Native fishes face imperilment due to habitat loss and fragmentation, megadrought, invasive species, and synergies amongst threats. These threats coalesce at Glen Canyon Dam (GCD), which impounds the Colorado River to create Lake Powell, a reservoir inhabited by a suite of nonnative fishes. Water level in Lake Powell has declined precipitously due to water overallocation and megadrought, increasing the risk of nonnative fish entrainment through the dam, as the penstocks draw water from the epilimnion. Our goal was to characterize the distribution and abundance of nonnative fishes in the vicinity of GCD and identify environmental conditions that may increase entrainment. We used active and passive sampling, hydroacoustics, and acoustic telemetry to assess the distribution and abundance of fishes. When the reservoir was stratified fish remained in the epilimnion, were present near the GCD penstocks, and thus at high risk of entrainment. When the reservoir was mixed, fish generally occupied shallower, warmer areas further from GCD, but this varied among species. The distribution of nonnative fish and accompanying risk of entrainment varied by season and epilimnion depth. Retaining more water in Lake Powell to draw water from below the thermocline would reduce the risk of nonnative fish entrainment into Grand Canyon.

Translocation is a common practice for wildlife conservation and restoration ecology, aimed at mitigating the impacts of anthropogenic pressures on native populations. Assessing animal behaviour prerelease could identify individuals that may improve translocation success. We examined whether behavioural assays correlate with dispersal distance of translocated American beavers, Castor canadensis, in a section of a degraded and simplified desert river in east-central Utah, U.S.A. We conducted behavioural assays on nuisance beavers captured for translocation. We tested their reactivity, exploration and boldness using captivity, novel object and release tests, respectively, while they were in captivity for quarantine, and then used VHF and passive integrated transponder (PIT) tags to measure dispersal distance for up to 30 days after release. Out of the 40 beavers included in our analysis, five remained alive and in our study site, five were alive but outside of the study site and seven died by predation. Others had unknown fate, primarily due to the loss of VHF tags. We found no significant relationships between reactivity or boldness and maximum dispersal distance, and too few beavers completed the novel object tests for statistical analysis. Our findings indicate that while these behavioural assays have shown promise in predicting the outcome of translocations in other species, some assays, such as captivity tests, are not appropriate for beavers and more research is needed to determine which assays would be useful in selecting individuals that best meet the varying priorities of beaver translocation programmes. (c) 2026 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

Water development and the proliferation of invasive riparian vegetation have led to widespread habitat loss and simplification of rivers in the western United States, contributing to the imperilment of native fishes. Here, we quantify channel narrowing and vegetation encroachment, which are conspicuous indicators of riverine habitat alteration, along ∼400 km of three dryland tributaries of the upper Colorado River. We conducted a comparative analysis of aerial photographs between the 1930s and 2010s/2020s time periods using visual interpretation and used Light Detection and Ranging (LiDAR) data along with Object-Based Image Analysis (OBIA) to quantify canopy cover of woody riparian species. All three rivers underwent substantial channel narrowing, coinciding with a general decrease in spring floods over time. However, the extent of narrowing varied among the rivers (78 %, 73 %, and 29 %) with greater narrowing corresponding to larger reductions in spring flows. In contrast, contemporary woody cover was similarly high among all three rivers (39 %, 41 %, and 36 %), and a woody vegetation analysis we conducted for one river indicated a substantial increase in vegetation along the active channel (4 %-74 %). These findings underscore a common pattern observed in rivers throughout the basin, where river channels often undergo narrowing and encroachment by invasive vegetation following dam construction and/or decreases in flows, ultimately leading to habitat simplification, with negative implications for native fishes and other riparian biota. Our findings also emphasize that, even in the presence of nonnative vegetation establishment, preserving or restoring large magnitude and long duration floods can help conserve diverse habitat in dryland rivers.

