Abstract Objective North-temperate fishes fluctuate in their energy status, metabolic rate, and behavior in response to seasonal changes in temperature and light, which could influence tissue concentrations of biomagnifying contaminants like mercury (Hg). Yet, repeated sampling within a single year is rarely conducted to quantify if contaminant concentrations vary seasonally across different fish populations. Methods We quantified liver total Hg (THg), muscle THg, and liver methylmercury of Lake Trout Salvelinus namaycush, a predatory fish that is harvested for sport and subsistence, from two different lakes in Ontario (Lake of Two Rivers and Lake Opeongo) across all four seasons over two consecutive years. Seasonal increases in Hg were expected to occur during fall, due to decreased Lake Trout energy status following fall spawning, and winter, due to increased winter trophic positions or temperature-driven decreases in elimination rates. Results Contrary to our expectations, liver Hg varied seasonally in only one of our study lakes (Lake Opeongo). In this lake, liver THg increased from summer to fall, but only in year 1, and liver methylmercury decreased from fall to winter. Also contrary to our expectations, muscle THg displayed more consistent seasonal patterns and achieved the lowest concentrations in summer in both lakes and years, which could relate to growth dilution. Muscle THg increased by 32% from summer to fall in Lake Opeongo (from 2.078 ± 0.094 to 2.75 ± 0.145 μg/g) and by 55% from summer to winter in Lake of Two Rivers (from 2.054 ± 0.09 to 3.198 ± 0.216 μg/g), which could relate to lower body condition after fall spawning (Lake Opeongo) and higher winter trophic positions (Lake of Two Rivers). Conclusions Inconsistent seasonal variation in liver Hg could arise because of dynamic processes of Hg accumulation and redistribution occurring in the liver. However, muscle tissue, which can be sampled nonlethally, could be useful for monitoring seasonal Hg trends based on our findings. Because muscle THg was lower in summer compared with fall and winter, fish Hg samples collected during summer alone may not reflect Hg concentrations during other seasons.
Abstract Objective Hypoxia (i.e., low dissolved oxygen availability) is a natural phenomenon but can also be induced by human activities (e.g., nutrient enrichment from runoff). Given that dissolved oxygen is essential for aquatic life, periods of hypoxia tend to have negative consequences (e.g., sublethal disturbances, mortality) for most fishes. Extensive laboratory research has documented hypoxia thresholds and physiological and behavioral outcomes for a variety of freshwater and marine fishes and there is also an extensive body of fieldwork assessing population-level responses (e.g., survival, distribution). Yet, studying how individual fish respond to hypoxia in the wild has proved challenging; electronic tagging and tracking tools (e.g., biotelemetry, biologging) have made it easier to study individual responses to hypoxia and complement other tools like hydroacoustics that tend to focus on population-level responses. Methods We review what has been learned from contemporary studies that employ electronic tagging and tracking tools to understand how fish respond to hypoxia in the wild. Topics explored include identifying and validating hypoxia thresholds, habitat compression, connectivity, mortality, physiological and bioenergetic consequences, and extreme weather conditions. We also consider what we have learned about evaluating various management plans for hypoxia and reflect on how electronic tags have also been used in aquaculture systems and in hybrid studies that combine laboratory and field research. We highlight fishes in Lake Erie as a research narrative to demonstrate how electronic tagging and tracking have markedly improved our understanding of a longstanding hypoxia issue. Results Our synthesis revealed that electronic tagging and tracking have provided critical information on how fish respond to hypoxia in the field, revealing complex trade-offs and compensatory mechanisms as well as cryptic hypoxia-induced mortality. Beyond just illuminating space use and mortality, tags equipped with various sensors are revealing how fish deal with hypoxia in real time in terms of physiology, bioenergetics, and behavior. Conclusions As electronic tagging and tracking methods experience further innovation and are increasingly applied to understand the effects of hypoxia on fish, we expect more unanticipated findings about the effects of hypoxia on fish in all aquatic ecosystems, which will strengthen our ability to manage and mitigate hypoxia. Combining tools and approaches (e.g., lab and field) is perhaps the best way to generate comprehensive understanding.
