Geology topics

Daniel J. Schill

Publications and source records attributed to Daniel J. Schill.

At least 19 recordsLinked to original sources

USGS2025

Why the whole is greater than the sum of its parts: A case for population-level management

Catch-and-release regulations in recreational fisheries have been implemented for decades with the intention of allowing fishing while reducing fishing mortality. In addition, voluntary catch-and-release behaviors are increasingly common. Social and scientific interest in fish handling practices in catch-and-release fisheries as they relate to stress, reflex impairment, reproductive success, and mortality has been expanding. The scientific literature is now replete with studies that examine these issues, and they have become a fixture in the public discourse on angling. Scientists and anglers are making fishing regulation proposals and suggestions for fish-handling practices. The proximal intent of these recommendations is to reduce excessive stress or mortality on a per capita basis with a belief that the reduction ultimately has a positive effect on the population. Whether the proximal intent achieves population goals depends on several factors, including effort, population dynamic rates, and stock-recruitment dynamics. This perspective reviews the state of the science relative to a hierarchical framework of fishery population dynamics, with a call for fishery scientists to consider their scope of inference and assumptions relative to conservation and population management.

Article↗

USGS2024

Nonnative Smallmouth Bass in the Snake River, Idaho: Population dynamics, demographics, and management options

The Snake River in Idaho, USA, supports a popular sport fishery for nonnative Smallmouth Bass Micropterus dolomieu , but there are limited studies on the population dynamics of this introduced species in Idaho and other water systems in the western United States. The purpose of this study was to describe the population dynamics and demographics of Smallmouth Bass in the Snake River, Idaho. In total, we sampled 4,929 Smallmouth Bass during electrofishing surveys on the Snake River (separated into nine segments) and three major tributaries (Boise, Payette, and Weiser rivers). We estimated age for 1,869 Smallmouth Bass sampled from the Snake River ( n = 1,433) and three tributaries ( n = 436). Catch-per-unit-effort for all nine segments combined on the Snake River was 36.6 fish/h (±4.4 SE). In the tributaries, catch-per-unit-effort varied from 43.6 to 125.0 fish/h. Relative weight of all Smallmouth Bass varied from 86 to 107, indicating that fish were in relatively good body condition. Fish in the system grew fast, with relative growth index values often near or exceeding 100 for all age classes. Total annual mortality for the Snake River was 45.1 ± 0.7%, and it was 36.8–40.5% in the tributaries. Furthermore, we estimated exploitation to be 5.3% (90% CI; ±2.2%) for the Snake River and tributaries combined. We used a yield-per-recruit population model to evaluate the effects of varying minimum length limits on the fishery. With the observed population demographics and exploitation rates, increasing the current minimum length limit from 305 mm to 356 or 406 mm would probably have little influence on the number of Smallmouth Bass available to anglers. However, increasing the length limit would result in reduced biomass available for harvest. The potential for recruitment overfishing was minimal for all minimum length limits and levels of exploitation. As such, changes to current harvest regulations do not appear warranted. Our findings provide important information on the population dynamics of Smallmouth Bass that can be useful in evaluating their management across Idaho and in similar systems in western North America.

Idaho↗

USGS2023

The effects of estradiol-17β on the sex reversal, survival, and growth of Red Shiner and its use in the development of YY individuals

Objective The Red Shiner Cyprinella lutrensis is one of the most prolific and ecologically destructive invasive fish species in the southwestern United States. The production and release of YY individuals as Trojan sex chromosome carriers can theoretically eradicate invasive fish populations by eventually eliminating phenotypic females. Methods The YY individuals are typically produced through hormonally induced sex reversals and selective breeding of subsequently feminized males. We tested three dosages of estradiol-17β (E2)-treated diets (50, 100, and 150 mg of E2 per kg of diet) administered to sexually immature Red Shiner for various durations to determine their effectiveness at feminizing Red Shiner cohorts. Survival, growth, and gonadal development were assessed for each treatment. Result All E2 treatments had minimal, if any, detrimental effects on the growth and gonadal development of Red Shiner. The 50-mg dosage lasting from 2 to 120 days posthatch achieved a 100% feminization rate while using the lowest amount of E2; therefore, this dosage and treatment interval are recommended when attempting Red Shiner feminization under these rearing conditions. Feminization of males allowed for the spawning of neofemales (F XY ) with wild-type males (M XY ), which resulted in the first putative YY Red Shiner. The YY verification crosses ( n = 20) resulted in predominately male offspring (189 males/191 offspring) except for (1) an intersex individual from an M YY × F XX cross with two previtellogenic oocytes in its testis and (2) a single female that may have resulted from an inbred cross between an XY male and a YY female or from an unknown autosomal or environmental effect on sexual phenotype. Conclusion More progeny tests with inbred and outbred crosses should be conducted to determine the prevalence of female offspring from YY individuals and how this may impact an eradication strategy featuring releases of YY Red Shiner.

