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At least 109 records · Page 6Linked to original sources
USGS2018

Effects of climate-related stream factors on patterns of individual summer growth of Cutthroat Trout

Coldwater fishes are sensitive to abiotic and biotic stream factors, which can be influenced by climate. Distributions of inland salmonids in North America have declined significantly, with many of the current strongholds located in small headwater systems that may serve as important refugia as climate change progresses. We investigated the effects of discharge, stream temperature, trout biomass, and food availability on summer growth of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri , a species of concern with significant ecological value. Individual size, stream discharge, sample section biomass, and temperature were all associated with growth, but had differing effects on energy allocation. Stream discharge had a positive relationship with growth rates in length and mass; greater rates of prey delivery at higher discharges probably enabled trout to accumulate reserve tissues in addition to structural growth. Temperature effects were positive but not significant, and support in growth models was limited, likely due to the cold thermal regimes of the study area. The strength of the discharge effect on growth suggests that climate adaptation strategies for coldwater fishes that focus solely on thermal characteristics may be misleading and highlights the importance of considering multiple factors, including hydrologic regimes, in conservation planning.

Montana, Wyoming↗
USGS2018

Posthatch development of otoliths and daily ring genesis in age-0 Spotted Gars

Gars (Lepisosteidae) are increasingly being managed as top-level predators that are important to overall ecosystem health. Given the paucity of information on early life history for many species, additional early life history data would aid in gar conservation and management. Daily rings in otoliths are useful for determining many early life history parameters, such as growth rates and the date of hatch, but properly interpreting these structures requires additional information on otolith formation. Gars represent an ancient lineage, and their otoliths are unlike those of most teleost fishes, having multiple nuclei and being covered on the surface with very small otoconia. We used computed tomography X-ray scanning and oxytetracycline (OTC) marking of a series of known-age fish from hatch through 10–12 d posthatch (dph) to understand the formation of otoliths in the Spotted Gar Lepisosteus oculatus , a species that is of management concern in several parts of its range in North America. The sagittae and lapilli each began as loose associations of otoconia at hatch; they fused and hardened into single crystals by 4 dph, in concert with the transition from attached larvae to free-swimming juveniles. Asteriscus otoliths were not observed in any of the individuals examined through 12 dph. The OTC marks were not observed on individuals treated at hatch, and variable marks were detected at 1–5 dph. By 6 dph, 100% of individuals exhibited OTC marks on sagittae and lapilli. Daily rings could not be discerned until fish were marked at 4 dph, after which daily age estimates increased linearly with fish age. Results of this study verify that in Spotted Gars, otoliths (sagittae and lapilli) and their daily rings form several days after hatch, in relation to the transition from sessile, attached larvae to free-swimming juveniles.

Oklahoma↗
USGS2018

An interferometric synthetic aperture radar (InSAR) habitat suitability model to identify overwinter conditions for coregonine whitefishes in Arctic lagoons

Lagoons provide critical habitats for many fishes, including coregonine whitefishes, which are a mainstay in many subsistence fisheries of rural communities in Arctic Alaska. Despite their importance, little is known about the overwintering habits of whitefishes in Arctic Alaska due to the challenges associated with sampling during winter. We developed a habitat suitability (HS) model to understand the potential range of physical conditions that whitefishes experience during the Arctic winter, using three indicator lagoons that represent a range of environmental characteristics. The HS model was built using a three-step approach. First, remote sensing that uses interferometric synthetic aperture radar (InSAR) identified areas of floating and bottomfast ice. Second, through in-field ground-truthing, we confirmed the presence and quality of liquid water (water depth, temperature, and dissolved oxygen) beneath the ice cover. Third, we assessed the suitability of that liquid water as habitat for whitefishes based on published literature and expert interpretation of water quality parameters. InSAR determined that 0, 65.4, and 88.2% of the three lagoons were composed of floating ice corresponding with areas of liquid water beneath a layer of ice. The HS model indicated that all three lagoons had reduced suitability as whitefish habitat in winter than in summer due to the loss of habitat because of the presence of bottomfast ice and a reduction in the quality of liquid water due to cold temperatures, high salinities, and low dissolved oxygen levels. However, only the shallowest lagoon had lethal conditions and zero suitability as whitefish habitat. The methods outlined here provide a simple, cost-effective method to identify habitats that consistently provide critical winter habitat and integrate remote sensing in a HS model framework.