Objective The first objective of this study was to develop electrofishing depletion models that allowed effort to vary among passes while accounting for capture probability that varied with fish size. The second objective of this study was to extend the models from estimating abundance at a single site to multiple sites using a hierarchical model.Methods Simulations were used to evaluate four potential models under four different scenarios of electrofishing effort and capture probability. Further, depletion models were fit to electrofishing data from five sites in the Henrys Fork Snake River basin and two sites in the Big Lost River basin, Idaho.Results The model that accounted for heterogeneity in effort among passes and capture probability among fish size (i.e., size-effort model) resulted in the lowest average bias and root-mean-squared error in the simulations. In addition, the size-effort model resulted in greater estimates of abundance in the simulations and the empirical analysis than the other models evaluated.Conclusions Results of this study suggest that bias that is common in electrofishing depletion estimates can be reduced by using models that explicitly account for assumptions that are often violated in practice, such as nonconstant effort and heterogeneity in capture probability among size-classes of fish and passes. In this study, we developed models that accounted for variable effort among passes and fish size in electrofishing depletion surveys. The models produced unbiased estimates of abundance when these assumptions were violated in a simulation. Further, estimates of abundance that accounted for effort and fish size were greater than models that did not when using empirical data.
Objective Although Alligator Gar Atractosteus spatula are known to inhabit river mouths and brackish areas at various life stages, little is known about the residency patterns of Alligator Gar in estuaries. We examined coastal habitat use and movement of Alligator Gar in Texas and analyzed potential environmental drivers of seasonal movement.Methods Alligator Gar were tracked with acoustic tags in the Cedar Lakes region of the Texas Gulf coast, which includes two substantial river mouths and the Intracoastal Waterway. Generalized additive mixed models were utilized to analyze the impacts of water temperature, river discharge, salinity, dissolved oxygen, turbidity, and individual size on movement.Results Individuals utilized both estuarine marsh habitat and riverine habitat seasonally throughout the study, with estuarine detections occurring more frequently in spring-summer (94.0% of detections) and riverine detections occurring more frequently in fall-winter (64.0% of detections). Alligator Gar were mostly absent from the estuary during the coldest months of the year, which was consistent with fishery catch data suggesting their relative absence from estuaries in winter. Fish frequently used the Intracoastal Waterway to travel between habitats (18.8% of detections). Relatively cool water temperatures were predictive of higher rates of movement, and temperature may be a cue for upriver migration in the fall and winter.Conclusions Coastal Alligator Gar overall showed a seasonal preference for productive, shallow estuarine habitat in spring and summer; however, in fall and winter, individuals generally made directed movements into riverine habitat. Our findings indicate that coastal Alligator Gar are highly mobile between estuarine and freshwater habitats and respond to environmental cues to transition between these habitats. Understanding the behaviors and habitat use of Alligator Gar in estuaries will inform future management of the species at the coastal edge of its range. Alligator Gar were tracked in estuarine areas of Texas using acoustic array technology. Individuals exhibited wide variation in movement distances and rates, with larger individuals typically traveling further distances. Temperature was a cue for movement, and Alligator Gar tended to leave the estuary during winter.
Objective Penetrability of streambed sediments is important for the ecological functioning of many benthic organisms, yet there is ambiguity in terminology associated with this concept and there is no standardized method of penetrability measurement available to scientists and resource managers. The goal of our study was to better define the terms of compaction and penetrability as they relate to streambed substrate and to advance the measurement techniques for assessing penetrability of bed sediments in gravel-bed streams.Methods We tested the performance of a modified penetrometer method in 12 different substrate configurations in the laboratory and used the data to determine the relationship between penetration depth and substrate characteristics. We then simulated additional sampling effort and used bootstrapping methods to provide guidance on the number of samples required to achieve consistent results. Finally, we tested the device in a field setting to validate our laboratory and simulation results.Results Depth of penetration was negatively related to increasing substrate particle diameter and number of particle size-classes in both laboratory and field tests. Mean depth of penetration and CV stabilized after five samples, but variation remained high in scenarios containing large-grained particles.Conclusions Our results show that the device has application in gravel-bed streams but may require high amounts of sampling effort to capture variation in streams dominated by large substrate. We discuss the uses and limitations of this method for estimating substrate penetrability and provide guidance on interpretation of the estimates that it produces. Streambed condition influences its suitability for aquatic organisms, but measuring it can be difficult. We developed a new way to measure streambed penetrability and found that it works well in some situations. The new method can help us to better understand streambed characteristics and manage aquatic organisms.