North American Journal of Aquaculture↗

USGS2023

The effects of estradiol-17β on the sex reversal, survival, and growth of green sunfish Lepomis cyanellus

The feminization of green sunfish Lepomis cyanellus could expand their utility as a game fish or aquacultured species by preventing overcrowding and precocious reproduction in stocked systems. Feminization of green sunfish could also help elucidate information on their sex determination system. We report the feminization of green sunfish cohorts via oral administration of estradiol-17β (E2) during early development. A low-dose (100 E2 mg per kg of diet) and a high-dose (150 E2 mg per kg of diet) experimental E2 treatment were fed to juvenile green sunfish from 30 to 90 days post-hatch. Fish were subsequently evaluated for any treatment effect on gonadal development, survival, and growth. Both E2 treatments resulted in 100% feminization, with no morphological or histological differences detected between E2 treated ovaries and those from a control group. The control group was composed mostly of males (82.61%). Overall, there was no effect of E2 on survival ( P = 0.310) and growth rate data suggested no statistical differences ( P = 0.0805). However, the growth rate of the high-dose group increased slightly higher after the treatment ended than the other treatments ( P = 0.042), suggesting that E2 might suppress growth in green sunfish. In addition, the control group did not exhibit a higher survival rate after the treatment period ended ( P = 0.266), whereas both E2 treated groups did ( P = 0.0003–0.0050). We found that the low dose, 100 E2 mg per kg of diet, was sufficient for fully feminizing green sunfish if administered during development from 30 to 90 days post-hatch and E2 dosages may result in deleterious effects on green sunfish's health and growth.

Aquaculture↗

USGS2022

The development of genetic sex identification markers and evidence of a male heterogametic sex determination system in Red Shiner

The Red Shiner Cyprinella lutrensis is of increasing management interest as an invasive species that negatively impacts many native fishes throughout North America. Trojan sex chromosome (TSC)-carrying individuals could theoretically control invasive fish populations by skewing the sex ratio to 100% male. The efficacy of TSC-based control programs requires an understanding of a population's sex determination system, yet such information is lacking for Red Shiner. We used single-digest restriction site-associated DNA sequencing to discover sex-linked single-nucleotide polymorphisms (SNPs), and we conducted a series of breeding experiments to uncover the sex determination system. All candidate sex-linked SNPs that fit our selection criteria exhibited a pattern of male heterogamety. We developed two sex-identification (sex-ID) marker assays, XY_248 and XY_170, which showed phenotype–genotype concordance scores of 77.00% and 84.35%, respectively. These sex-ID markers exhibited relatively high phenotype–genotype concordance in females (XY_248 = 96.30%; XY_170 = 98.61%), which allowed for selective breeding of phenotypically feminized genetic males. We observed a 3:1 male : female sex ratio in spawns from feminized males crossed with wild-type males, indicative of a male heterogametic sex determination system (i.e., XY male/XX female). The discovery of a male heterogametic sex determination system, in combination with our two markers, increases the likelihood of developing an effective TSC eradication strategy for invasive Red Shiner populations.

North American Journal of Aquaculture↗

USGS2022

Development of aquaculture protocols and gonadal differentiation of green sunfish (Lepomis cyanellus)

We provide detailed rearing methods and describe green sunfish ( Lepomis cyanellus ) gonadal development and histological differentiation for both sexes. Developing in-depth aquaculture protocols and describing the gonadal differentiation of green sunfish could facilitate strategies to control nuisance populations, enhance stocking programs, and provide information for this species' use in bioassay trials or toxicology studies. Our methods resulted in consistent year-round production of green sunfish and allowed us to identify the timing of their gonadal differentiation through histological assessment. Our spawning methods provided year-round volitional spawns from green sunfish broodstock. Our rearing methods involved weaning larval green sunfish off live nauplii and onto only artificial diets by 37 days post-hatch (dph). Most of the offspring generation reached sexual maturity by 213 dph. Green sunfish are gonochoristic, with testes and ovaries differentiating directly from undifferentiated gonads. Ovaries begin to differentiate by 39 dph and testes begin to differentiate by 69 dph. This information can provide biologists consistent means to produce this Centrachid and understand their gonadal development.