Alaska↗
USGS2018

Behavior and survival of stocked trout in southern Appalachian Mountain streams

Stocking of trout to support recreational fisheries is a common practice among state and federal agencies to meet angling and harvest demands. Success of stocking efforts relies upon fish behavior and survival to maximize the availability of fish to anglers. We quantitatively described the movement behavior and survival of stocked Brook Trout Salvelinus fontinalis , Brown Trout Salmo trutta , and Rainbow Trout Oncorhynchus mykiss in three southern Appalachian Mountain streams in western North Carolina, USA, that were managed under delayed harvest regulations. Hatchery trout were tagged with a combination of PIT tags and radio transmitters (radio tags); stocked into “Delayed Harvest Trout Waters” of the North Toe, East Prong Roaring, and Little rivers; and monitored during the catch-and-release season from October to June. Assessed according to river and species, 19–65% of trout emigrated from the delayed harvest study reaches, while 1–29% died within the reaches. The majority of radio-tagged fish (71%; 59–85% by river) remained within 2 km of the stocking location, whereas 6% migrated over 10 km from the stocking location. Few trout stocked during fall (October and November) were available to anglers the following June due to a combination of migration and mortality. Emigration from delayed harvest study reaches was associated with stocking and high-flow events. Multi-state modeling detailed these observations with weekly estimates of migration and survival rates. River-specific differences in emigration and mortality suggested that emigration was a greater source of trout loss than mortality in all rivers; no pattern related to river size was apparent in emigration, but mortality was greater in small streams. Brook Trout mortality rates were highest among the three species, and large fish of most species showed higher emigration and mortality than catchable-sized trout. Fisheries managers can apply our results to alter stocking regimes so as to enhance the efficiency of stocking and the acclimation of stocked trout to instream environments.

North Carolina↗
USGS2018

Acoustic telemetry and benthic habitat mapping inform the spatial ecology of Shortnose Sturgeon in the Hudson River, New York, USA

A history of overexploitation and industrialization of riverine habitats has impacted the Shortnose Sturgeon Acipenser brevirostrum , leading this species to become one of the earliest listed under the U.S. Endangered Species Act. The present understanding of Shortnose Sturgeon spatial ecology is based on observations from a limited number of Atlantic coastal rivers. To better understand Shortnose Sturgeon in the Hudson River, New York, we used acoustic telemetry to characterize seasonal habitat use and to identify regions of the river where seasonal sturgeon activity occurred. From 2012 to 2016, 101 adult fish were tagged and tracked, and sturgeon detections per unit effort (a metric of fish observation standardized by search effort) were evaluated against benthic habitat variables by using generalized additive regression models. Models indicated strong habitat associations in the spring season defined by gravel-dominated substrates and specific depth ranges, presumably associated with spawning activity. During summer, Shortnose Sturgeon were more dispersed, associating with muddy habitats, whereas in fall/winter, sturgeon congregated in specific regions of the river. These data demonstrate that river use and habitat associations vary seasonally and identify important areas for managing overlap between seasonal habitat use by Shortnose Sturgeon and human activity on the river.