Objectives: Restoration efforts for Alligator Gar Atractosteus spatula include stocking programs in multiple states; however, limited efforts have occurred to document poststocking survival. Acoustic telemetry can be used to estimate survival of fish, but age-0 Alligator Gar have not been acoustically tagged previously. Our objectives in this study were to (1) develop a protocol for acoustic transmitter implantation in age-0 Alligator Gar and (2) determine the appropriate size of acoustic transmitter for future field studies by assessing tag retention and posttagging survival and growth of age-0 Alligator Gar in a hatchery setting. Methods: A total of 125 age-0 Alligator Gar were equally sorted into five treatment groups: (1) tagged with Innovasea (Halifax, Nova Scotia, Canada) V6-2x dummy tags, (2) tagged with V7-4x dummy tags, (3) tagged with V9-1x dummy tags, (4) sham surgery fish that underwent the entire surgical process but did not have a tag inserted, and (5) control fish that were not anesthetized and were only measured for total length, body width anterior to the pelvic fins, and weight. Following handling and/or surgery, Alligator Gar were monitored in hatchery holding tanks each day for 14 to 15 d postsurgery for evidence of tag shedding, mortality, cannibalism, or other adverse effects. Results: During the monitoring period, all tagged Alligator Gar retained the dummy tags, and incisions had completely healed by the end of the trial. We documented three mortalities (n = 2 with V6 tags, n = 1 with a V9 tag), resulting in 4% mortality of fish with transmitters. Based on logistic regression analysis, treatment group, total length, weight, tag burden, or relative condition did not have a significant effect on survival of age-0 Alligator Gar. We also did not detect any significant differences in growth between treatment groups during the monitoring period. Conclusions: Our results suggest that short-term tag retention, survival, and growth of age-0 Alligator Gar (similar to 200-400 mm TL) are not significantly altered by surgical implantation of acoustic transmitters. Both V6 and V7 transmitters would be suitable for use in future studies of Alligator Gar >180 mm TL, while V9 transmitters could be used in Alligator Gar >250 mm TL. The use of acoustic telemetry in future field studies of Alligator Gar will allow researchers and managers to answer key questions related to movements, habitat use, and survival, improving the outcomes of restoration programs throughout the range of the species.
Objective The decline of native Walleye Sander vitreus populations in inland lakes raises concerns about conservation and continued exploitation in these fisheries. Although Walleye have been intensively studied, available data on the long-term trajectories of exploited populations are limited. We tested for trends in the dynamics of an exploited Walleye population in Escanaba Lake, Wisconsin, following the implementation of a harvest-eliminating regulation in 2003.Methods We used long-term fisheries-dependent and -independent surveys of the Walleye population of Escanaba Lake during 1982-2022 to test for regulation effects. We compared sex-specific mortality and growth before and after the no-harvest regulation was enacted. We also performed breakpoint analyses on annual estimates of density and age-0 relative abundance and Walleye-specific angler effort, angler catch per unit effort, and distance traveled by Walleye anglers.Results Total mortality declined and there was a lower growth potential (von Bertalanffy L infinity) for both sexes following the elimination of harvest. A rapid increase in density occurred during an initial recovery phase (2004-2008), which was followed by a decade of variability (2009-2022) that appeared to stabilize toward the end of the data set. The recruitment dynamics were decoupled from adult population responses, and no trends over time were detected with the breakpoint analysis. Walleye-specific effort declined following the elimination of harvest, yet catch rates remained dynamic and were not related to the regulation change.Conclusions Our findings emphasize the importance of density-dependent processes and interannual variability when evaluating responses to regulations. Walleye populations may be capable of recovery when they maintain their productive capacity despite high harvest. We found that Walleye catch rates and age-0 recruitment may not be the best indicators of adult Walleye population dynamics. Implementing management strategies to ensure the resilience of Walleye populations under changing ecological and anthropogenic stressors will be critical to their long-term persistence. The responses of an exploited Walleye population to the elimination of harvest were variable. Walleye recovery from harvest is a stochastic process and it may take many years for a population to regain relatively predictable dynamics.
