Bull Trout Salvelinus Confluentus Research Articles

Seasonal Movement and Habitat Use by Subadult Bull Trout in the Upper Flathead River System, Montana

AbstractDespite the importance of large‐scale habitat connectivity to the threatened bull trout Salvelinus confluentus, little is known about the life history characteristics and processes influencing natural dispersal of migratory populations. We used radiotelemetry to investigate the seasonal movements and habitat use by subadult bull trout (i.e., fish that emigrated from natal streams to the river system) tracked for varying durations from 1999 to 2002 in the upper Flathead River system in northwestern Montana. Telemetry data revealed migratory (N = 32 fish) and nonmigratory (N = 35 fish) behavior, indicating variable movement patterns in the subadult phase of bull trout life history. Most migrating subadults (84%) made rapid or incremental downriver movements (mean distance, 33 km; range, 6–129 km) to lower portions of the river system and to Flathead Lake during high spring flows and as temperatures declined in the fall and winter. Bull trout subadults used complex daytime habitat throughout the upper river system, including deep runs that contained unembedded boulder and cobble substrates, pools with large woody debris, and deep lake‐influenced areas of the lower river system. Our results elucidate the importance of maintaining natural connections and a diversity of complex habitats over a large spatial scale to conserve the full expression of life history traits and processes influencing the natural dispersal of bull trout populations. Managers should seek to restore and enhance critical river corridor habitat and remove migration barriers, where possible, for recovery and management programs.

Stable isotopes and gut content show diet overlap among native and introduced piscivores in a large oligotrophic lake

Abstract – In past dietary studies kokanee Oncorhynchus nerka were prominent in the diet of Pend Oreille Lake's large piscivores: native bull trout Salvelinus confluentus, cutthroat trout O. clarki and northern pikeminnow Ptychocheilus oregonensis, and introduced lake trout S. namaycush and Kamloops rainbow trout O. mykiss gairdneri. However, kokanee have declined to 10–20% of their former abundance. We therefore initiated this study to understand current predation demands on kokanee and diet overlap among piscivores, using gut content samples and analysis of stable nitrogen (δ15N) and carbon (δ13C) isotopes from the lake's fish and invertebrate community. In gut content samples, kokanee were the main prey item of large [i.e., ≥400 mm total length (TL)] bull and lake trout; a conclusion that was affirmed by stable isotope analysis. Rainbow trout >500 mm TL consumed mostly kokanee, thus there was a high degree of diet overlap among large bull, lake and rainbow trout. Small (i.e., <400 mm TL) rainbow and cutthroat trout diets overlapped, and were composed mostly of littoral benthic invertebrates. However, gut content and stable isotope analysis did not accord for 400–500 mm TL rainbow trout, small lake trout, and large cutthroat trout. In these instances, a linear mixing model using stable isotope results predicted kokanee consumption for each species, but no kokanee were identified in rainbow or lake trout gut content. Gut content and stable isotope analysis of native northern pikeminnow indicated a diet of mostly littoral benthic invertebrates at smaller (100–150 mm TL) lengths, with kokanee becoming more prominent in the diet of individuals >300 mm TL. Percent of kokanee in the diet of northern pikeminnow has declined from a prior study; otherwise piscivore diets have apparently remained unchanged. In this study, judgments as to the feeding of some piscvores, based on gut content alone, would be tenuous because of small sample sizes, but stable isotope analysis provided an efficient means for confirming diets.