Fish culture can be employed to study impacts of environmental stressors on early life success of fish. A lack of documented culture techniques for native fish of the family Leuciscidae therefore limits study on the impacts of stressors on these fish. Even when culture techniques have been developed, they are designed for production and not necessarily suited for experimentation. We developed a culture technique suitable for experimentation for an understudied native leuciscid, Tui Chub Siphateles bicolor, and used the technique to investigate the impacts of a common environmental stressor, total dissolved solids (TDS), on early life stages. To fertilize eggs, we strip-spawned mature Tui Chub harvested from Pyramid Lake, Nevada. We incubated embryos in small plastic storage containers at 5 ecologically relevant TDS concentrations using only sponge filters for biofiltration. We controlled for all other incubation conditions as best as possible. We only observed larvae in the 2 lowest concentrations tested (130 and 5888 mg/L), with no hatch observed at TDS concentrations higher than current conditions in Pyramid Lake (>= 9044 mg/L). We observed elevated total ammonia (>0 ppm) in most of our aquaria due to embryo mortality, which confounded our ability to attribute differences in hatch to TDS alone. Larvae in the highest TDS concentration where hatch was observed (the current TDS concentration of Pyramid Lake) did not survive as long as larvae in the lowest concentration (fresh water from the Truckee River, Nevada). The differences in larval success across treatments imply that Tui Chub early life stages are already negatively impacted by TDS concentrations in Pyramid Lake. We present a low-cost, small-scale culture technique suitable to evaluate the effects of environmental stressors on early life stages, or to investigate early life requirements, of Tui Chub and related species.

Objective Lake elevation decline is a global phenomenon with pronounced effects in arid regions that changes the characteristics of nearshore habitat area available to lacustrine spawners, potentially impacting recruitment and whole-lake food web dynamics. Our objective was to understand the potential effects of lake elevation decline on spawning habitat for the Tui Chub Siphateles bicolor, a lacustrine spawner and critical component of the native food web in Pyramid Lake, Nevada.Methods We explored the distribution of ripe Tui Chub in nearshore habitat by associating habitat characteristics to ripe Tui Chub CPUE from a custom gill-net configuration, with data analyzed using generalized linear mixed-effects models. We then explored potential spawning habitat availability at all potential lake elevations using an elevation-explicit model of the basin that we developed based on several bathymetric and geospatial data sets and the knowledge of spawner distribution gained in the first component of the study.Results Ripe Tui Chub catch was primarily predicted by temperature, reaching a maximum between 14.2 degrees C and 24.8 degrees C found at less than 15 m of depth in Pyramid Lake throughout the summer spawning period. We estimated that with a contemporary decline in lake elevation of 8 m, Pyramid Lake will host the minimum area of spawning habitat based on morphometry alone at a 40% decrease from a theoretical maximum.Conclusions A decrease in lake elevation or an increase in lake temperatures-both of which are probable events based on future climate scenarios and estimates of water extraction upstream of Pyramid Lake-is likely to further restrict Tui Chub spawning habitat area. Our results have important implications for ecological water demand in Pyramid Lake and provide managers with information facilitating a science-based approach to managing the fish community. Water management and climate influence the surface elevation of lakes. We found that the quality and area of nearshore spawning habitat for a keystone fish, the Tui Chub, are impacted by water level fluctuations and warming trends.

Fisheries management has historically focused conservation efforts on game or sport species. However, most species are nongame-those not traditionally captured for sport or harvest in countries where recreational fisheries predominate. Greater conservation of nongame species could help ensure that population declines do not go unnoticed. Unfortunately, fisheries managers already manage complex ecosystems with limited resources, and they frequently are directed to focus on game fishes. However, game fish populations can also be tightly coupled to nongame fishes, so nongame management can sometimes also benefit game species. We reviewed functional roles of freshwater nongame fishes and suggest categories that may be especially important for conservation. Of note, nongame fishes are more imperiled than game fishes and fill largely distinct functional roles. These roles include food-web impacts, ecosystem engineering, and mussel hosting. Management priorities could include nongame piscivores and species with high biomass, especially herbivores, nest builders, and imperiled mussel hosts. We provide practical options for including nongame fishes in current management, many of which require little additional funding. These include recognizing when sport fish funding and conservation can also benefit nongame species, whole-community sampling at some monitoring locations, collecting catch data for select species observed during game fish surveys, embracing environmental DNA sampling, and making presence-absence record keeping the default option.