Aquaculture↗

USGS2021

Development of aquaculture protocols and gonadal differentiation of red shiner

Developing detailed rearing methods and describing the onset of gonadal differentiation in Red Shiners Cyprinella lutrensis could facilitate the development of novel techniques to control or enhance populations, enable toxicology studies, and help construct bioassays. In this study, we develop and report aquaculture practices for Red Shiner that ensure consistent year-round production in laboratory settings and evaluate the timing of sexual differentiation via histological gonad examinations. Our methods resulted in a mean of 56.00% (SD = 8.98%) survival through the larval stages of development, and we obtained spawns from captive-reared Red Shiners 138 d posthatch. Red Shiners are gonochoristic, and both ovaries and testes differentiate directly from undifferentiated gonads. Ovaries begin to differentiate in females 45 d posthatch, while testes begin differentiating in males 105 d posthatch. This study provides in-depth protocols for the closed-cycle aquaculture of Red Shiners and describes the gonadal differentiation and development of both sexes.

Arizona↗

USGS2020

Influence of population density and length structure on angler catch rate in kokanee fisheries

Management agencies are often charged with providing fisheries that lead to angler participation. Catch rate is one of the primary drivers of angler participation but can be influenced by a suite of factors, including population structure (e.g., density and size structure). The complexity of understanding how population structure influences angler catch rate is typified in kokanee Oncorhynchus nerka fisheries. Previous research suggests that angler catch rates of kokanee are positively influenced by fish density and total length. However, that research was based on data collected using size-selective midwater trawls. Due to the potential limitation of previous research, we sought to (1) understand the relative bias of midwater trawls and gill nets for describing the size structure of kokanee available to anglers and (2) re-evaluate the influence of fish density and fish length on angler catch rates in kokanee fisheries. Midwater trawl, gill-net, and creel data were collected on five prominent kokanee fisheries throughout Idaho in 2016 and 2017. Catch composition and percent overlap of midwater trawls, gill nets, and angler-caught fish were compared to understand the efficacy of midwater trawls and gill nets for representing the size structure of kokanee available to anglers. In addition, the influence of kokanee density and length on angler catch rates was evaluated. Midwater trawls primarily sampled small kokanee (<330 mm) and exhibited little overlap with angler-caught fish, whereas gill nets sampled more large fish (>330 mm) and exhibited higher overlap with angler-caught fish when compared to midwater trawls. Fish length was not positively associated with angler catch rates. However, fish density exhibited a positive relationship with angler catch rates. Our results highlight the importance of gear choice for understanding how kokanee populations function and elucidate the tradeoffs associated with population density, fish length, and resulting kokanee fisheries.

Idaho↗

USGS2019

Movement dynamics of Smallmouth Bass in a large western river system

The Snake River, Idaho, between Swan Falls and Brownlee dams supports a popular fishery for Smallmouth Bass Micropterus dolomieu . Recently, anglers have expressed concern about harvest of Smallmouth Bass associated with seasonal congregations in and near the lower reaches of several major tributaries. Little is known about Smallmouth Bass movement in the system, and a better understanding of movement dynamics will help to guide future management. From March to August 2016, Smallmouth Bass (≥260 mm; n = 1,131) were tagged with T-bar anchor tags to evaluate large-scale movement patterns. Movement was estimated from 63 angler-reported tags for which area descriptions provided sufficient detail to assign a recapture location. Extent of fish movement varied among segments and tributaries from 0 to 128 river kilometers (rkm). From March to May 2017, Smallmouth Bass (≥305 mm; n = 149) in the Snake, Boise, Payette, and Weiser rivers and in Brownlee Reservoir were implanted with radio transmitters. Of the 149 Smallmouth Bass that were released with radio transmitters, 107 were relocated at least once. Additionally, 79.6% of fish with radio transmitters had a maximum extent of movement of 5 rkm or greater and 42.6% had a maximum extent of 30 rkm or greater; one radio-tagged fish moved 167 rkm upstream. Average daily movement of Smallmouth Bass varied among river segments and was greatest in the spring and summer. Fish from the Snake River, tributaries (e.g., Boise River), and Brownlee Reservoir moved all around the study area, indicating an absence of clear population boundaries. As such, Smallmouth Bass in the study area appear to function as one large population as opposed to multiple subpopulations, thereby indicating that management as one population is likely appropriate.