Are Block Nets Necessary?: Movement of Stream‐Dwelling Salmonids in Response to Three Common Survey Methods

AbstractFish movement during sampling may negatively bias sample data and population estimates. We evaluated the short‐term movements of stream‐dwelling salmonids by recapture of marked individuals during day and night snorkeling and backpack electrofishing. Bull trout Salvelinus confluentus and rainbow trout Oncorhynchus mykiss were captured in sufficient numbers to evaluate instream movements and the influence of stream habitat on movement. Salmonids moved upstream more often than downstream, which suggests that fish fled in response to sampling procedures. Fish movement rates were related to sampling method and stream habitat characteristics. The proportion of bull trout (mean total length, 148 mm) moving upstream from a 50‐m reach was 28% for day snorkeling, 25% for night snorkeling, and 18% for single‐pass electrofishing. The likelihood of rainbow trout movement upstream during sampling declined as the percentage of rubble substrate increased. Rainbow trout (mean total length, 139 mm) traveled short distances; 67% of mobile rainbow trout traveled 25 m, whereas 59% of mobile bull trout traveled more than 50 m. Our results suggest that fish movement out of sample sites during sampling may negatively bias abundance estimates and positively bias estimates of detection probability. We suggest that block nets are necessary to ensure population closure when estimating population abundance for stream‐dwelling salmonids. We further caution that, when fish flee in response to sampling, observations of habitat use may only apply to the proportion of fish that remained stationary and that were observed.

An Evaluation of Multipass Electrofishing for Estimating the Abundance of Stream-Dwelling Salmonids

Failure to estimate capture efficiency, defined as the probability of capturing individual fish, can introduce a systematic error or bias into estimates of fish abundance. We evaluated the efficacy of multipass electrofishing removal methods for estimating fish abundance by comparing estimates of capture efficiency from multipass removal estimates to capture efficiencies measured by the recapture of known numbers of marked individuals for bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi. Electrofishing capture efficiency measured by the recapture of marked fish was greatest for westslope cutthroat trout and for the largest size-classes of both species. Capture efficiency measured by the recapture of marked fish also was low for the first electrofishing pass (mean, 28%) and decreased considerably (mean, 1.71 times lower) with successive passes, which suggested that fish were responding to the electrofishing procedures. On average, the removal methods overestimated three-pass capture efficiency by 39% and underestimated fish abundance by 88%, across both species and all size-classes. The overestimates of efficiency were positively related to the cross-sectional area of the stream and the amount of undercut banks and negatively related to the number of removal passes for bull trout, whereas for westslope cutthroat trout, the overestimates were positively related to the amount of cobble substrate. Three-pass capture efficiency measured by the recapture of marked fish was related to the same stream habitat characteristics that influenced (biased) the removal estimates and did not appear to be influenced by our sampling procedures, including fish marking. Simulation modeling confirmed our field observations and indicated that underestimates of fish abundance by the removal method were negatively related to first-pass sampling efficiency and the magnitude of the decrease in capture efficiency with successive passes. Our results, and those of other researchers, suggest that most electrofishing-removal-based estimates of fish abundance are likely to be biased and that these biases are related to stream characteristics, fish species, and size. We suggest that biologists regard electrofishing-removal-based estimates as biased indices and encourage them to measure and model the efficiency of their sampling methods to avoid introducing systematic errors into their data.

Swimming Endurance of Bull Trout, Lake Trout, Arctic Char, and Rainbow Trout following Challenge with Renibacterium salmoninarum

AbstractWe tested the swimming endurance of juvenile bull trout Salvelinus confluentus, lake trout S. namaycush, Arctic char S. alpinus, and rainbow trout Oncorhynchus mykiss at 9°C and 15°C to determine whether sublethal infection from a moderate challenge of Renibacterium salmoninarum administered months before testing affected the length of time fish could maintain a swimming speed of 5–6 body lengths per second in an experimental flume. Rainbow trout and Arctic char swam longer in trials than did bull trout or lake trout, regardless of challenge treatment. When we tested fish 14–23 weeks postchallenge, we found no measurable effect of R. salmoninarum on the swimming endurance of the study species except for bull trout, which showed a mixed response. We conducted additional trials with bull trout 5–8 weeks postchallenge to determine whether increasing the challenge dose would affect swimming endurance and hematocrit. In those tests, bull trout with clinical signs of disease and those exposed to the highest challenge doses had significantly reduced swimming endurance compared with unchallenged control fish. Fish hematocrit levels measured at the end of all swimming endurance tests varied among species and between test temperatures, and patterns were not always consistent between challenged and control fish.

