat trout are extremely hard to determine as many factors affect species survival at different life history stages (e.g., Johnston and Mercer 1976). The time of greatest mortality likely occurs early in life; from the egg to juvenile stage. Eggs and newly hatched alevins are highly sensitive to environmental degradation, particularly sedimentation and dewatering. Physical injury and competition for rearing habitat is likely significant where such habitat is limited. For fry and larger juveniles, competition with each other and sympatric species for food and areas of refuge may be significant. As well, they may be heavily preyed upon by piscivorous fishes (e.g., cottids, bull trout (Salvelinus confluentus), brook trout, northern pikeminnow (Ptychocheilus oregonensis) and other salmonids). Adults are susceptible to a number of terrestrial predators (raptors, mustelids, etc.) where sufficient cover is lacking. Recreational harvesting may also represent a significant source of mortality for adults, even where fisheries restrictions have been implemented (see LIMITING FACTORS).

Movements/dispersal

Cutthroat trout exhibit one of the broadest and most variable spectra of migratory behaviours of all the salmonids, owing perhaps to the diversity of life history types and habitats occupied by the species (Northcote 1997; Hilderbrand and Kershner 2000a). Cutthroat trout undergo a series of different types of movement during their lifetime: seasonal movements (feeding, overwintering), spawning runs, and those associated with life history regime shifts. Importantly, mixed migratory strategies for different life history types is likely an adaptation to buffer periodic environmental disturbances (e.g., Rieman and Clayton 1997).

During their first year of life, fry disperse from areas of high density to low density; generally into lateral habitats with sufficient cover. Juveniles reside in natal streams from 1 to 4 years depending on stream productivity and the particular life history type involved. During this time, individuals may be relatively sedentary, remaining in the vicinity of the same stream reach or pool. Older juveniles and sub-adults may range further in response to changing water levels, stream temperatures, or the availability of food. Individuals from headwater streams in Montana, for example, have been observed to move less than 1 km (Jakober et al. 1998) while fluvial and adfluvial WCT may migrate over large distances (in excess of 100 km) to find suitable feeding grounds or overwintering habitat (Schmetterling 2001). Recent telemetric data from the lower Elk River in southeastern BC suggests that home ranges of WCT in that system average ~ 11 km. Home ranges in the upper river were nearly twice that (averaging ~ 23 km) and likely reflect a lack of suitable overwintering pool habitat in the upper river (Prince and Morris 2003). The age of outmigration for migratory forms typically appears to be 2-3 years of age (95-170 mm FL; McIntyre and Rieman 1995). Again, timing depends on local conditions, but peaks early to mid-summer with migrants leaving natal streams at night. Movement during the summer will often cease once suitable feeding habitat has been found.

In late summer and early fall, WCT begin to seek suitable overwintering sites in response to decreasing water temperatures and ice formation. Again, individuals may travel considerable distances to find suitable habitat but may remain sedentary through winter months in stream sections lacking anchor ice. In streams with dynamic ice conditions, movement can continue throughout the winter (Brown and Mackay 1995b; Schmetterling 2001, Prince and Morris 2002).  In response to lengthening days and increasing water temperatures, WCT will often rapidly leave their overwintering habitat in late winter-early spring to return to small natal tributaries to spawn. This may occur between March and July, but most typically between May and June. Having arrived at the natal system, there are typically a large number of small movements within a small section of stream (i.e., within breeding territory). Following spawning, there may or may not be a sudden movement to summer habitat (again depending on its location/ availability) followed by little subsequent movement during the summer.

Nutrition and interspecific interactions

Cutthroat trout tend to be highly opportunistic in terms of their diet, often feeding voraciously on whatever prey item is seasonally abundant. Unlike the coastal variety, WCT are not highly piscivorous and tend rather to specialize as invertebrate feeders, even where forage fish are abundant (Shepard et al. 1984). This is likely a result of their sympatric evolution with two highly piscivorous species, the bull trout and the northern pikeminnow (Behnke 1992). For young-of-the-year fry, chironomid larvae in lateral habitats are an important food source. Older juveniles and adults feed both on terrestrial insects and planktivorous invertebrates; dipterans (true flies, other than chironomidae such as crane flies, fruit flies, etc.) and ephemeropterans (mayflies) are the most important dietary components. Trichopterans (caddisflies) are important for fish 110 mm long or longer (reviewed by Liknes and Graham 1988). Winged insects are not important in the diets of smaller fish, but the diversity of food items increases with increasing size. For adfluvial forms, zooplankton are an important food source, particularly during winter months (Shepard et al. 1984).