Due to rapid climate change, arctic ecosystems are experiencing an increase in disturbances including localized land-surface failures caused by melting ground ice (thermokarst failures). These failures result in the mass transport of sediment and organic materials into surface waters, with the potential to dramatically alter aquatic ecosystem function and biotic interactions. We coupled direct comparisons and long-term data of a suite of abiotic and biotic variables in a thermokarst-impacted lake and nearby reference lake to assess the impacts of thermokarst failure. After the thermokarst failure and relative to long-term averages, water transparency was substantially reduced. We hypothesized there would be subsequent changes to lower trophic levels and profound declines in fish foraging efficiency. However, these characteristics were within the range of natural variability and/or rapidly recovered to values within the range of long-term variability. In addition, although there was limited evidence of taxa-specific changes, we did not observe any strong changes in the total relative densities, growth rates, or composition of the bacterioplankton and zooplankton communities, benthic macroinvertebrates, or changes in fish diet, that could be attributed statistically to the thermokarst event. In sum, the thermokarst disturbance had substantial effects on water transparency and some lower trophic levels, which surprisingly were not manifested in higher trophic levels. Overall, the lake ecosystem appeared resistant to thermokarst disturbance with rapid recovery within two years after the disturbance.

Stream restoration programs employ beaver‐related restoration techniques, including beaver translocations and installation of beaver dam analogs (BDA), to create complex in‐stream habitat. We investigated whether BDA installations improved the probability of translocated beavers surviving and colonizing a section of a degraded desert river. We translocated beavers fitted with tracking devices to the Price River, Utah, United States, for 2 years before and after BDAs were installed. We monitored survival and site fidelity of beavers to estimate apparent survival ( φ ), using model selection to evaluate models with BDA, flow, and other factors hypothesized to relate to apparent survival. We found similar apparent survival 8 weeks post‐release of pre‐BDA ( φ = 0.50 ± 0.08 SE) and post‐BDA beavers ( φ = 0.41 ± 0.06 SE). There were 15 predator‐caused mortalities and 39 beavers emigrated out of the study site. Top models indicated apparent survival was negatively related to mean flow. Of the 70 BDAs constructed, beaver activity was detected on only two structures and the number of intact natural dams decreased due to monsoon floods. Our results suggest BDAs may not improve survival and site fidelity of translocated beavers in desert river systems. Instead, the dynamic flow of desert rivers and negative relationship between flow and apparent survival suggest the timing of release may be an important consideration for successful beaver translocation. Additional research is needed to understand how habitat, food availability, individual behavior, and resident conspecifics influence beaver translocation success.

Widespread hydrologic alterations have simplified in-stream habitats in rivers globally, driving population declines and local extirpations of many native fishes. Here, we examine how rapid geomorphic change in a historically degraded desert river has influenced habitat diversification and ecosystem persistence. In 2010, a large reach of the degraded and simplified lower San Rafael River (SRR), Utah, was impacted by the formation of a valley plug and began to shift from a homogenous, single-thread channel to a complex, multi-threaded riverscape. We combined field measurements and drone-collected imagery to document habitat changes due to the valley plug. Our results demonstrate that in 2021, the valley plug reach was more diverse than any other stream reach along the SRR, containing 641% more diverse habitat (e.g., pools, riffles, backwaters) than what was measured in 2015. The plug reach also retained water for periods beyond what was expected during seasonal drying, with the total extent of inundation within the riverscape increasing by over 2,800%. Since the formation of the valley plug, riparian habitat has increased by 230% and channel networks have expanded to more than 50 distinct channels throughout the zone of influence. Our results provide evidence of successful self-restoration in a formerly highly degraded reach of desert river, and encourage new methods of desert river restoration. We aim to inform the use of large-scale, disruptive restoration actions like intentional channel occlusions, with the goal of mitigating the impacts of simplification and increasing habitat persistence in the face of exacerbated aridity in the desert Southwest.

