Geology topics

Matthew P. Corsi

Publications and source records attributed to Matthew P. Corsi.

16 recordsLinked to original sources

USGS2025

Food habits of Yellow Perch, Smallmouth Bass, and Northern Pikeminnow in Lake Cascade, Idaho

Objective Yellow Perch Perca flavescens was first encountered in Lake Cascade, Idaho, in 1957. Since its introduction, the abundance of Yellow Perch in Lake Cascade has been highly variable. Historically, declines in Yellow Perch abundance were attributed to predation by Northern Pikeminnow Ptychocheilus oregonensis . In the 1990s, Smallmouth Bass Micropterus dolomieu became established in Lake Cascade. Although Yellow Perch abundance has been high and currently supports a world-class fishery, the Idaho Department of Fish and Game has documented poor Yellow Perch recruitment in recent years, concerning regional managers. An investigation into the food habits of Yellow Perch, Smallmouth Bass, and Northern Pikeminnow was conducted to better understand the importance of juvenile Yellow Perch to predator diets. Methods In Lake Cascade, gill nets were deployed monthly from April 2022 to May 2023. Ageing structures and stomach contents were removed from Yellow Perch, Smallmouth Bass, and Northern Pikeminnow. Diet composition was quantified and then summarized by species, cohort, and season. Bioenergetics modeling, coupled with estimates of predator abundance, was used to evaluate overall consumption of Yellow Perch. Results In total, stomach contents were extracted from 1,099 Yellow Perch, 440 Smallmouth Bass, and 980 Northern Pikeminnow. Food habits varied by season and predator length, but Yellow Perch was a primary prey item of all study species. The energetic contribution of Yellow Perch to Yellow Perch varied from 10% to 80% across seasons. Smallmouth Bass consumed fewer Yellow Perch than either Yellow Perch or Northern Pikeminnow. The highest average per capita consumption of Yellow Perch (i.e., across seasons) was observed for 500–600-mm Northern Pikeminnow (five Yellow Perch/Northern Pikeminnow). Energetic contribution of Yellow Perch to Northern Pikeminnow diets was highest in spring 2023, yet Northern Pikeminnow consumed the highest average number of Yellow Perch in July. Bioenergetics modeling estimated that per-capita annual consumption of Yellow Perch was 6.0 kg for Yellow Perch, 3.4 kg for Smallmouth Bass, and 9.9 kg for Northern Pikeminnow. In total, estimated consumption of all study species was as high as 37% of the estimated total number age-0 Yellow Perch produced. Conclusions Yellow Perch was an important prey item for all three species, particularly Yellow Perch and Northern Pikeminnow. Results from this study provide important information on the predation of Yellow Perch in Lake Cascade. Further, this work will bolster a growing body of research on the food habits of and interactions between native and nonnative predators in western reservoir systems.

Idaho↗

USGS2025

Population dynamics of White Sturgeon in the upper Snake River, Idaho: Evaluation of management options for a harvest fishery