New York↗
USGS2018

Neutral genetic and phenotypic variation within and among isolated headwater Brook Trout populations

Isolated populations are challenging to manage and conserve as they are particularly vulnerable to genetic drift, allelic fixation, inbreeding, and may express markedly reduced phenotypic variability. We sought to improve our understanding of how spatial isolation, occupancy range, and restricted gene flow influence contemporary phenotypic variation within and among native populations of Brook Trout Salvelinus fontinalis by examining the neutral genetic and phenotypic characteristics of 35 isolated headwater populations from Great Smoky Mountains National Park. Across a suite of 13 neutral microsatellite loci, we observed high levels of allelic fixation and considerable genetic differentiation among populations, subwatersheds, and watersheds that were consistent with patterns of isolation. We observed significant, positive correlations between allelic diversity and estimates of effective population sizes. In contrast, we observed considerably less phenotypic structure among streams, subwatersheds, and watersheds. Much of the phenotypic variation observed occurred among individuals within populations. Pairwise Mann‐Whitney tests revealed no significant phenotypic differences among the populations of Brook Trout we examined. Similarly, we observed no significant relationship between the amount of phenotypic variation within populations and any of the examined measures of genetic diversity or the amount of occupied habitat sampled, which suggests that unmeasured variables may be influencing morphometric and meristic variation within isolated populations. The observed patterns of isolation, genetic drift, and allelic fixation highlight the importance of enhancing population connectivity, but also suggest considerable phenotypic variability may persist within small, fragmented populations. Our results elucidate some challenges associated with managing and conserving isolated populations of Brook Trout, and reinforce the importance of conducting genetic studies on fragmented populations to inform management decisions.

Transactions of the American Fisheries Society↗
USGS2018

River reach restored by dam removal offers suitable spawning habitat for endangered Shortnose Sturgeon

The lowermost dam on the Penobscot River, Maine, was removed in 2013, making new habitat available for migratory fish. There is no evidence that endangered Shortnose Sturgeon Acipenser brevirostrum have spawned in the Penobscot River in recent years, but dam removal has facilitated access to potential freshwater habitat essential for spawning. Spawning success also depends on the quality of the available habitat. We sought to describe the distribution and amount of suitable spawning habitat in the first 5-km reach upstream of the removed dam. Previously collected river elevation and bottom substrate data were used to create two-dimensional hydrodynamic simulations of the reach for spring discharges ranging from 310 to 1480 m3 s-1 using the program River2D. Simulations were validated and adjusted using field-collected data. Suitable spawning habitat was predicted based on literature-informed suitability curves of depth, velocity, and bottom substrate. Between 41% and 63% of the study area offered usable spawning habitat, depending on river discharge. Velocity was the most limiting characteristic to overall suitability at all modeled discharges. Embeddedness was minimal at suitable sites. Based on the habitat characteristics considered, the newly accessible reach of the Penobscot River could support Shortnose Sturgeon spawning, offering critical habitat for this endangered species.

Transactions of the American Fisheries Society↗
USGS2019

Freshwater tidal forests and estuarine wetlands may confer early life growth advantages for delta-reared Chinook Salmon

Large river deltas are complex ecosystems that are believed to play a pivotal role in promoting the early marine growth and survival of threatened Chinook Salmon Oncorhynchus tshawytscha . We used a fish bioenergetics model to assess the functional role of multiple delta habitats across a gradient of salinities and vegetation types, where consumption and growth rate potential (GRP) were considered as proxies for habitat quality. We subsequently compared our model output to empirical, or realized , growth estimates from scale circuli. In terms of consumption, prey energy density (ED prey ) was 46–86% higher in tidal freshwater forest than in any other habitat type, while estimated consumption rates (expressed as proportion of maximum daily consumption; P cmax ) were positively correlated with FL. These size‐related differences in P cmax led to a noticeable increase along a freshwater‐to‐saline gradient from roughly 0.25 in tidal freshwater forest to 0.55 in the offshore subtidal zone, yet despite higher observed P cmax values in nearshore and offshore habitats, the tidal freshwater forest and emergent salt marsh demonstrated the highest modeled GRP values. Similarly, realized growth rates for fish caught in tidal freshwater forest were up to 0.5% higher per day than for fish caught in the offshore area, but habitat‐level differences were overshadowed by allometry and rearing origin. Scales from unmarked fish (assumed to be of wild origin) indicated that they grew, on average, 11% faster than did hatchery fish; however, these differences were subtle and were more obvious at fork lengths <100 mm. Our results suggest that tidal forests and emergent marshes may offer early life growth advantages for wild Chinook Salmon, but that wild and hatchery fish can compensate as they move seaward by opportunistically consuming greater quantities of low‐energy density prey, taking advantage of pulses of larval forage fish, or by spending time in multiple interconnected habitat types.