Submerged macrophytes have complex effects on spatiotemporal characteristics of river ecosystems, including trout habitat. We investigated the impact of submerged macrophyte coverage on trout habitat in the Henrys Fork of the Snake River, Idaho, USA. We hypothesized that higher submerged macrophyte coverage would create new habitat types beneficial for trout growth. We assessed river physical and biotic attributes, trout habitat preferences, and estimated trout growth potential with bioenergetics models across a gradient of submerged macrophyte coverage (32–94

Abstract Background Unfettered movement among habitats is crucial for fish to access patchily distributed resources and complete their life cycle, but many riverscapes in the American Southwest are fragmented by dams and dewatering. The endangered Rio Grande silvery minnow (Hybognathus amarus, RGSM) persists in a fragmented remnant of its former range (ca. 5%), and its movement ecology is understudied. Methods We tracked movements of hatchery-reared RGSM, tagged with passive integrated transponder tags, using stationary and mobile antennas from 2019 to 2022. We quantified probability of movement and total distance moved by RGSM released above and below a dam. We then assessed how well two prevailing riverine movement theories (i.e., restricted movement paradigm [RMP] and colonization cycle hypothesis [CCH]) explained RGSM movement patterns. Results We detected 36.8% of released RGSM (n = 37,215) making at least one movement. Movements were leptokurtic and substantially greater than expected based on the RMP for both stationary (1.7–5.9 m) and mobile (30.3–77.8 m) individuals. On average, RGSM were detected at large for 75 days and moved a total of 12.2 rkm within a year. The maximum total distance moved by RGSM was 103 rkm. Similarly, we observed a multimodal distribution of detected range sizes with a mean detected range of 2.4 rkm and a maximum detected range of 78.2 rkm. We found little support for an upstream movement bias, as expected under the CCH, and most movements (74%) were directed downstream. Conclusions Our data suggest RGSM are highly mobile, with the ability to make long-distance movements. Neither movement theory adequately described movement patterns of RGSM; instead, our findings support a nomadic movement pattern and an apparent drift paradox matching recent studies of other pelagic-broadcast spawning minnows where populations persist upstream despite experiencing downstream drift as larvae. Resolution of the drift paradox may be achieved through further, targeted studies into different aspects of the species’ life history. Quantification of RGSM movement provides crucial insights into the species’ movement ecology and may help define the appropriate scale of recovery efforts.

Flow alteration and riparian vegetation encroachment are causing habitat simplification with severe consequences for native fishes. To assess the effectiveness of enhancing simplified habitat in a large dryland river, we experimentally added invasive wood at 19 paired treatment and reference (no wood added) subreaches (50–100 m) within the main channel of the San Juan River. Using a before‐after‐control‐impact design, we sampled fishes and macroinvertebrates, and quantified habitat complexity. After wood addition, total native fish densities were 2.2× higher in treatments compared with references, whereas total nonnative fish densities exhibited no response. Macroinvertebrate densities were 6.8× higher, and habitat complexity increased in treatments. Counts of geomorphic features in treatments increased from 1 to a maximum of 11 following wood addition, while the number of features in references remained unchanged. Wood addition has potential to instigate natural riverine processes, ultimately enhancing native fish habitat by increasing macroinvertebrate densities and habitat complexity in dryland rivers. Water overallocation and increasing aridity will continue to challenge efforts to improve habitat conditions with environmental flows alone, and managers might consider integrating non‐flow alternatives like addition of abundant, invasive wood to reduce habitat simplification.

