Thomas E. McMahon

Effect of Temperature on Growth and Survival of Bull Trout, with Application of an Improved Method for Determining Thermal Tolerance in Fishes

Abstract Elevated temperature is considered an important factor in the decline of the threatened bull trout Salvelinus confluentus, but the thermal requirements of this species have not been defined. We used the acclimated chronic exposure (ACE) method to assess the upper thermal limits and growth optima of bull trout fed daily to satiation over test temperatures ranging from 8°C to 28°C during 60-d trials. Survival of age-0 bull trout was at least 98% at 8, 10, 12, 14, 16, and 18°C, but 0% at 22, 24, 26, and 28°C after 60 d. The predicted ultimate upper incipient lethal temperature for these trout was 20.9°C. Peak growth, as estimated by regression analysis, occurred at 13.2°C (95% confidence interval, 10.9–15.4°C). Feed consumption declined significantly (P < 0.001) at temperatures greater than 16°C, and fish held at temperatures of 22°C and above did not feed. Feed, lipid, and protein efficiencies were similar at 8–18°C but declined significantly (P < 0.001) at 20°C. Our results corroborate field inves...

Role of stream ice on fall and winter movements and habitat use by bull trout and cutthroat trout in Montana headwater streams

Abstract We used radiotelemetry and underwater observation to assess fall and winter movements and habitat use by bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi in two headwater streams in the Bitterroot River drainage, Montana, that varied markedly in habitat availability and stream ice conditions. Bull trout and cutthroat trout made extensive (>1 km) downstream overwintering movements with declining temperature in the fall. Most fish remained stationary for the remainder of the study (until late February), but some fish made additional downstream movements (1.1–1.7 km) in winter during a low-temperature (⩽1°C) period marked by anchor ice formation. Winter movement was more extensive in the mid-elevation stream where frequent freezing and thawing led to variable surface ice cover and frequent supercooling (<0°C). Habitat use of both species varied with availability; beaver ponds and pools with large woody debris were preferred in one stream, and pools with boul...

Hybridization rapidly reduces fitness of a native trout in the wild

Human-mediated hybridization is a leading cause of biodiversity loss worldwide. How hybridization affects fitness and what level of hybridization is permissible pose difficult conservation questions with little empirical information to guide policy and management decisions. This is particularly true for salmonids, where widespread introgression among non-native and native taxa has often created hybrid swarms over extensive geographical areas resulting in genomic extinction. Here, we used parentage analysis with multilocus microsatellite markers to measure how varying levels of genetic introgression with non-native rainbow trout (Oncorhynchus mykiss) affect reproductive success (number of offspring per adult) of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) in the wild. Small amounts of hybridization markedly reduced fitness of male and female trout, with reproductive success sharply declining by approximately 50 per cent, with only 20 per cent admixture. Despite apparent fitness costs, our data suggest that hybridization may spread due to relatively high reproductive success of first-generation hybrids and high reproductive success of a few males with high levels of admixture. This outbreeding depression suggests that even low levels of admixture may have negative effects on fitness in the wild and that policies protecting hybridized populations may need reconsideration.

Comparative Thermal Requirements of Westslope Cutthroat Trout and Rainbow Trout: Implications for Species Interactions and Development of Thermal Protection Standards

Abstract Water temperature appears to play a key role in determining population persistence of westslope cutthroat trout Oncorhynchus clarkii lewisi, but specific thermal performance and survival criteria have not been defined. We used the acclimated chronic exposure laboratory method to determine upper thermal tolerances and growth optima of westslope cutthroat trout and rainbow trout O. mykiss, a potential nonnative competitor that occupies much of the former range of westslope cutthroat trout. Rainbow trout had a distinct survival advantage over westslope cutthroat trout at water temperatures above 20°C. The ultimate upper incipient lethal temperature of rainbow trout (24.3°C; 95% confidence interval [CI] = 24.0–24.7°C) was 4.7°C higher than that of westslope cutthroat trout (19.6°C; 95% CI = 19.1–19.9°C). In contrast, both species had similar growth rates and optimum growth temperatures (westslope cutthroat trout: 13.6°C; rainbow trout: 13.1°C) over the temperature range of 8–20°C, although rainbow tr...

Spatial Variation in Spawning Habitat of Cutthroat Trout in a Sediment-Rich Stream Basin

Abstract We examined distribution and habitat characteristics of spawning sites of cutthroat trout Oncorhynchus clarki at various spatial scales to assess effects of sedimentation within a large basin in Montana. Redd density varied widely across the basin; nearly all (99%) of the 362 redds observed occurred in two high-elevation headwater tributaries. Redd density at the reach scale was positively correlated (r 2 = 0.72, P = 0.001) with abundance of spawning gravels. Other habitat variables, such as gradient, width, depth, embeddedness, bank stability, and percent riffle, were not significantly correlated to redd density. Taylor Fork redds contained some of the highest proportions of fine sediments (<6.35 mm, mean = 41.6%; <0.85 mm, 17.9%) observed in egg pockets of salmonid redds in the Rocky Mountain region. Cache Creek, a highly disturbed subbasin, had significantly greater proportions of fine sediments smaller than 0.85 mm in redds than the undisturbed Wapiti Creek subbasin. High fine-sediment levels...