Objective Understanding how fish populations will respond to management actions is critical for making effective management decisions. This study provides important information regarding population demographics for a nonnative, hatchery-implemented population of White Sturgeon Acipenser transmontanus . We investigated the population dynamics of White Sturgeon in the upper Snake River, Idaho, and developed an age-structured population model to evaluate potential stocking and harvest scenarios (e.g., length limits and annual quotas). Methods White Sturgeon were sampled from June to October 2022 and from June to August 2023 using angling (i.e., rod and reel) and setlines from a 260-km-long section of the Snake River. Capture histories from 261 known-age White Sturgeon informed age and growth analysis and an evaluation of movement trends. A closed-population capture–recapture model and an estimate of setline-specific catchability were used to estimate the total abundance of White Sturgeon in the upper Snake River. Apparent survival for the population was estimated using a Cormack–Jolly–Seber model. Finally, a population model was parameterized using information on the population dynamics of White Sturgeon in the upper Snake River. The model was used to estimate the effects of varying stocking rates and harvest scenarios (i.e., harvest slot of 76–122 cm fork length [FL] and annual quotas of 0–25 White Sturgeon harvested) on the population. Results In total, 340 individual White Sturgeon were captured throughout the study area, with 181 recapture events. Individuals varied in FL from 54 to 205 cm, and the mean relative weight for captures was 105.2 (SD = 14.4), suggesting relatively high body condition. Age varied from 2 to 25 years, and White Sturgeon moved an average of 8.1 km (SD = 23.5) downstream from stocking locations. Estimated abundance of White Sturgeon in the tailwaters of American Falls Dam was 428 fish (95% CI = 403–463). That abundance estimate was used to inform a total abundance estimate of 887 White Sturgeon (95% CI = 835–960) in the study area. Apparent annual survival was 0.79 (95% CI = 0.64–0.89). A stocking rate of 285 age-2 White Sturgeon/year was necessary to maintain current abundance. For every five fish harvested (harvest slot = 76–122 cm FL) per year, estimated abundance decreased by about 2.2% over 20 years. Conclusions Our research identified fast growth of White Sturgeon relative to other populations and relatively high mortality for a White Sturgeon population without exploitation. Also, like other studies evaluating harvest, a population model was used to illustrate the effect of varying rate functions on a fishery. The age-structured population model suggested that a harvest fishery is possible while still meeting management goals for the upper Snake River White Sturgeon fishery.

Idaho↗

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↗

USGS2025

Points of consensus on catch-and-release: Considerations for science, ethics, and fisheries management

Catch-and-release (C&R), whether via regulations or voluntary actions, is typically employed with the intent of reducing fishing mortality while maintaining recreational angling opportunities (Isermann & Paukert, 2010), but there has been significant discourse about the relative importance of individual-level (see Cooke et al., 2025) vs. population-level (see Corsi et al., 2025) effects of C&R. We hope to offer the angling, scientific, and management communities some points of consensus from which this multifaceted dialogue on C&R can productively build. We preface this by noting that our collective views were informed by this process and the opportunity to share and learn from each other. What is apparent from this exercise is the value of embracing a holistic, cooperative approach. From a regulatory perspective, C&R is typically implemented in high-effort fisheries that are popular due to the quality of the fishing. Catch-and-release regulations are also applied in fisheries where populations have declined and fishing mortality is thought or, most appropriately, known to be a limiting factor for the population. When practiced voluntarily, anglers engaging in C&R can be characterized by high avidity and specialization, and angling is often a central component of their identity (e.g., Fisher, 1997). Due to their passion, avid anglers tend to spotlight the ethical, scientific, and regulatory considerations of C&R. As such, there is a need in fisheries management and science to understand in what situations these considerations transcend biological scales and how to navigate the gamut of voluntary actions and regulatory mandates. In this essay, we offer a distillation of the emergent consensus concepts that we hope managers and researchers will consider at both individual and population scales for legal, ethical, and scientific deliberations.

Fisheries↗

USGS2025

Comparison of lapilli otoliths and pectoral fin rays for estimating age of Northern Pikeminnows

The Northern Pikeminnow Ptychocheilus oregonensis is a piscivorous cyprinid native to western North America. Information on the best structure for estimating the age of Northern Pikeminnows is a key knowledge gap that may limit inquiries on management efforts. Thus, the objective of this study was to evaluate between-reader precision and concordance between age estimates for lapilli otoliths and pectoral fin rays from Northern Pikeminnows. Age estimates from lapilli otoliths were compared to those from pectoral fin rays of 150 Northern Pikeminnows captured from Lake Cascade, Idaho, in April–May 2022. Exact percent agreement of estimated ages between the readers was higher for fin rays (75.3%) than otoliths (50.0%), with a mean coefficient of variation of 3.5 and 8.7, respectively. Readers also assigned a confidence rating (0–3; higher value reflects higher confidence in age estimate) to each structure. Confidence ratings were higher for fin ray age estimates (mean ± SD; 1.6 ± 0.6) than otolith estimates (1.1 ± 0.7) between readers. A consensus age was estimated for each structure and fish. Agreement between consensus age estimates for otoliths and fin rays was 26.7% with a coefficient of variation of 14.0. Our findings suggest that fin rays were easier to collect, process, and read than otoliths, and resulted in more precise age estimates than otoliths. Results from our study provide guidance on the best structures for estimating the age of Northern Pikeminnows that can be used to inform management efforts.