Idaho↗

USGS2019

Effects of air exposure on survival of Yellowstone Cutthroat Trout angled from a stream with warm water temperatures

We evaluated the effects of air exposure on Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri caught and released in a cold-water stream with elevated water temperatures (i.e., > 14°C) in southeastern Idaho. Anglers caught fish in a 2.3-km section of Fall Creek, Idaho, during August 2018. Sampled fish remained underwater while we measured and then tagged them with T-bar anchor tags. We exposed fish to air for 0, 30, or 60 s and then released them at the point of capture. We continuously monitored temperature during the study period. Water temperatures during the study varied from 10.0 to 19.7°C and averaged 14.9°C (SE = 0.08). In total, anglers caught 161 Yellowstone Cutthroat Trout over 10 d. Of those fish, we did not expose 54 to air; we exposed 54 to air for 30 s, and 53 for 60 s. We used electrofishing to recapture tagged fish and estimate relative survival. Relative survival was highest for fish exposed to air for 60 s (0.40 [SE = 0.25]) followed by 0 s (0.35 [SE = 0.25]) and 30 s (0.30 [SE = 0.27]), but differences were not statistically significant. Results from this study are consistent with other air-exposure studies suggesting that air exposure of 60 s or less is not likely a concern in Yellowstone Cutthroat Trout fisheries. Releasing fish as quickly as possible is always encouraged, but management regulations restricting air exposure seem unnecessary given the collective body of field-based research on air exposure. Nevertheless, similar studies on other systems and species are warranted.

Idaho↗

USGS2019

Effectiveness of lake trout (Salvelinus namaycush) suppression in Lake Pend Oreille, Idaho: 2006–2016

The nonnative lake trout ( Salvelinus namaycush Walbaum, 1792) population in Lake Pend Oreille, Idaho increased exponentially during 1999–2006. This led to an unsustainable level of predation mortality on kokanee ( Oncorhynchus nerka Walbaum, 1792), increased the conservation threat to native bull trout ( Salvelinus confluentus Suckley, 1859), and jeopardized the popular recreational fishery for kokanee and rainbow trout ( Oncorhynchus mykiss Walbaum, 1792). In response, lake trout were suppressed since 2006 using incentivized angling, gill netting, and trap netting. From 2006 through 2016, 193,982 lake trout were removed (50% by gill netting; 44% by angling; 6% by trap netting). During this period, age-8 + (adult) lake trout abundance declined by 64%, age-3 (recruit) abundance declined by 56%, and mean total annual mortality ( A ) was 31.1%. Lake trout did not show evidence of a density-dependent response. Kokanee did not collapse and rebounded to abundances not observed since before lake trout expansion. Bull trout abundance declined during suppression, but the population was sustained. Lake trout suppression allowed a harvest fishery for kokanee and trophy fishery for rainbow trout to be restored. We conclude that suppression can be an effective management action for mitigating effects of nonnative lake trout in a large, deep lake.

Hydrobiologia↗

USGS2019

Influences of Lake Trout (Salvelinus namaycush) and Mysis diluviana on Kokanee (Oncorhynchus nerka) in Lake Pend Oreille, Idaho

Research on Lake Pend Oreille, Idaho, has focused on the influence of two potential limiting factors for kokanee Oncorhynchus nerka (Walbaum, 1792): competition for food with Mysis diluviana (Loven, 1862, hereafter Mysis ) and predation by lake trout Salvelinus namaycush (Walbaum, 1792). Population fluctuations of Mysis and lake trout have resulted in substantial heterogeneity in food web conditions, apparently altering both bottom-up and top-down dynamics. Therefore, relative importance of predation and competition were evaluated as drivers of kokanee abundance, biomass, and production. A series of general linear models was used to evaluate relative influences of Mysis and lake trout on kokanee. Kokanee production was a density-dependent process and the collapse of Mysis corresponded to an increase in the modeled maximum annual production of kokanee from 224 tonnes to 408 tonnes. Lake trout also negatively influenced kokanee biomass. A Mysis -mediated, predator-induced kokanee biomass collapse occurred when lake trout and Mysis abundances were both high. Sustainable management of this fishery requires recognition that competition with Mysis will define the scope of kokanee production and therefore the scope of sustainable predation.