Influences of Temperature and Environmental Variables on the Distribution of Bull Trout within Streams at the Southern Margin of Its Range

The bull trout Salvelinus confluentus is believed to be among the most thermally sensitive species in coldwater habitats in western North America. We conducted a comprehensive field assessment of thermal habitat associations throughout the southern margin of the species' range. We developed models of thermal habitat associations using two data sets representing a geographically diverse range of sites and sampling methods. In both data sets, maximum temperature was strongly associated with the distribution of bull trout. In spite of the potential biases in these data sets, model predictions were similar. In both cases, the probability of the occurrence of bull trout exceeded 50% when the maximum daily temperature was less than 14-16°C, a result that is consistent with recent laboratory-based thermal tolerances. In one data set, we modeled the association between the distribution of bull trout and environmental variables, including temperature, instream cover, channel form, substrate, and the abundance of native and nonnative salmonid fishes. Only temperature was strongly associated with the distribution of bull trout. Our results and related studies of landscape habitat associations suggest that conservation efforts for bull trout would benefit from a focus on maintaining and restoring large and interconnected coldwater habitats.

Winter Diel Habitat Use and Movement by Subadult Bull Trout in the Upper Flathead River, Montana

We evaluated the diel habitat use and movement of subadult bull trout Salvelinus confluentus by use of radiotelemetry during winter in the upper Flathead River, Montana. Of the 13 monitored bull trout, 12 (92%) made at least one diel movement to other habitat locations during their respective day–night tracking surveys and moved an average of 73% of the time. The median distance moved from day to night locations by the mobile fish was 86 m (range, 27–594 m). Diel shifts in habitat use by nine of the tagged fish were related to light intensity; nocturnal emergence generally commenced immediately after the onset of night, and daytime concealment occurred at daybreak. When diel shifts in microhabitat use occurred, subadult bull trout moved from deep, midchannel areas during the day to shallow, low-velocity areas along the channel margins without overhead cover at night. Resource managers who wish to protect the overwintering habitat features preferred by subadult bull trout in the upper Flathead River should use natural flow management strategies that maximize and stabilize channel margin habitats at night.

Nuclear DNA identification of migrating bull trout captured at the Puget Sound Energy diversion dam on the White River, Washington State.

Bull trout (Salvelinus confluentus) is a char listed as threatened under the United States Endangered Species Act throughout its range in the coterminous United States. Substantial morphological similarities between bull trout and Dolly Varden (S. malma) make field identification difficult. This has resulted in an incomplete understanding of their distribution and abundance in Washington State where these two species occur sympatrically. We used three diagnostic nuclear loci to determine the species of char collected at a trap on the White River in southern Puget Sound (Washington State, USA). Each of the 104 samples revealed the expected bull trout genotype at all three loci. This work presents three principle results: (i) the presence of a migratory bull trout population in southern Puget Sound; (ii) no evidence of migratory Dolly Varden over 3 years; and (iii) no evidence of hybridization was detected. These results also demonstrate how molecular markers can provide information essential to the conservation and management of these species.

Effect of Bull Trout and Brook Trout Interactions on Foraging Habitat, Feeding Behavior, and Growth

Abstract Observations of free-ranging sympatric bull trout Salvelinus confluentus and nonnative brook trout S. fontinalis in two eastern Oregon headwater streams provided little evidence of habitat partitioning. Both species held focal feeding points in similar microhabitats and fed primarily from the water column rather than from the surface or benthos. In an instream experiment, 20 enclosures were assigned one of three treatments: two bull trout, four bull trout, or a mix of two bull trout and two brook trout. In the enclosures, macroinvertebrate drift was restricted and trout densities were elevated, creating an environment of reduced food and habitat resources. Under these conditions, there was no indication of a niche shift by bull trout; feeding behavior and habitat use by bull trout did not differ depending on the presence or absence of brook trout. Brook trout in the mixed-species treatment were the most aggressive, maintained dominance in 75% of the enclosures, and exhibited significantly higher ...