Idaho↗

USGS2024

Food habits of nonnative Walleyes in Lake Pend Oreille, Idaho

Objective Walleye Sander vitreus is an important species that has been widely introduced outside of its native distribution. The goal of this study was to assess the effects of an established Walleye population in the Lake Pend Oreille (LPO) system, Idaho. Methods Food habits of Walleyes were described using stomach contents and stable isotopes (δ 15 N, δ 13 C). Trophic structure of the LPO system's food web was identified using stable isotopes. Annual consumption by Walleyes of important prey items was estimated using a bioenergetics model. Result Walleyes consumed a diversity of prey items, including macroinvertebrates and fishes. Kokanee Oncorhynchus nerka , the most frequently consumed prey item, occurred in 23% of all Walleye diets. Combined, native cyprinids and catostomids occurred in 31% of all Walleye stomachs. Select taxa (e.g., native cyprinids, kokanee) were consistently consumed by Walleyes across seasons, regions, and cohorts, whereas other taxa (e.g., Westslope Cutthroat Trout O. lewisi , Smallmouth Bass Micropterus dolomieu ) were consumed inconsistently. Stable isotope analysis suggested that Walleyes occupied similar trophic positions as other top‐level piscivores in the system . As Walleye age increased, δ 15 N increased and δ 13 C decreased, indicating increased consumption of pelagic prey resources and prey at higher trophic positions. The estimated biomass of kokanee consumed annually by Walleyes was 27,121 kg (95% confidence interval = 9178–61,603). Comparatively, native cyprinids represented about 46% of the total biomass of kokanee consumed by Walleyes, whereas native catostomids represented about 11% and native salmonids represented about 15% of the total biomass of kokanee consumed by Walleyes. Conclusion This study revealed that Walleyes consumed various fishes across the LPO system. Although kokanee were the most frequently consumed prey item, native cyprinids and catostomids (combined) occurred at similar proportions. This study contributes to our growing knowledge of the effects of nonnative Walleyes on important salmonids and native fishes in western systems.

Idaho↗

USGS2024

Abiotic and biotic factors related to growth of non-native Walleyes in Lake Pend Oreille, Idaho

Objective Growth is one of the primary drivers of fish population dynamics and understanding factors influencing growth is vital to effective management of fish populations. This study investigated potential factors influencing growth of a recently established, non-native population of Walleye Sander vitreus in the Lake Pend Oreille system in northern Idaho. Methods We used relative growth index to describe growth of Walleyes relative to populations across North America. Mixed‐effects modeling was used to relate growth to abiotic (i.e., mean summer water temperature, river inflow) and biotic (i.e., kokanee Oncorhynchus nerka abundance and biomass; opossum shrimp Mysis diluviana density) variables. Models were ranked using Akaike's information criterion corrected for small sample size. Individual variability in growth was related to diet represented by stable isotopes (i.e., δ 15 N, δ 13 C) using linear regression for age‐1, age‐2, age‐3, and age‐5 individuals. Subsequently, for each age‐class, we evaluated differences in δ 15 N and δ 13 C between fast‐growing (i.e., 75th and higher percentiles of growth) and slow‐growing (i.e., 25th and lower percentiles of growth) individuals. Results The relative growth index suggested that Walleye grew fast relative to other populations, particularly those at similar latitudes to the Lake Pend Oreille system. Mixed-effects regression modeling indicated that growth of Walleyes was positively associated with temperature as well as abundance and biomass of kokanee; growth was negatively associated with inflow from the Clark Fork River and Mysis diluviana density. The top model explaining growth of Walleyes contained temperature and abundance of kokanee as environmental variables. The second equally plausible (i.e., within 2 AICc) model contained temperature. Growth of Walleyes varied among individuals. Generally, fast-growing Walleyes had higher δ15N than slow-growing Walleyes. Similarly, δ13C was more depleted in the fast-growing individuals for all age classes, except age 1, suggesting that age-1 individuals used higher proportions of littoral prey items compared to other age classes. Conclusion This study showed that kokanee abundance and temperature appeared to be important factors influencing growth of Walleyes in the Lake Pend Oreille system. Additionally, variability in growth appeared to be related to variability in diet, particularly for age-1 Walleyes. Impact statement Growth of Walleyes has been extensively studied, yet few studies have evaluated growth of Walleyes in novel systems or assessed individual variability in growth. Our research adds to the understanding of individual variability in growth and factors influencing population dynamics of non-native Walleyes.