Transactions of the American Fisheries Society↗
USGS2019

Acidification impacts and goals for gauging recovery of Brook Trout populations and fish communities in streams of the Western Adirondack Mountains, New York, USA

Results from several long‐term monitoring programs in the western Adirondack Mountains, New York, indicate that acid–base chemistry of headwater streams has remained unchanged or improved only marginally since the 1990s. A paucity of quantitative fishery data, however, limits our understanding of the pre‐acidified communities as well as present‐day impacts of acidification on fish assemblages, which impedes efforts to evaluate temporal trends and biological recovery in streams of the region. Fish communities were characterized at 48 streams (chemistry was assessed at 47 streams) in the western Adirondacks at least once during summer 2014–2016 to assess present‐day effects of acidification on fish assemblages, refine important relations, and identify biological targets and chemical effect thresholds that could help gauge biological recovery across the region. Concentrations of inorganic aluminum (Al i ) exceeded chronic and acute toxicity thresholds (1.0 and 2.0 μmol/L) in 21.3% and 8.5%, respectively, of 47 study streams sampled during summer 2014–2016 and in 64.0% and 44.0% of 25 streams sampled during spring 2014–2015. In streams with summer Al i concentrations less than 1.0 μmol/L, community richness, density, and biomass averaged 2.0 species, 444.2 fish/0.1 ha, and 1,924.4 g/0.1 ha, respectively, whereas density and biomass of Brook Trout Salvelinus fontinalis populations averaged 280.8 fish/0.1 ha and 1,384.0 g/0.1 ha, respectively. These findings identify defensible targets for biological recovery and show that Al i toxicity is not a major concern for fish assemblages in most streams during summer base flow periods but is potentially a serious issue for fish in as many as two‐thirds of streams during spring high flows. Though additional data are needed to address several limitations and information gaps, results from this study provide a sound foundation to gauge biological recovery, detect future effects of climatic stressors, and help ensure that functional stream ecosystems can be sustained or restored in parts of the Adirondacks.

New York↗
USGS2019

A natural‐origin steelhead population's response to exclusion of hatchery fish

It is asserted that reduction or elimination of hatchery stocking will increase natural‐origin salmon Oncorhynchus spp. and steelhead O. mykiss production. We conducted an analysis of steelhead population census data (1958–2017) to determine whether elimination of summer steelhead stocking in the upper Clackamas River in 1998 increased the productivity of natural‐origin winter steelhead. A Bayesian state–space stock–recruitment model was fitted to the adult steelhead data set, and productivity was estimated as a function of hatchery‐origin spawner abundance as well as other environmental factors. When used as a predictive variable in our model, the abundance of hatchery summer steelhead spawners (1972–2001) did not have a negative effect on winter steelhead recruitment. However, spill at North Fork Dam (the gateway to the upper Clackamas River basin) and the Pacific Decadal Oscillation (an index of ocean conditions) were both negatively associated with winter steelhead recruitment. Moreover, winter steelhead abundance in the upper Clackamas River basin failed to rebound to abundances observed in years prior to the hatchery program, and fluctuations in winter steelhead abundance were correlated with those of other regional winter steelhead stocks. Our assessment underscores the need for studies that (1) directly quantify the effects of hatchery fish on the production of natural‐origin salmon and steelhead, (2) empirically test published theories about mechanisms of hatchery fish impacts on natural‐origin populations, and (3) document population responses to major changes in hatchery programs.