Native Species Conservation Through Education: The Adopt-A-Trout program in Montana

Abstract In order to enhance the understanding and conservation of native fish in the West, we developed an educational program (Adopt-A-Trout) to introduce fisheries research to students in rural Montana schools. This innovative project provides data on fish migrations to schools in the Blackfoot watershed through teacher training, a website, and field trips. These data are from an ongoing study conducted by Montana Fish, Wildlife and Parks (MFWP) biologists at Milltown Dam in western Montana. As part of this research project, fluvial westslope cutthroat trout (Oncorhynchus clarki lewisi) and bull trout (Salvelinus confluentus) that migrated to Milltown Dam were captured and released upstream from the dam so they could complete their spawning migrations. Radio transmitters were implanted in many of the transferred fish to monitor their movements to upriver areas, and classes followed their migrations along with biologists. MFWP biologists recorded data weekly on the migrations on a website that students ...

Efficacy of Three Denil Fish Ladders for Low-Flow Fish Passage in Two Tributaries to the Blackfoot River, Montana

We evaluated the efficacy of three Denil fish ladders retrofitted to three irrigation diversions in two tributaries to the Blackfoot River, Montana. Fish ladders were 2.4–6.1 m long, with slopes of 9.6–15.8% and mean velocities of 18–140 cm/s. We captured westslope cutthroat trout Oncorhynchus clarki lewisi, brown trout Salmo trutta, bull trout Salvelinus confluentus, and slimy sculpins Cottus cognatus upstream of the fish ladders (treatment groups), and in three nearby stream reaches without a ladder (control groups). Fish were marked and then released downstream of the fish ladders (treatment) or immediately downstream of their capture reaches (control). Fish lengths were similar between treatment and control sites (t-tests; P > 0.05). We resampled the reaches 5–7 d after marking and recaptured similar numbers of marked westslope cutthroat trout and brown trout in both treatment and control sections (χ2 = 0.13; df = 2; P = 0.94). No slimy sculpins were recaptured in treatment or control sections. Lengths of recaptured westslope cutthroat trout ranged from 92 to 305 mm after the trout ascended the ladders and were similar to lengths of marked fish in those sections (t-tests; P > 0.05). Although it was the longest, the fish ladder with the lowest slope and velocities appeared to limit small (<300 mm) brown trout movement. Recaptured brown trout in the treatment reach were significantly larger (P = 0.05; range = 215–352 mm) than marked brown trout. Fish ladders on irrigation diversions require frequent maintenance. When we evaluated the three structures, only one was operating correctly. Nevertheless, the ladders demonstrate promise for passing small stream-resident or juvenile fishes over irrigation diversions.

Evidence of Introgressive Hybridization between Bull Trout and Brook Trout

Abstract Hybridization between native bull trout Salvelinus confluentus and introduced brook trout S. fontinalis occurs over a wide geographic area in the western United States. We described the extent to which introgressive hybridization has occurred between these species using biochemical and molecular genetic techniques in samples collected from five streams in western Montana. We found that about three-quarters of the hybrids detected were male, first-generation (F1) hybrids. Most of the rest were backcrosses to the parental species, indicating that F1 hybrids can reproduce. We found no evidence of hybrid swarms in which all individuals were of hybrid origin. Our results suggest that both the reduced fertility of F1 hybrids and the reduced survival of their progeny prevents these species from forming such hybrid swarms. We also found that hybridization between bull and brook trout tends to occur predominantly between female bull trout and male brook trout, indicating that hybridization represents grea...