Idaho↗

USGS2018

Effects of air exposure during simulated catch-and-release angling on survival and fitness of Yellowstone Cutthroat Trout

Concerns have been raised regarding the practice of exposing fish to air during catch-and-release (C&R) angling. The purpose of this study was to evaluate the effects of air exposure on short- and long-term survival and progeny production of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri . Prespawn adults were sampled at a weir during upstream migration in 2016 and 2017, exposed to a simulated angling event of 102 s, and then exposed to air for a randomly selected duration of 0, 30, or 60 s. An additional control group was added during 2017 in which fish were not exposed to simulated angling or air. In total, 1,519 fish were sampled in 2016, and 744 fish were sampled in 2017. Additionally, age-0 fish (2016: n = 2,924; 2017: n = 1,492) were collected to evaluate the effects of air exposure on the production of progeny. No effect of angling itself or of angling and air exposure was observed on short-term (≤60 d posttreatment) or long-term (>1 year posttreatment) survival of adults, with one exception. During 2016, fish that had been air exposed for 60 s had a statistically higher short-term survival rate than fish that received no air exposure. Air exposure had no effect on the proportion of fish that successfully spawned. Regression analysis revealed that neither angling nor air exposure affected progeny production. Considering that much of the literature, as well as this study, reports little to no influence of air exposure on salmonid mortality or reproductive success, it seems highly unlikely that air exposure of less than 60 s during C&R angling would have negative population-level effects.

Idaho↗

USGS2018

Effects of summer air exposure on the survival of caught-and-released salmonids

Despite the success of catch‐and‐release regulations, exposing fish to air during release has emerged as a growing concern over the past two decades. We evaluated the effect of air exposure during midsummer on survival of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri , Bull Trout Salvelinus confluentus , and Rainbow Trout O. mykiss exposed to catch‐and‐release angling. Fish were sampled by angling on Palisades Creek (August 2016), Sawmill Creek, and the Main Fork of the Little Lost River, Idaho (July−August 2017). After capture, fish were kept underwater while they were measured and individually tagged. Anglers, in groups of two to four, caught study fish and gave them an air exposure treatment of 0, 30, or 60 s. Single‐pass backpack electrofishing was then used to recapture tagged fish and estimate relative survival. In total, 328 Yellowstone Cutthroat Trout were sampled (0 s: n = 110; 30 s: n = 110; 60 s: n = 108), 278 Bull Trout (0 s: n = 92; 30 s: n = 94; 60 s: n = 92), and 322 Rainbow Trout (0 s: n = 103; 30 s: n = 106; 60 s: n = 113). The majority of fish were caught using artificial flies (≥92%) and were hooked in the corner of the mouth, lower jaw, or upper jaw (≥78%) in all three species. No difference in survival was observed among air exposure treatments for all three species. Results from the present study along with those from prior field studies of air exposure times during angling suggest that mortality from exposing fish to air for ≤60 s is not likely a population‐level concern in catch‐and‐release fisheries for these species.

Idaho↗

USGS2018

Fight and air exposure times of caught and released salmonids from the South Fork Snake River

Catch-and-release regulations are among the most common types of fishing regulations. In recent years, concerns have arisen regarding the exposure of fish to air during catch-and-release angling. The purpose of our study was to quantify the length of time angled fish were exposed to air by anglers in a typical catch-and-release fishery and relate it to the lengths of time reported to produce negative effects. In total, 312 individual anglers were observed on the South Fork Snake River, Idaho, from May through August 2016. Fight time varied from 1.1 s to 230.0 s, and average fight time was 40.0 s (SD = 36.8). Total air exposure times varied from 0.0 s to 91.8 s and averaged 19.3 s (SD = 15.0). Though not statistically significant, a trend in reduced fight times was observed when anglers were guided and increased air exposure times when a net was used and a picture was taken. Results of the current study suggest that anglers expose fish to air for periods that are much less than those reported to cause mortality.