Understanding the processes governing population dynamics is important for effective conservation and environmental management. Disentangling the relative role of density-dependent versus density-independent processes on population dynamics is often made difficult by the inability to control for abiotic or biotic factors, but long-term datasets are invaluable in this pursuit. We used a 14-year dataset from the Logan River, Utah, to assess long-term trends in abundance and evidence of density-dependent and density-independent effects on population dynamics of Paiute sculpin (Cottus beldingii) across six sites. Additionally, we evaluated the feeding ecology of sculpin over 4 years. Sculpin densities generally increased from upstream to downstream, and the annual per capita rate of increase was negatively and significantly correlated with sculpin density at four of six sites. We observed a negative relationship between total gut content and sculpin density but did not observe a negative relationship between relative condition and density. Sculpin displayed a generalist feeding strategy, and interannual differences in diet composition appeared to be influenced by interannual differences in flow, particularly years with higher magnitude flow. The observed spatial patterns in sculpin abundance throughout the watershed matched those of invasive brown trout (Salmo trutta), the top piscivore in the Logan River, and likely represent affinities for the suite of ecological conditions associated with downstream sections of the Logan River. Our results suggest that sculpin populations are regulated largely by density-dependent processes and match those from other studies on sculpin population dynamics including a range of species and habitats that differ vastly in abiotic conditions.

Declining body size is believed to be a universal response to climate warming and has been documented in numerous studies of marine and anadromous fishes. The Salmonidae are a family of coldwater fishes considered to be among the most sensitive species to climate warming; however, whether the shrinking body size response holds true for freshwater salmonids has yet to be examined at a broad spatial scale. We compiled observations of individual fish lengths from long-term surveys across the Northern Hemisphere for 12 species of freshwater salmonids and used linear mixed models to test for spatial and temporal trends in body size (fish length) spanning recent decades. Contrary to expectations, we found a significant increase in length overall but with high variability in trends among populations and species. More than two-thirds of the populations we examined increased in length over time. Secondary regressions revealed larger-bodied populations are experiencing greater increases in length than smaller-bodied populations. Mean water temperature was weakly predictive of changes in body length but overall minimal influences of environmental variables suggest that it is difficult to predict an organism's response to changing temperatures by solely looking at climatic factors. Our results suggest that declining body size is not universal, and the response of fishes to climate change may be largely influenced by local factors. It is important to know that we cannot assume the effects of climate change are predictable and negative at a large spatial scale.

Wildlife translocation facilitates conservation efforts, including recovering imperiled species, reducing human-wildlife conflict, and restoring degraded ecosystems. Beaver (American, Castor canadensis; Eurasian, C. fiber) translocation may mitigate human-wildlife conflict and facilitate ecosystem restoration. However, few projects measure outcomes of translocations by monitoring beaver postrelease, and translocation to desert streams is relatively rare. We captured, tagged, and monitored 47 American beavers (hereafter, beavers) which we then translocated to two desert rivers in Utah, USA, to assist in passive river restoration. We compared translocated beaver site fidelity, survival, and dam-building behavior to 24 resident beavers. We observed high apparent survival (i.e., survived and stayed in the study site) for eight weeks postrelease of resident adult beavers (0.88 +/- 0.08; standard error) and lower but similar apparent survival rates between resident subadult (0.15 +/- 0.15), translocated adult (0.26 +/- 0.12), and translocated subadult beavers (0.09 +/- 0.08). Neither the pre- nor the post-translocation count of river reaches with beaver dams were predicted well by the Beaver Restoration Assessment Tool, which estimates maximum beaver dam capacity by river reach, suggesting beaver-related restoration is not maximized in these rivers. Translocated beavers exhibited similar characteristics as resident subadult beavers during dispersal; they were more vulnerable to predation and many emigrated from the study sites. High mortality and low site fidelity should be anticipated when translocating beavers, but even so, translocation may have contributed to additional beaver dams in the restoration sites, which is the common goal of beaver-assisted river restoration. Multiple releases at targeted restoration sites may eventually result in establishment and meet conservation objectives for desert rivers.