Idaho↗

USGS2020

Ontogenetic diet shifts with potential ramifications for resource competition in a kokanee – Mysis diluviana system

Ontogenetic shifts represent important transitions that can influence how fish interact with their environment. However, ontogenetic shifts are rarely placed into a population context due to the difficulty of incorporating the vagaries of size-mediated interactions. As such, we evaluated the role of ontogenetic shifts in diet as they relate to potential competitive interactions between kokanee Oncorhynchus nerka and Opossum Shrimp Mysis diluviana (hereafter Mysis) in Lake Pend Oreille, Idaho. Contemporary data were used to understand diet patterns of Mysis and kokanee. Historical data were evaluated within the context of ontogenetic shifts to better understand the long-term, population-level ramifications of interactions between Mysis and kokanee. Diet analysis revealed age-specific divergences in diet whereby juvenile kokanee primarily consumed copepods and adult kokanee preferentially consumed cladocerans. When placed in a historical context, age-specific patterns in kokanee diet likely led to increases in adult growth following declines in Mysis abundance. Improved fitness of adult fish likely resulted in record high abundances of kokanee in Lake Pend Oreille thereby shifting the balance from inter- to intraspecific competition.

Idaho↗

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

Distribution and movement of steelhead and anglers in the Clearwater River, Idaho

Steelhead Oncorhynchus mykiss is a species that is of high economic value that supports popular sport fisheries across the Pacific Northwest. The Clearwater River in Idaho provides a trophy steelhead fishery, and it is home to both wild- and hatchery-origin steelhead. To manage the fishery effectively, information is needed about the spatial and temporal overlap of wild and hatchery steelhead in the Clearwater River, as well as the activity of anglers. We conducted a radiotelemetry study to describe the distribution of steelhead and their final fate in the Clearwater River, and creel surveys were used to describe the distribution of anglers. In total, 289 wild (Potlatch River and Lochsa River) and hatchery (from Dworshak National Fish Hatchery and South Fork Clearwater River) steelhead were radio-tagged at Lower Granite Dam, 51 river kilometers (rkm) downstream from the mouth of the Clearwater River. Fish were monitored upon their entry into the Clearwater River by using mobile tracking surveys (boat and vehicle) and stationary antennas. The majority of wild and hatchery steelhead arrived in the Clearwater River in the fall with the exception of those from the Lochsa River, which arrived in the fall and following spring. Average daily movement of the fish was minimal (range = 0.3–4.7 km/d) and dependent on water temperature and flow. The fates of wild and hatchery steelhead varied. Fish returned to spawning grounds, were harvested by anglers (hatchery fish only), or had unknown fates. Both wild and hatchery steelhead returned at high rates to their natal tributaries and release locations. No straying was observed in either group; however, occasions when steelhead have overshot their natal tributaries and release locations were documented. Spatial and temporal overlap of the distributions of wild and hatchery steelhead was minimal. The distribution of anglers overlapped with that of hatchery steelhead in the fall, winter, and spring. The distributional overlap of anglers and wild steelhead was minimal and largely occurred in September in the lower Clearwater River. This suggests that the Clearwater River has a highly compartmentalized fishery and that current fishing regulations in the Clearwater River are providing for a diversity of angling opportunities while conserving wild steelhead and offering harvest of hatchery fish. The results from this study have important implications for the conservation and management of wild and hatchery steelhead.