Objectives Our objectives were to compile observations of Pink Salmon Oncorhynchus gorbuscha in California rivers over the past three decades, to evaluate whether sightings are increasing, and to understand the factors associated with their patterns of occurrence.Methods We compiled Pink Salmon records from 1996 to 2023 from agency monitoring programs, gray literature, and outreach to more than 30 organizations monitoring salmonids across California. We then modeled the probability of Pink Salmon presence as a function of time (year), watershed characteristics, proximity to potential source populations, and timing (month) of monitoring efforts.Results We recorded Pink Salmon observations in 18 watersheds and subbasins in California. Our findings indicate an increase in Pink Salmon detections corresponding to increasing Pink Salmon abundances across the whole North Pacific. Both adults and fry were observed in California rivers, indicating successful spawning. All 18 watersheds had odd-year observations of Pink Salmon, reflecting the dominance of odd-year Pink Salmon in their southern range. However, five of these watersheds also had observations of Pink Salmon during even years. Overall, Pink Salmon were more likely to be detected in California when monitoring began earlier in the spawning season (i.e., late summer to early fall).Conclusions Our observations raise the possibility that Pink Salmon have a growing presence in California, in contrast to severe and widespread declines of most other Pacific salmon runs in California. Our work highlights the importance of inclusive monitoring frameworks to detect rare or emerging salmonid populations at range boundaries and to inform species management strategies. Pink Salmon sightings across California have risen in recent years, corresponding with increased abundances across the North Pacific. Pink Salmon are expanding at the northern end of their range in the Arctic, and our work suggests their presence is also growing at their southern range, where other salmon species are in decline.
Objective Age of maturation in Chinook Salmon Oncorhynchus tshawytscha is phenotypically plastic, influenced by both genotype and environmental factors. Salmon hatchery programs often rear fish under accelerated growth regimes using high-lipid diets that can result in younger age at maturity, including increased prevalence of age-2 males (minijacks). In the first phase of this investigation (previously published), we demonstrated that minijack prevalence could be decreased by up to 65% through a combination of reduced feeding frequency and dietary lipid. The goal of the second phase of this investigation was to report on the dietary treatment effects on (1) juvenile travel time and apparent river survival, (2) adult recovery rates and demography, and (3) the relationship between the prevalence of minijacks and adult recovery.Methods Juvenile Umatilla River hatchery fall Chinook Salmon were reared at Bonneville Hatchery, Oregon, under four dietary treatments across four replicate rearing cohorts. The dietary treatments included two feeding frequencies (standard [fed 7 d/week] and reduced [fed 4 d/week]) and two dietary lipid levels (standard [18%] and reduced [12%]) in a 2 & times; 2 factorial design. The fish were differentially implanted with coded wire tags according to treatment, and some of the fish were tagged with passive integrated transponder tags. The downstream passive integrated transponder tag detections at John Day Dam were used to determine juvenile travel time and river survival. The coded wire tags were used to determine adult recoveries in the marine and freshwater environment. The recovery data were analyzed within the context of significantly variable ocean conditions over the course of the four ocean entry years.Results The downstream travel time of the out-migrating juveniles was significantly affected by the dietary treatments, with the smallest treatment fish traveling slower (mean travel time = 68.2 d) than the largest treatment fish (mean travel time = 45.2 d). However, juvenile downstream survival to John Day Dam was not statistically different between treatments. Dietary treatment had a significant effect on total adult recovery. More adult fish (ages 4-6) were recovered from the low feeding frequency-low dietary lipid treatment (0.51%) than from the high feeding frequency-high dietary lipid treatment (0.42%) over the four ocean entry years. In addition, despite being smaller at release, this treatment produced older and larger adults (mean age at recovery = 4.46 years) than were produced by the largest smolts at release (mean age at recovery = 3.72 years). However, interannual variation in the ocean conditions had a more substantial effect on adult recovery than did any hatchery-rearing regime.Conclusions The results of this investigation provide insights for customizing feeding regimes to reduce precocious male maturation while increasing overall adult size, recovery rates, and age at recovery in salmon hatchery programs within the context of variable ocean conditions. Reduced feeding frequency and dietary lipid in hatchery Chinook Salmon decreased early male maturation and increased the recovery of larger, older adults. These findings provide insight into how variations in juvenile rearing have far-reaching effects on adult recovery for hatchery programs.