Oregon↗
USGS2019

Using the Distinct Population Segment concept to protect fishes with low levels of genomic differentiation: conservation of an endemic minnow (Hitch, Lavinia exilicauda)

In the United States, the Endangered Species Act (ESA) of 1973 was enacted to conserve species which are endangered or threatened throughout all or a portion of their range. The definition of ‘species’ includes subspecies and distinct population segments (DPSs). In freshwater fishes, use of DPS designations has largely been restricted to salmonid fishes (Salmonidae), although the DPS concept is increasingly applied to other fishes as well. As more taxa approach threatened status, the difficult question becomes what to do when genetic evidence does not strongly support formal taxonomic designations (full species or subspecies). We examine the potential use of the DPS concept to protect fishes using the example of Hitch ( Lavinia exilicauda ), a cyprinid fish endemic to California. The Hitch is divided up into three formally described, geographically separated subspecies. However, genomic studies (RADseq), presented here with three independent analyses using a large data set, only weakly support subspecies designations. Results suggest population but not subspecies structure. Nevertheless, conventional taxonomic methods, strong contemporary isolation, the importance of protecting genetic diversity, and high cultural values still qualify all three designated subspecies for DPS status and demonstrate how taxa such as Clear Lake Hitch can warrant protection under the ESA.

Transactions of the American Fisheries Society↗
USGS2019

Assessment of operational and structural factors influencing performance of fish collectors in forebays of high-head dams

Providing efficient downstream passage is critical for improving populations of migratory fishes in impounded river systems. High‐head dams, such as those used for water storage or flood‐risk management, pose unique passage challenges requiring unique solutions. Systems to collect fish in dam forebays (“forebay collectors”) for transport to downstream release locations have been used at some high‐head dams in the western United States since the 1950s. Collection efficiency of these facilities has ranged from nearly 0% to 100%, suggesting the need for a better understanding of factors affecting performance in these complex environments if they are to be designed and deployed at new sites. We compiled information on environmental, structural, and performance characteristics of seven existing forebay collectors to quantify factors affecting their performance based on a meta‐analysis using a data set containing 52 separate collection estimates. Covariates included species type (steelhead Oncorhynchus mykiss, Chinook Salmon O. tshawytscha, Coho Salmon O. kisutch, and Sockeye Salmon O. nerka), collector inflow, collector entrance area, relative size of the dam forebay, and whether or not nets were used to enhance collection. We found that inflow, the use of lead nets, the size of the collector entrance area, the relative size of the dam forebay, and the interaction between collector entrance and forebay areas were significant predictors of collection performance. There was also evidence for differences between species. Chinook Salmon exhibited the lowest collection rates among the projects we examined, while steelhead collection rates were highest. These results provide guidance to design more efficient forebay collectors and improve the success of existing systems.

Oregon, Washington↗
USGS2019

Fish growth changes over time in a Midwestern U.S. lake

Growth of Walleyes Sander vitreus, Yellow Bass Morone mississippiensis, Common Carp Cyprinus carpio, and Black Bullheads Ameiurus melas was assessed in Clear Lake, Iowa, over several decades and in relation to environmental variables. Growth of Common Carp was positively correlated with phytoplankton concentration. Recent Black Bullhead growth was faster than in the 1950s and 1990s, which may be a consequence of their recent decline in abundance. Growth of Common Carp and Yellow Bass was faster in the 1940s than in more recent time periods. Relative to their entire range, Common Carp first‐year growth was below average, whereas length at later ages was above average. Walleye relative growth showed a similar pattern. The large changes in growth over several decades suggest that as the Clear Lake ecosystem continues to change, growth rates of its important fish species are also likely to continue changing.