Cutthroat trout possess traits that appear to reduce their interspecific interactions with other salmonids. The small size of cutthroat trout at maturity allows them to utilize smaller streams than those typically inhabited by larger salmonids. 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, RBT or brook trout. These densities may reflect that such habitats are less optimal for other salmonids, not necessarily that they are preferred by WCT. Their preference for cooler water temperatures appears to make WCT a superior competitor at higher elevation stream reaches and supports the idea of a “temperature/elevation refugia” for WCT (Griffith 1988, Fausch 1989, Paul and Post 2001). It appears, however, that WCT populations are less likely to coexist with introduced brook trout than with other native salmonids (Griffith 1988). In Yellowstone National Park, the introduction of brook trout has nearly always resulted in the disappearance of WCT (Varley and Gresswell 1988). Brook trout have a competitive advantage over WCT at warmer temperatures (De Staso and Rahel 1994) and mature earlier in life than WCT. It may be that WCT are marginalized by other mechanisms such as habitat degradation or overfishing, and are then replaced by brook trout. Once a WCT population is replaced by another salmonid species, however, it is unlikely that that space will ever be regained by WCT (e.g., Moyle and Vondracek 1985).

Finally, WCT are subject to introgressive hybridization when closely related species (i.e., RBT, other cutthroat subspecies) are introduced into their range. Several factors appear to contribute to the breakdown in species barriers. Firstly, RBT and the various interior subspecies of cutthroat trout appear to have evolved in relative isolation from one another (Behnke 2002). As such, only weak ethological isolating mechanisms have evolved to maintain separation between the different species. Secondly, the similarity in chromosome number between species can, in many cases, allow for fertile crosses between species (Thorgaard 1983; Allendorf and Leary 1988).

While the relative fitness of hybrids remains uncertain, the ongoing spread of introgression in the wild (e.g., Rubidge et al. 2001; Hitt et al. 2003) suggests that at least some hybrids do survive and are capable of successful reproduction. While some first-generation (F1) hybrids have been identified, most appear to be later generation hybrids and backcrossed individuals (Rubidge 2003, Hitt et al. 2003; Janowicz 2004). The majority of backcrossed hybrids appear to involve RBT males and WCT females although reciprocal crosses have been observed. The apparent absence of selection against hybrids suggests that introgressed genotypes can persist in wild populations, and have the potential to ultimately lead to the formation of hybrid swarms. Hybrid swarms present a significant threat to the persistence of native species and have been perceived as a “genomic extinction” or “extinction in progress” because the unique genotypes characteristic of the pure parental species are lost once randomly mating hybrid swarms are formed (Leary et al. 1995; Allendorf et al. 2003).

Behaviour/adaptability

All available information suggests that cutthroat trout are highly susceptible to habitat perturbation, particularly processes affecting water quality, temperature, or the amount of instream structure (Liknes and Graham 1988; Reeves et al. 1997; Porter et al. 2000). In several long-term studies, the loss of riparian buffer integrity generally leads to a dramatic decline in trout biomass, and populations remain suppressed for 5-20 years until the riparian zone regenerates (Hartman et al. 1996; Reeves et al. 1997). Such habitat perturbations involve a complex series of changes that disrupt natural growth processes within populations and cause increased mortality at certain age classes (Hartman et al. 1996). It further disrupts normal habitat partitioning and leads to increased competition for resources. Westslope cutthroat trout may be particularly sensitive to changes in natural flow regimes (Brown and MacKay 1995a; Downs et al. 1997). In agricultural or urbanized areas where water has been appropriated for irrigation or domestic use, WCT populations suffer dramatic declines as the loss of water affects all life history stages (e.g., Joseph Creek example, HABITAT TRENDS).

A popular sport fish in western Canada, cutthroat trout are perhaps second only to RBT in terms of angler interest throughout their range. This may be, in part, because they are more easily caught than other species (MacPhee 1966, Paul and Post 2001; Paul 2003). Their sometimes voracious feeding habits and accessibility in small streams make cutthroat trout subject to overharvesting (Giger 1972; Varley and Gresswell 1988). In a recent creel survey in the Elk River, WCT made up 94.5% of the total catch of 98,031 fish (Heidt 2002). While this could simply suggest greater relative abundance, it is likely that fish can be caught numerous times in a season and often more than once on the same day. In Yellowstone National Park, for example, studies have shown that cutthroat trout were caught an average of 9.7 times in a heavily fished catch-and-release section of the Yellowstone River during one 3.5 month fishing season (Schill et al. 1986).