Although submerged macrophyte (hereafter, “macrophyte”) communities are globally prevalent in low-gradient rivers, the net reach-scale effect of macrophytes on drift-feeding fish microhabitat preference is poorly understood. We used snorkeling and bioenergetics to study fish habitat selection for rainbow trout ( Oncorhynchus mykiss) in the Henrys Fork, ID, USA, investigating microhabitat preference across a reach-scale gradient of macrophyte growth. Fish preferred microhabitats with deep water, low velocity, and low macrophyte coverage. Preferences for microhabitats with higher net rate of energy intake (NREI) were modulated by reach-scale macrophyte coverage, higher coverage increasing preferences for higher NREI. Macrophyte coverage was a weak positive predictor for depth and NREI, and a weak negative predictor for water velocity and median substrate. Our results suggest trade-offs between fish predation risk and bioenergetic food intake, with macrophytes modulating these trade-offs across scales by affecting reach-scale geomorphology, bioenergetics, and predation risk. As such, this study highlights the important and dynamic role that macrophytes can play in fish population dynamics in rivers, with important implications for management decisions.

Greater drought frequency and severity due to climate change will result in greater drawdown of water storage reservoirs. However, changes to oxythermal regimes due to drawdown are reservoir specific and interface with fish species-specific habitat requirements, producing varying effects on coldwater fish populations. We examined the effect of drawdown on the oxythermal habitat and relative abundance of kokanee Oncorhynchus nerka, a coldwater salmonid, in Island Park Reservoir on the Henrys Fork of the Snake River, Idaho. A measure of relative kokanee abundance was negatively, exponentially related to drawdown. Oxythermal patterns measured in the reservoir during 2021, a severe drought year, revealed that drawdown reduced kokanee habitat by increasing water temperatures and decreasing dissolved oxygen concentrations. Oxythermal refugia for kokanee appeared to relate to inflow from the spring-fed Henrys Fork and other groundwater inflows. However, we did not quantify groundwater flow or connections, and we did not study kokanee population demographics or mortality. Reducing these sources of uncertainty is a priority for future study. Still, our study highlights a potential mechanism connecting reservoir drawdown to fish populations and the unique yet predictable mechanisms by which reservoir drawdown interacts with reservoir morphometry to affect fish habitat availability.

Water development has threatened the ecological integrity of riverine ecosystems. Increasing water demand, persistent drought, and climate change exacerbate the effects of habitat degradation and loss in altered systems such as the Colorado River basin. Today, biologists are challenged to identify management actions that benefit native fishes while not hindering water development or management. Herein, we discuss the importance of the natural flow regime for functioning riverine ecosystems and provide examples from four tributaries to the Green River, a major headwater branch of the Colorado River. These tributaries represent a gradient of impacts ranging from water abstraction to the point of complete seasonal desiccation to a relatively natural flow regime, and consequently have maintained different levels of instream habitat complexity and native fish persistence. Despite decades of management, endangered species lack self-sustaining populations and other native species have been extirpated from over half their ranges, which begs the question: can water development and fish conservation be balanced under current water laws and climate change-driven declines in runoff? Given the continued decline in freshwater biodiversity and abundance occurring across the globe, we contend that immediate designation of rivers with natural flow regimes as freshwater conservation areas will enhance native species recovery.

Desert riverscape rehabilitation practitioners must contend with compounding effects of increasing human water demand, persistent drought, non-native species establishment, and climate change, which further stress desert riverine ecosystems such as rivers in the Colorado River basin, United States. Herein, we provide our perspective on the importance of natural flows, large floods in particular, for successful conservation and rehabilitation of riverscapes. We present ideas developed from our experience with rehabilitation projects across multiple desert tributary rivers with varying levels of habitat degradation and water abstraction. We propose spatially extensive measures such as protection of in-stream flows, tailoring rehabilitation efforts to available annual water availability, and working with nature using low-tech process-based techniques to more completely address the mechanisms of habitat degradation, such as flow reduction and vegetation-induced channel narrowing. Traditionally, rehabilitation efforts in the Colorado River basin take place at relatively small spatial extents, at convenient locations and, largely focus on reducing non-native plant and fish species. We suggest that we need to think more broadly and creatively, and that conservation or recovery of natural flow regimes is crucial to long-term success of almost all management efforts for both in-stream and riparian communities.