Idaho↗

USGS2019

Growth disparity in sympatric kokanee breeding groups

Growth is arguably the most important dynamic rate function due to its interaction with survival and recruitment. As such, understanding the mechanisms underlying growth is a primary focus of fisheries research. Kokanee Oncorhynchus nerka in Lake Pend Oreille, Idaho, provide an interesting case study for investigating the factors that influence growth. Early‐run and late‐run kokanee occur in Lake Pend Oreille, but early‐run fish generally grow faster than late‐run fish. The observed growth disparity between early‐ and late‐run fish could be due to genetic differences between the two groups. Conversely, a common hatchery practice of slowing growth by reducing feed has been hypothesized to elicit a compensatory growth response in early‐run fish and to explain the size difference between breeding groups. Using two different experiments, we tested the hypotheses that (1) early‐run kokanee are genetically disposed to grow faster than late‐run kokanee at identical water temperatures; and (2) feed restriction elicits a compensatory growth response in early‐run kokanee that explains the observed size difference between breeding groups. Estimates of mean FL, weight, Fulton's condition factor ( K ), and specific growth rate (SGR) were not significantly different ( P ≥ 0.05) between early‐run and late‐run fish in the first experiment. However, water temperature was positively related to mean FL, weight, K , and SGR for both breeding groups. Fish that were subjected to food deprivation exhibited an increased growth rate and obtained weights similar to those of control fish. Overall, our results suggest that early‐ and late‐run fish have similar growth potential, but certain hatchery practices likely provide early‐run fish with an initial advantage in growth, size, or both.

Idaho↗

USGS2019

Size selectivity of sampling gears used to sample Kokanee

Kokanee Oncorhynchus nerka provide valued recreational fisheries and also serve as a prey resource for economically, socially, and ecologically important fishes. As such, management of kokanee is a major focus of natural resource agencies. Kokanee are typically monitored using midwater trawls, but the interpretation of data collected using midwater trawls is difficult due to the unknown size selectivity of the gear. We sought to assess the length selectivity of midwater trawls by comparing estimates obtained from midwater trawls with estimates obtained from gill nets adjusted for size selectivity. Experimental curtain gill nets and midwater trawls were used in conjunction to sample kokanee in seven lentic systems in Idaho. The size selectivity of gill nets was estimated by accounting for the probability of encounter and the probability of retention. Estimates of size selectivity were then used to adjust the length distribution of fish sampled in gill nets. The adjusted length distribution of fish sampled in gill nets was compared with estimates obtained from midwater trawls to identify potential size selectivity of midwater trawls. A pattern of size selectivity was apparent for both sampling techniques. The average length of kokanee sampled with midwater trawls was 111 mm; whereas, kokanee sampled with gill nets had a mean length of 235 mm. Our results suggest experimental gill nets are useful for common sampling of kokanee (e.g., trend monitoring) because the gear is less size selective than midwater trawls and is adjustable for size selectivity. However, midwater trawls are likely the best gear for addressing questions associated with early life history. Overall, our results provide a better understanding of gill-net and midwater trawl selectivity and ultimately improve the ability to sample and manage the species.

North American Journal of Fisheries Management↗

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↗

USGS2019

Long-term suppression of the Lake Trout (Salvelinus namaycush) population in Lake Pend Oreille, Idaho

A simulation model of lake trout Salvelinus namaycush (Walbaum 1792) population dynamics in Lake Pend Oreille, Idaho, was used to estimate (1) the optimal allocation of effort among gillnet mesh sizes that minimizes abundance in the shortest time; (2) the number of years needed to suppress the population to 90% of peak abundance; and (3) once suppressed, how much effort could be reduced to sustain abundance indefinitely. A density-dependent stochastic simulation model was parameterized from data in 2006–2016, including parameter uncertainty and implementation error. Time to suppression could be reduced by using more large-mesh gillnet than was used during 2007–2016. Continued fishing at the peak level of total gillnetting effort, but using an optimal effort allocation among meshes, would suppress abundance to the target level within 7–13 years. Once suppressed, gillnet effort could be reduced 76–86% (157,000 m, 95% CI 116,000–199,000 m) to sustain abundance at the target level. Our findings suggest that time to suppression of lake trout populations in other systems may be able to be reduced by optimizing gillnet effort allocation among mesh sizes, and that total effort can be greatly reduced to sustain abundance at the reduced level thereafter.

Idaho↗

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↗