Effects of Copper Exposure on Growth and Survival of Juvenile Bull Trout

Abstract Metal toxicity from mine wastes has been identified as a potential contributor to declining populations of the federally threatened bull trout Salvelinus confluentus. Although several studies have been published on the effects of long-term Cu exposure on growth in salmonid fish, none have been conducted with bull trout. We evaluated the effects of a 60-d Cu exposure on bull trout in water at 220 mg/L hardness (as CaCO3) and pH 7.9. The endpoints measured were growth, whole-body Cu concentration, and mortality. Exposure to Cu at 179 μg/L resulted in 7% mortality (compared with a mortality of 2% in controls), but no effects of Cu exposure on growth were observed at any Cu concentration tested. Measured tissue residues of Cu in these bull trout were higher than those associated with reduced growth in rainbow trout Oncorhynchus mykiss, indicating that bull trout may be more tolerant of long-term Cu exposure than rainbow trout. This greater tolerance to Cu may be associated with a lower rate of Cu acc...

Upstream Movement of Residual Hatchery Steelhead into Areas Containing Bull Trout and Cutthroat Trout

Hatchery-reared steelhead Oncorhynchus mykiss that do not emigrate as smolts shortly after release may harm wild fish communities through ecological interactions. We used systematic, stratified snorkeling surveys to document the relative abundance of wild rainbow trout (nonanadromous O. mykiss), bull trout Salvelinus confluentus, and westslope cutthroat trout O. clarki lewisi as well as the upstream limit of residual hatchery steelhead (hatchery-reared steelhead that had failed to emigrate by June 1). Our objective was to determine whether residual hatchery steelhead had migrated upstream from their release point into an area containing westslope cutthroat trout and a threatened population of bull trout. Hatchery steelhead made up a larger proportion of the salmonid community in the sites near their release location and constituted a lower proportion as distance upstream from the release location increased. However, residual hatchery steelhead had migrated over 12 km upstream into an area containing westslope cutthroat trout and a threatened stock of bull trout.

Effective Population Size and Genetic Conservation Criteria for Bull Trout

Abstract Effective population size (Ne ) is an important concept in the management of threatened species like bull trout Salvelinus confluentus. General guidelines suggest that effective population sizes of 50 or 500 are essential to minimize inbreeding effects or maintain adaptive genetic variation, respectively. Although Ne strongly depends on census population size, it also depends on demographic and life history characteristics that complicate any estimates. This is an especially difficult problem for species like bull trout, which have overlapping generations; biologists may monitor annual population number but lack more detailed information on demographic population structure or life history. We used a generalized, age-structured simulation model to relate Ne to adult numbers under a range of life histories and other conditions characteristic of bull trout populations. Effective population size varied strongly with the effects of the demographic and environmental variation included in our simulation...

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

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 investigations suggesting that bull trout have among the lowest upper thermal limits and growth optima of North American salmonids. The slower acclimation times and long-term duration of the ACE method resulted in a more realistic measure of thermal tolerance in natural situations than would have been obtained with traditional methods and afforded sufficient time for sublethal differences in growth rate, feed consumption, and feed efficiency to become apparent.

Spatial Distribution of Native and Nonnative Salmonids in Streams of the Eastern Slopes of the Canadian Rocky Mountains

Abstract We surveyed 53 stream reaches from the eastern slopes of the Canadian Rocky Mountains and examined the distribution of native and nonnative salmonids as related to habitat variables measured at the reach scale (100 m). The most common fishes encountered in these surveys were cutthroat trout Oncorhynchus clarki, bull trout Salvelinus confluentus, brook trout S. fontinalis, and rainbow trout O. mykiss. Of these salmonids, only cutthroat and bull trout are native to Kananaskis Country; however, cutthroat trout have also been extensively stocked throughout the region. Reach elevation, which strongly influenced mean summer stream temperatures, was the only habitat variable that was significantly related to the presence of all four salmonids. Both cutthroat and bull trout were more likely to occur in the higher elevations, whereas brook and rainbow trout were more likely to occur in the lower elevations. Because the distribution of stocked fishes is not independent of their original stocking locations,...