Transactions of the American Fisheries Society↗
USGS2019

Vertical habitat use of adult Walleye conflicts with expectations from fishery-independent surveys

Stock assessments of Walleyes Sander vitreus in Lake Erie rely on a combination of suspended and bottom overnight gill‐net surveys to provide population and demographic information. However, the assumption that Walleyes undertake diel vertical migrations and become available to the suspended gill nets at night has never been validated. To understand how vertical habitat use affects the availability of Walleyes to fishery‐independent surveys, we compared individual behaviors observed by means of acoustic depth telemetry tags (20 individuals; 2013–2016) with catches in paired suspended and bottom gill‐net sets (273 paired sets; 2013–2016) used by management agencies. In contrast to our expectations and observations in other lakes, the mean depths for Walleyes most often occurred in the lower one‐half to one‐third of the water column, and at lake depths <25 m the fish tended to be close to the bottom. The relationship between fish and lake depth was dependent on year and season. At lake depths >15 m, Walleyes were found at shallower depths during stratified periods (i.e., summer) than during unstratified periods (autumn). They were also found at shallower depths in 2015 and 2016 than in 2013 and 2014. In paired autumn gill‐net surveys, (1) the overall proportion of Walleye catch was nearly equal in suspended and bottom gill nets and (2) the proportion of the catch in suspended gill nets declined with fish length. The pattern of decline was dependent on how deep the suspended net was fished and the year in which the data were collected. These results provide evidence that the suspended gill‐net surveys currently being used to assess Walleye demographics in Lake Erie are biased toward the capture small Walleyes, while bottom nets are biased toward the capture of large ones. Future telemetry investigations will be needed to understand whether these biases reflect differences in the depth of habitat between small and large Walleyes.

Lake Erie↗
USGS2019

Temperature–not flow–predicts native fish reproduction with Implications for climate change

Habitat alterations and introduction of nonnative fishes reduced the distributions of the Flannelmouth Sucker Catostomus latipinnis , Bluehead Sucker C. discobolus , and Roundtail Chub Gila robusta to less than 50% of their historical ranges. Climate change models generally predict decreased streamflows and increased water temperatures that may further affect these species. Understanding the effects of flow and water temperature on their life histories should lead to better assessments of climate change impacts on extant populations and more informed management for species conservation. Basinwide larval fish sampling and hatch dates derived from otolith daily increment counts showed that water temperature was the dominant environmental factor cueing reproduction in the upper White River basin, Colorado. Reproduction for all three species began in spring, occurring first at warmer, lower-elevation, downstream locations and progressing upriver to higher elevations as water temperatures increased. Warmer water temperatures in tributaries initiated earlier reproductive activity compared to adjacent cooler main-stem habitat. Presence of larvae in samples and estimated hatch dates demonstrated a distinct, predictable upstream progression of reproduction associated with warming water and clear upstream limits to reproduction for all three species. Larval presence and hatching dates revealed earlier reproductive activity in 2012 than in 2013, driven by lower flow and earlier stream warming. A regression model predicted stream temperature during fish spawning seasons under different climate change scenarios and showed expanded upstream limits of thermally suitable reproductive habitat and earlier reproduction for our study species. The long-term implications of climate change are unknown, but managers should strive to perpetuate the valuable and relatively pristine native fish community in the upper White River drainage as a vestige of those that formerly existed throughout the Colorado River basin.

Colorado↗
USGS2019

Integrating fish assemblage data, modeled stream temperatures, and thermal tolerance metrics to develop thermal guilds for water temperature regulation: Wyoming case study

Many streams are experiencing increased average temperatures due to anthropogenic activity and climate change. As a result, surface water temperature regulation is critical for preserving a diverse stream fish species assemblage. The development of temperature regulations has generally been based on laboratory measurements of individual species' thermal tolerances rather than community response to temperature in the field, despite multiple limitations of using laboratory data for this purpose. Using field data to develop temperature regulations may avoid some of the limitations of laboratory data, but the use of field data comes with additional challenges that prevent its widespread adoption. We used Wyoming stream fish assemblages as a case study to examine the feasibility of addressing the limitations of field and laboratory data through a hybrid approach that integrates both types of data to classify species into thermal guilds that can potentially inform regulatory standards. We identified coldwater, coolwater, and warmwater classes of sites with modeled mean August temperatures of <15.5, 15.5–19.9, and >19.9°C, respectively. We used species' associations with these temperature classes to place species into site‐groups. Finally, we used standardized laboratory measures of species' upper acute and chronic thermal tolerances to identify and reclassify species with unusual thermal distributions. Through this process we classified species into five thermal guilds that may be useful for surface water temperature regulation in Wyoming. Our approach addresses the limitations identified for field and laboratory data and demonstrates a framework that could be used for incorporating multiple types of data to develop temperature standards.