Objective Rainbow Trout Oncorhynchus mykiss were originally stocked in Missouri streams in the late 1800s, and several spring-fed streams retain self-sustaining populations, presumably derived from the original stocking events. The phylogenetic and geographic origins of Missouri Rainbow Trout are unclear and based on historical records that have not been substantiated by genetic techniques.Methods Fin clips were collected from Rainbow Trout in 10 Missouri streams, eight of which contain self-sustaining wild stocks. Single-nucleotide polymorphisms (SNPs) and microsatellites were used to compare the wild-caught fish with hatchery strains from Missouri as well as hatchery and wild populations of Rainbow Trout from California.Results Phylogenetic analyses, which included Golden Trout O. aguabonita and Coastal Cutthroat Trout O. clarkii as outgroups, indicated that Missouri Rainbow Trout were most similar to Coastal Rainbow Trout O. m. irideus from California's Sacramento River drainage, with perhaps some introgression from other stocks. Wild and hatchery populations from Missouri were all more similar to each other than to any sampled population outside the state, indicating mixing of stocks among Missouri streams and hatcheries since the original stocking events, coupled with genetic drift between founder and source populations over the past century. At the population level, STRUCTURE analysis of SNPs and microsatellites found Mill Creek to be most distinct, but all wild populations were distinct from each other and from hatchery strains at higher levels of resolution. The SNPs associated with anadromy and the ancestral form of the Omy05 and Omy20 chromosomal inversions were at high frequencies in Missouri streams, indicating that the original source contained offspring of steelhead (anadromous Rainbow Trout). Effective population sizes in the largest stream populations were small, but we recommend that stocking should cease, thereby allowing local adaptation to continue shaping their evolutionary trajectory.Conclusions Missouri Rainbow Trout of both wild-caught and hatchery-propagated lines are most closely related to each other in this study. Overall, they are similar to the populations of the Coastal Rainbow Trout subspecies from the Sacramento River drainage, supporting their origin from California in the 1800s. Since that time, they have maintained polymorphisms relating to either temperature variation or migration tendency in the form of chromosomal inversion variations on Omy05 and Omy20. The genetic background of these fish is distinctive, probably representing both a sampling of unique genetic variation from the original source population and the effects of isolation in Missouri since the founding of populations in that state. We examined DNA from wild and hatchery-origin Rainbow Trout from Missouri and determined that they are descendants of coastal Rainbow Trout and steelhead from the Sacramento River drainage in California. They are not McCloud River Redband Trout as is sometimes assumed.
Objective To enhance monitoring efforts and provide new insights into the population status and ecology of the Rio Grande Silvery Minnow Hybognathus amarus, we designed two sets of hydrolysis probe quantitative real-time PCR assays to be combined with environmental DNA (eDNA) sampling to provide a noninvasive method for detecting this endangered species.Methods Each assay was validated with eDNA samples that were collected from (1) tanks holding Rio Grande Silvery Minnow, (2) locations that the species is known to occupy within the middle Rio Grande (New Mexico, United States), and (3) locations where the species is presumed to be absent in New Mexico.Results Both quantitative real-time PCR assays are highly sensitive, with limits of detection at two and eight copies of target DNA per reaction. Tandem use of the two assays increases detection power and reduces the risk of false negative results.Conclusions Our newly developed noninvasive eDNA tool for the detection of Rio Grande Silvery Minnow will support recovery efforts and adaptive management by augmenting the current methods that are used to monitor the population status and distribution of the species. Demonstrating the value of environmental DNA tools in aquatic endangered species monitoring, we developed genetic assays that accurately reveal the likely presence or absence of the Rio Grande Silvery Minnow by using noninvasive water sampling across study areas, providing managers with a time- and cost-saving population monitoring method.