Temperature and competition between bull trout and brook trout : A test of the elevation refuge hypothesis

Abstract We tested the elevation refuge hypothesis that colder temperatures impart a competitive advantage to bull trout Salvelinus confluentus and thus account for increased biotic resistance to invasion and displacement by brook trout S. fontinalis in headwater streams. Growth, survival, and behavior were compared in allopatry and sympatry at temperatures of 8-20°C in the laboratory. In allopatry, age-0 bull trout and brook trout grew at similar rates at temperatures of 8.0-14.3°C, but brook trout grew significantly faster at higher temperatures. In sympatry, bull trout grew significantly less than brook trout at all test temperatures, with growth differences increasing linearly with increased temperature. Age-1 brook trout had significantly higher feeding and aggression rates than did similar-sized bull trout at 8°C and 16°C. The modeled growth of age-0 bull trout and brook trout based on tributary temperature data from a high-elevation site (mean summer temperature, 10°C) and a low-elevation site (14°...

Local-Habitat, Watershed, and Biotic Features Associated with Bull Trout Occurrence in Montana Streams

Abstract We evaluated the association of local-habitat features, large-scale watershed factors, the presence of nonnative brook trout Salvelinus fontinalis, and connectivity to neighboring populations with patterns of occurrence of threatened bull trout S. confluentus in 112 first-order to fourth-order streams in the Bitterroot River drainage in western Montana. Species presence or absence was estimated via single-pass electrofishing, local-habitat features were measured in 500-m sampling reaches, watershed variables were obtained from topographic maps, and potential demographic support from nearby bull trout populations occupying larger main-stem streams was estimated from electrofishing data records. We defined a set of nine candidate models that represented various combinations of these four main factors and used an information-theoretic approach to evaluate the relative plausibility of competing models. Models combining local habitat (width, gradient, and woody debris) with brook trout presence and th...

Diel Habitat Partitioning by Bull Charr and Cutthroat Trout During Fall and Winter in Rocky Mountain Streams

We used underwater observation to determine diel habitat partitioning between bull charr, Salvelinus confluentus, and cutthroat trout, Oncorhynchus clarki, during fall and winter (0.1–8.3°C) in two Rocky Mountain streams that differed in habitat availability. The majority (>70%) of both species emerged from concealment cover at night, though bull charr exhibited a greater tendency for nocturnal behavior than cutthroat trout. Differences in day and night counts were most pronounced at temperatures <3°C, when very few fish of either species were observed in the water column during the day, but both species were common at night. Both species used concealment cover of large woody debris and boulder substrate crevices in deep pools during the day. At night, fish emerged from cover and habitat use shifted to shallow water with low cover. Microhabitat partitioning among species and size classes occurred at night, cutthroat trout moving into shallower, faster water that was farther from cover compared to bull charr. Smaller fish of both species occupied focal positions in slower, shallower water closer to the substrate than larger fish. Large, mixed-species aggregations also were common in beaver ponds both day and night. High variation in diel and site-specific winter habitat use suggests the need for caution in developing habitat suitability criteria for salmonids based solely on daytime observations or on observations from a few sites. Our results support the need to incorporate nocturnal habitat use and partitioning in studies of salmonid ecology.

Decline of the migratory form in bull charr, Salvelinus confluentus, and implications for conservation

Large-bodied, migratory life history forms of bull charr, Salvelinus confluentus, were historically abundant in northwestern North America, but many remaining populations of this now-threatened species presently persist as small-bodied residents isolated in headwater streams. We examined whether the migratory form has been lost from headwater populations of bull charr and their potential for re-establishment. Upstream and downstream movement of bull charr and other salmonids from three tributary populations in the Bitterroot River drainage, Montana, was measured with weirs over a 17-month period. The migratory life history was rare or absent in two tributaries but still present at a low level in a third. In contrast, substantial numbers (n = 1745) of juvenile and adults of other salmonids (brown trout, Salmo trutta, cutthroat trout, Oncorhynchus clarki, and mountain whitefish, Prosopium williamsoni) were captured near tributary mouths, indicating a migratory life history was common in other species. Apparent decline of the migratory life history in bull charr was not directly related to damming suggesting other downstream mortality factors (predation, temperature) also are involved. Isolated, nonmigratory forms have increased risk of extinction, and restoration of the population connectivity via the re-establishment of migratory stocks is an important conservation goal for bull charr recovery. However, the factors governing migratory tendency remain unclear.

Local Habitat, Watershed, and Biotic Factors Influencing the Spread of Hybridization between Native Westslope Cutthroat Trout and Introduced Rainbow Trout

Abstract The invasion of nonnative fishes in freshwater systems is often facilitated by the interaction of biotic and abiotic factors operating at multiple spatial and temporal scales. We evaluated the association of local habitat features (width, gradient, and elevation), watershed characteristics (mean and maximum summer water temperatures, the number of road crossings, and road density), and biotic factors (the distance to the source of hybridization and trout density) with the spread of hybridization between native westslope cutthroat trout Oncorhynchus clarkii lewisi and introduced rainbow trout O. mykiss in the upper Flathead River system in Montana and British Columbia. The presence of hybridization and the proportion of rainbow trout admixture were estimated using seven diagnostic microsatellite loci. We defined logistic and linear regression models including various combinations of spatial and environmental factors and used an information-theoretic approach to evaluate the relative plausibility o...