Sources and Magnitude of Sampling Error in Redd Counts for Bull Trout

Abstract Monitoring of salmonid populations often involves annual redd counts, but the validity of this method has seldom been evaluated. We conducted redd counts of bull trout Salvelinus confluentus in two streams in northern Idaho to address four issues: (1) relationships between adult escapements and redd counts; (2) interobserver variability in redd counts; (3) sources of interobserver variability; and (4) temporal and spatial variation in spawning activity. We found that estimated adult escapements and redd counts were strongly correlated on a logarithmic scale, but both sources of data probably contained large estimation or observation errors. In particular, redd counts varied significantly among observers in replicate counting trials. Observer counts ranged between 28% and 254% of the best estimates of actual redd numbers. Counting errors included both omissions and false identifications. Correlations between counting errors and redd and habitat characteristics were highly variable and provided lim...

Modeling Seasonal Trophic Interactions of Adfluvial Bull Trout in Lake Billy Chinook, Oregon

We examined the trophic interactions of bull trout Salvelinus confluentus in Lake Billy Chinook, Oregon, using a bioenergetics model combined with data on annual growth, seasonal diet, distribution, and thermal experience to determine the seasonal and size-specific prey requirements of bull trout and the influence of bull trout predation on some of their major prey species in the reservoir. Per capita estimates of consumption were expanded to population-level estimates based on estimates of size structure and the abundance of the bull trout population. Bull trout became progressively more piscivorous with increasing size, and fish were the primary prey of predators of 450 mm fork length (FL) or more. Kokanee (lacustrine sockeye salmon Oncorhynchus nerka) and other salmonids (predominantly bull trout and rainbow trout O. mykiss) represented the largest fraction of fish prey in the diet, although cyprinids, cottids, and catostomids were also consumed. Predation on kokanee occurred primarily in autumn and secondarily in winter−spring. Predation by bull trout of less than 450 mm FL was greatest on smaller bull trout and rainbow trout in the reservoir during June−September, whereas predation by bull trout of 450 mm or more on salmonids other than kokanee was most prevalent during winter−spring. Bull trout of all sizes were capable of eating fusiform fish up to 50% of their own length. Given the size structure and estimated abundance of bull trout of 200 mm FL or more in Lake Billy Chinook during 1993 (3,600) and 1994 (8,400), model simulations indicated that annual predation removed large fractions of the bull trout (19–44% of age-0 fish and 7–16% of age-1 fish) and kokanee (5–11% of age-0 fish, 1–2% of age-1 fish, and 9–59% of fish of ages 2–3) populations. Thus, cannibalism and prey supply are potentially important factors limiting the abundance and production of bull trout in Lake Billy Chinook.

Summer and fall microhabitat utilization of juvenile bull trout and cutthroat trout in a wilderness stream, Idaho

Microhabitat use and availability were evaluated and compared between different size classes of juvenile resident bull trout (Salvelinus confluentus) and cutthroat trout (Oncorhynchus clarki) in a small wilderness stream within the South Fork Clearwater River basin, Idaho. The objective was to determine if utilization of measured habitat characteristics changed from summer to late fall. Sampling of fish was conducted with night snorkeling. During the summer, smaller juvenile bull trout (<66 mm) total length (TL) were associated with shallow stream margins over coarse substrates. In the fall, they moved to significantly deeper, lower velocity water, and closer to cover (p<0.05), but maintained their association with coarse substrates. During the summer, larger juvenile bull trout and larger juvenile cutthroat trout (66–130 mm TL) occupied significantly deeper water than smaller juvenile bull trout (p<0.05). Generally, larger juvenile bull trout were found closer to the bottom and in lower velocity water than larger juvenile cutthroat trout (p<0.05). In the fall, larger juvenile bull trout and larger juvenile cutthroat trout were associated with significantly deeper, lower velocity water located closer to cover than in summer (p<0.05). However, cutthroat trout occupied slightly deeper water over finer substrates than bull trout. Deep water with low velocities evidently provide important rearing areas for large bull trout and large cutthroat trout in the fall. Land management practices that maintain such environments will benefit these species.