Objective Monitoring and assessment of nongame native fishes is limited, but conservation interest in these species is growing. Freshwater Drum Aplodinotus grunniens are a wide-ranging species that serve important functional roles and could serve as an indicator for similar but less common species. Our overall objectives were to quantify and compare population dynamic rates and life history of Freshwater Drum among study reaches in the upper Mississippi and Illinois rivers and relate these metrics to hypothesized environmental and anthropogenic factors.Methods We integrated recently collected age data with monitoring data to estimate age and size distributions, growth curves, maturation schedules, mortality rates, and young-to-adult ratios of Freshwater Drum in six study reaches spanning 1,500 km of river. Principal component analyses and linear regression were used to relate environmental and anthropogenic gradients (latitude, commercial harvest, hydrologic dynamics, primary productivity) to life history traits and population dynamic rates.Results We found latitudinal gradients in life history traits and population dynamic rates whereby Freshwater Drum in upstream, higher-latitude study reaches generally exhibited later maturity, slower growth, smaller maximum size, and lower mortality rates compared with those in lower-latitude study reaches. Further, young-to-adult ratios positively corresponded with chlorophyll-a concentration. No clear relationships were apparent between population dynamic rates and hydrologic variation or commercial harvest.Conclusions Latitude is an important structuring component of life history traits and population dynamics of Freshwater Drum in the upper Mississippi and Illinois rivers likely due to both temperature seasonality and disturbance regimes. The presence of demographic structure in a widespread, common species such as Freshwater Drum suggests similar patterns likely exist in other long-lived native fishes. Latitude is a key environmental gradient driving fast versus slow life histories in Freshwater Drum in the upper Mississippi River and Illinois River. Freshwater Drum at lower latitudes matured earlier, grew faster and larger, and had higher mortality than those at higher latitudes.
Objective Field observations in a Lake Ontario embayment suggested that there is a phototaxic response of larval coregonines that changes with development.Methods We used a controlled laboratory experiment with hatchery-reared larvae exposed to three different light regimes (24-h light, 24-h dark, or 12-h light followed by 12-h dark) to test for phototaxis in Cisco Coregonus artedi.Results Larval depth distributions were not uniform, and contrasts between vertical distributions within early and late portions of the time series when larvae were exposed to light showed evidence of phototaxis and transition to a preference for the bottom around 68 d posthatch. Larval behavior in dark treatments contrasted with those exposed to light and also suggested that other influences on depth distribution could be operating.Conclusions As observed in other species, larval Cisco phototaxis response would likely result in diel vertical migrations and may be an adaptation to changes in food distributions balanced with predation risk as the larvae grow and become stronger swimmers. This has implications for the success of Cisco restoration in the Great Lakes, particularly as climate change affects environmental conditions and phenology. Changes in larval Cisco response to light as they as grow indicate that their successful conservation in Lake Ontario will likely depend on proper daily environmental conditions, such as food, water currents, depth, and presence of predators, as they age.
Objective Juvenile Chinook Salmon Oncorhynchus tshawytscha that are released upstream of Shasta Reservoir migrate more than 35 km to reach Shasta Dam, although survival through this system is poorly understood. We conducted a reservoir-scale acoustic telemetry study to quantify downstream movement and survival under seasonally variable environmental conditions to inform decisions about juvenile collection strategies for Chinook Salmon reintroduction above Shasta Dam.Methods A total of 656 hatchery-origin juvenile Chinook Salmon were acoustic-tagged, released near the mouth of the McCloud River, and monitored in Shasta Reservoir and the Sacramento River from September 2024 through March 2025 using an array of telemetry receivers.Results Most tagged fish failed to move downstream through the McCloud River Arm of Shasta Reservoir and arrive at Shasta Dam. Survival probabilities were estimated at 0.268 to the downstream end of the McCloud River Arm and 0.119 to Shasta Dam. For fish that did reach the dam, elapsed time from release to arrival was 66.4 d, and fish typically arrived and departed during daylight hours. Nine tagged juveniles were detected downstream of the dam, and three were later detected more than 500 km downstream.Conclusions The consistently low survival and restricted downstream movement provide important information indicating that downstream collection of juvenile Chinook Salmon should be focused in the lower McCloud River and the upper portion of the McCloud River Arm of Shasta Reservoir rather than at Shasta Dam. Most tagged juvenile Chinook Salmon did not reach Shasta Dam, and survival through the system was low. These results inform whether collection upstream or dam passage approaches better support reintroduction efforts for winter-run Chinook Salmon.