Idaho↗

USGS2017

Simulated effects of YY-male stocking and manual suppression for eradicating nonnative Brook Trout populations

Eradication of nonnative Brook Trout Salvelinus fontinalis populations is difficult to achieve with standard techniques, such as electrofishing removal or piscicides; new approaches are needed. A novel concept is to stock “supermale” hatchery fish with wild conspecifics. Supermales (M YY ) have two Y-chromosomes, resulting in offspring that are all males; over time, successful supermale reproduction could eradicate the wild population. We constructed an age-structured stochastic model to investigate the effects of manually suppressing wild fish and stocking M YY fingerlings on the long-term viability of hypothetical nonnative Brook Trout populations. In streams, an annual stocking rate of supermales equivalent to 50% of wild age-0 Brook Trout density combined with an annual selective suppression rate equivalent to 50% of wild Brook Trout density resulted in a time to extirpation of only 2–4 years if supermale fitness was equivalent to wild male fitness. However, time to extirpation in streams was 5–15 years if supermale fitness was 80% lower than wild male fitness. In alpine lakes, higher supermale stocking rates and nonselective gillnetting were required to eradicate Brook Trout populations. If supermales were assumed to be as fit as wild males, however, any supermale stocking rate greater than 49% in alpine lakes or 60% in streams achieved eradication in 10 years or less, regardless of the suppression rate. Because manual suppression and the stocking of M YY fingerlings can readily be conducted at the levels assumed in our simulations, use of such an integrated pest management (IPM) approach could extirpate undesirable Brook Trout populations within reasonably short periods of time. Given the recent successful development of an M YY Brook Trout broodstock capable of producing large numbers of M YY fingerlings and given the positive results of the present simulations for both streams and alpine lakes, field testing of M YY stocking is warranted within an IPM program that includes manual suppression for eradicating undesirable Brook Trout populations.

North American Journal of Fisheries Management↗

USGS2017

Production and evaluation of YY-male Brook Trout to eradicate nonnative wild brook trout populations

Nonnative Brook Trout Salvelinus fontinalis were introduced throughout western North America in the early 1900s, resulting in widespread self-sustaining populations that are difficult to eradicate and often threaten native salmonid populations. A novel approach for their eradication involves use of YY male (MYY) Brook Trout (created in the hatchery by feminizing XY males and crossing them with normal XY males). If MYY Brook Trout survive after stocking, and reproduce successfully with wild females, in theory this could eventually drive the sex ratio of the wild population to 100% males, at which point the population would not be able to reproduce and would be eradicated. This study represents the first successful development of a FYY and MYY salmonid broodstock, which was produced in four years at relatively low cost. Field trials demonstrated that stocked hatchery MYY Brook Trout survived and produced viable MYY offspring in streams, although reproductive fitness appeared to have been lower than their wild conspecifics. Even if reduced fitness is the norm in both streams and alpine lakes, our population simulations suggest that eradication can be achieved in reasonable time periods under some MYY stocking scenarios, especially when wild Brook Trout are simultaneously suppressed in the population.

Conference Paper↗

USGS2016

Influences of summer water temperatures on the movement, distribution, and resources use of fluvial Westslope Cutthroat Trout in the South Fork Clearwater River basin

Although many Westslope Cutthroat Trout Oncorhynchus clarkii lewisi populations in Idaho are robust and stable, population densities in some systems remain below management objectives. In many of those systems, such as in the South Fork Clearwater River (SFCR) system, environmental conditions (e.g., summer temperatures) are hypothesized to limit populations of Westslope Cutthroat Trout. Radiotelemetry and snorkeling methods were used to describe seasonal movement patterns, distribution, and habitat use of Westslope Cutthroat Trout in the SFCR during the summers of 2013 and 2014. Sixty-six radio transmitters were surgically implanted into Westslope Cutthroat Trout (170–405 mm TL) from May 30–June 25, 2013, and June 20–July 6, 2014. Sedentary and mobile summer movement patterns by Westslope Cutthroat Trout were observed in the SFCR. Westslope Cutthroat Trout were generally absent from the lower SFCR. In the upper region of the SFCR, fish generally moved from the main-stem SFCR into tributaries as water temperatures increased during the summer. Fish remained in the middle region of the SFCR where water temperatures were cooler than in the upper or lower regions of the SFCR. A spatially explicit water temperature model indicated that the upper and lower regions of the SFCR exceeded thermal tolerance levels of Westslope Cutthroat Trout throughout the summer. During snorkeling, 23 Westslope Cutthroat Trout were observed in 13 sites along the SFCR and at low density (mean ± SD, 0.0003 ± 0.0001 fish/m 2 ). The distribution of fish observed during snorkeling was consistent with the distribution of radio-tagged fish in the SFCR during the summer. Anthropogenic activities (i.e., grazing, mining, road construction, and timber harvest) in the SFCR basin likely altered the natural flow dynamics and temperature regime and thereby limited stream habitat in the SFCR system for Westslope Cutthroat Trout.

Idaho↗