Wyoming↗
USGS2019

Growth, condition, and trophic relations of stocked trout in southern Appalachian mountain streams

Stream trout fisheries are among the most popular and valuable in the United States, but many are dependent on hatcheries to sustain fishing and harvest. Thus, understanding the ecology of hatchery‐reared trout stocked in natural environments is fundamental to management. We evaluated the growth, condition, and trophic relations of Brook Trout Salvelinus fontinalis , Brown Trout Salmo trutta , and Rainbow Trout Oncorhynchus mykiss that were stocked in southern Appalachian Mountain streams in western North Carolina. Stocked and wild (naturalized) trout were sampled over time (monthly; September 2012–June 2013) to compare condition and diet composition and to evaluate temporal dynamics of trophic position with stable isotope analysis. Relative weights ( W r ) of stocked trout were inversely associated with their stream residence time but were consistently higher than those of wild trout. Weight loss of harvested stocked trout was similar among species and sizes, but fish stocked earlier lost more weight. Overall, 40% of 141 stomachs from stocked trout were empty compared to 15% of wild trout stomachs ( N = 26). We identified a much higher rate of piscivory in wild trout (18 times that of stocked trout), and wild trout were 4.3 times more likely to consume gastropods relative to stocked trout. Hatchery‐reared trout were isotopically similar to co‐occurring wild fish for both δ 13 C and δ 15 N values but were less variable than wild trout. Differences in sulfur isotope ratios (δ 34 S) between wild and hatchery‐reared trout indicated that the diets of wild fish were enriched in δ 34 S relative to the diets of hatchery‐reared fish. Although hatchery‐reared trout consumed prey items similar to those of wild fish, differences in consumption or behavior (e.g., reduced feeding) may have resulted in lower condition and negative growth. These findings provide critical insight on the trophic dynamics of stocked trout and may assist in developing and enhancing stream trout fisheries.

North Carolina↗
USGS2019

Densities, diets, and growth rates of larval Alewife and Bloater in a changing Lake Michigan ecosystem.

Variability in abiotic and biotic factors during larval stages has profound impacts on fish recruitment. In Lake Michigan, where the composition of lower trophic levels has undergone considerable changes in the past decade, managers are concerned that fish recruitment could be negatively affected. We hypothesized that spatial variation in Lake Michigan larval fish density and growth can be explained by various environmental predictor variables. In July 2015, we sampled larval fish and zooplankton at 24 sites (distributed among eight transects) around Lake Michigan. We measured larval fish densities and estimated growth rates and diets of the two most abundant species: the Alewife Alosa pseudoharengus and Bloater Coregonus hoyi (prey fish that represented 89% and 4% of the total catch, respectively). Larval Alewife densities at a given site ranged from 0 to 42.57 larvae/100 m 3 , but no explanatory variables explained the variation. Alewife mean growth rate equaled 0.50 mm/d, and fish age and zooplankton density best explained growth variation across sites. Larval Bloater densities ranged from 0 to 1.16 larvae/100 m 3 , and mean growth rate was 0.21 mm/d. Across all sites, 67% of larval Alewife stomachs were empty, whereas only 16% of Bloater stomachs were empty. Larval fish growth rates observed in our study were at least 40% slower than those reported in previous decades for both Alewife and Bloater. Worsening prey environment for pelagic larvae, such as Alewife and Bloater, during the era of abundant dreissenid mussels could reduce the probability of strong year‐classes, which in turn may affect growth and survival of recreationally important salmonine predators.

Illinois, Michigan, Wisconsin↗