Objective The objective of this study was to evaluate statistical evidence for relationships between flow event characteristics and annelid host distribution, an important and necessary step toward providing and justifying management actions. Declines in Klamath River salmon have been attributed to infection and disease that are caused by the myxozoan parasite Ceratonova shasta. Flow manipulation has been used to manage risk of C. shasta for juvenile salmonids in this system. One mechanism by which flow-related disturbance can reduce the risk of C. shasta infection for salmon is by reducing populations of the obligate invertebrate host Manayunkia occidentalis. We previously demonstrated that hydraulic conditions during peak discharge events drive the distribution of M. occidentalis and suggested that high-magnitude flow events would be effective for reducing the distribution of the annelid host. However, evidence of the effects of flow events on M. occidentalis was needed to support the use of flow management.Methods To address this knowledge gap, we leveraged a multiyear data set to estimate relationships between the distribution the annelid host and the characteristics of each discharge event. We measured the presence or absence of M. occidentalis at spatially georeferenced sampling locations, stratified across the range of hydraulic and substrate conditions in three reaches annually from 2012 to 2020.Results During the study period, the magnitude of peak discharge ranged from 52.4 to 314.3 m3/s. The inclusion of seven additional years' data in the base model (hierarchical; annual-level covariate + previous "single year" model) resulted in parameter estimates that were similar to those that were derived for the previous (2016) model, supporting that substrate, depth, and velocity during peak discharge predicted annelid distribution. The magnitude model (base model + annual-level covariate for peak discharge) showed evidence of a strong negative association with annelid presence (95% of all Markov chain-Monte Carlo draws were negative). Every 101.9 m3/s increase in peak discharge was associated with an estimated 39% decrease in the odds of annelid presence. The estimated effect of magnitude on the probability of annelid presence was illustrated in the contrast between low- and high-magnitude peak discharge scenarios. For both scenarios, the effects of depth, velocity at peak discharge, and substrate, consistent with the previous model (2016), were evident, as was the further reduction in probability of annelid presence at the higher peak discharge. However, under the low-magnitude discharge scenario, M. occidentalis were predicted to be present on smaller less stable substrates and at a wider range of depths and velocities than they were under the high-magnitude discharge scenario. In contrast to the covariate for magnitude, the inclusion of a duration covariate did not show a strong relationship with annelid distribution, which we attributed to the lack of variation in this covariate during the study period.Conclusions This work provides quantitative evidence that high-magnitude peak discharge flow events lead to lower probabilities of annelid host presence. Lower probabilities of annelid hosts are indicative of a reduced distribution of the M. occidentalis host and in turn reduced risk of C. shasta for salmon. This study investigates how water management can be used to disrupt the life cycle of a harmful parasite that affects salmon populations. We analyzed the relationship between flow event characteristics and the distribution of the parasite's obligate (required) worm host. Higher magnitude flow events significantly reduced the host's presence and distribution, in turn lowering risk of infection for juvenile salmon.
Objective: Fish stocking is used to manage recreational fisheries and conserve native species; however, stocking success can be reduced if predators consume the stocked fish at high rates. We evaluated how different stable isotopes and predator tissue types influenced estimates of predator reliance on stocked fish. Methods: We measured delta C-13, delta N-15, and delta S-34 stable isotope values in Walleye Sander vitreus, stocked salmonids, and naturally reproduced prey fish before and after large salmonid stocking events in two Wyoming reservoirs. Shifts in the values for delta C-13, delta N-15, and delta S-34 stable isotopes across Walleye tissue types were assessed before, 1 month after, and 8 months after stocking using analyses of covariance, with length as a covariate. The contribution of stocked salmonids to Walleye diets was quantified using stable isotope mixing models in R (simmr) that were determined from Walleye muscle and liver tissues and combinations of stable isotopes. Results: Sulfur stable isotopes differentiated stocked salmonids and natural prey sources for over 8 months poststocking. Liver tissues documented Walleye consumption of stocked salmonoids that was not detected by analyses of muscle tissue alone. Including delta S-34 values increased the precision of the mixing-model outputs and led to more consistent results across modeling approaches that were based on different combinations of stable isotopes. Conclusion: Our results highlight the value of delta S-34 analyses and multiple tissues, with short- and long-term turnover rates, to identify the consumption of stocked fish by predators and describe the fates of stocked fish.
Objective Understanding whether fishes quickly respond to shifting temperatures and flows, especially as they pass through river reaches that may be thermally unsuitable, may help to prioritize climate-informed management strategies.Methods Here, we use 15 years of daily fish passage data (2005-2020) from the Leaburg Dam on the McKenzie River, Oregon, USA, with water temperatures and river flows from two associated gauges. We examine the relative influence of temperature, flow, and calendar date on fish moving upstream, the range of conditions experienced by each species, and long-term patterns in timing, supported by annual count data from the years 1971-2020.Results Comparisons of timing and conditions while each species passed upstream through the Leaburg Dam fish ladders revealed that some taxa were more consistent seasonally (e.g., Pacific Lamprey Entosphenus tridentatus and Largescale Sucker Catostomus macrocheilus), experiencing a more restricted range of conditions, while others moved throughout the year under highly variable environmental conditions (e.g., trout). For both groups, calendar date appeared to be a primary driver of movement timing, even when local environmental factors of temperature and flow were considered. We note broad trends toward earlier passage across all species except Chinook Salmon Oncorhynchus tshawytscha. Notable declines in movement of Mountain Whitefish Prosopium williamsoni and Largescale Suckers occurred during years of extreme weather events, indicating that they may be particularly sensitive to the combined impacts of water temperature and flow and could serve as sentinel taxa.Conclusions Although timing is recognized as a driver for the onset of migrations, this suggests that most fish may continue to move upriver during consistent time periods, potentially increasing their risk of exposure to suboptimal environmental conditions. Our results demonstrate the utility of long-term passage data for detecting patterns in local timing, environmental conditions co-occurring with fish movement, and the sensitivity of different fish species in responding to environmental extremes during upstream migrations. Assessment of this long-term data set reveals environmental factors associated with upstream fish movement, trends toward earlier passage, and species that may warrant continued monitoring.
Objective Energetic reserves are important indicators of the relative health of fish and fish populations. Body condition indices that relate fish weight to length are commonly used as quick, noninvasive methods for approximating lipid content and condition. A microwave meter (i.e., fat meter or energy meter) is a noninvasive method found to be more accurate in some species. The objective of this study was to evaluate the suitability of nonlethal techniques for estimating muscle lipid content in Lake Whitefish Coregonus clupeaformis.Methods We compared the sensitivity of three nonlethal indicators of lipid content to laboratory-extracted muscle lipid content in Lake Whitefish, including readings from a handheld microwave meter at several positions, Fulton's condition factor, and relative weight.Results We found significant, positive relationships between lipid content and each estimation method, except relative weight, with weak to moderate correlations. The microwave meter was moderately correlated to lipid content when positioned anterior to the dorsal fin above the lateral line (r2 = 0.50), while other positions and combinations of positions had weaker correlations (r2 range = 0.27-0.45). Correlation was only slightly improved by including additional model variables (i.e., length and weight). Fulton's condition factor was weakly correlated with lipid content (r2 = 0.19), while relative weight was not significantly correlated with lipid content.Conclusion The microwave meter provides an improvement to muscle lipid estimation compared with length-weight body condition indices; however, microwave meter readings alone do not constitute a reliable predictive measure for true muscle lipid content under the conditions tested here. We hypothesize that the low strength of correlation may be due to low muscle lipid content or the presence of thick scales in Lake Whitefish. Further investigation is needed to understand the mechanisms negatively affecting the predictive performance of the microwave meter in Lake Whitefish and other species. A microwave meter provides an improvement to muscle lipid estimation compared with traditional length-weight body condition indices in Lake Whitefish. However, microwave meter readings are not a reliable predictive measure for true lipid content.
Objective Body condition is often used as a surrogate of individual fitness in fisheries ecology and management, but empirical data demonstrating this linkage are few. Here, we investigated whether weight at length (i.e., body condition) explained individual variation in survival.Methods We conducted a mark-recapture survey of three native fishes subseasonally (every 2 months) over 28 months in a coolwater Piedmont stream in South Carolina, United States. We used a Cormack-Jolly-Seber model in the Bayesian state-space framework to characterize whether survival was explained by body condition.Results Body condition changed subseasonally, with its peak in spring (May) and trough in fall (November) in all three species (Bluehead Chub Nocomis leptocephalus, Creek Chub Semotilus atromaculatus, and Mottled Sculpin Cottus bairdii). Survival decreased in warmer subseasons for all species. Additionally, individuals with higher body condition survived better than those with lower condition only in these warmer subseasons and only in the Mottled Sculpin, a species that is territorial and the most intolerant of warmer temperatures among the study species.Conclusions Our findings show that body condition is a predictor of survival only when populations undergo high mortalities in some but not all species, which are likely caused by a combination of factors, such as physiological stress, diminished food resources, and increased competition for resources. Fish body condition is often assumed to indicate the well-being of individuals. By linking body condition and survival, we show that this relationship depends on season and species in a coolwater stream and highlights the context dependency.
