Objective Marking fish allows biologists to track individuals or cohorts through time and space and is often a requirement for fishery evaluations; however, the type of mark used may vary. Chemical marking of hard structures with calcein is one method used by biologists, and this technique benefits from the ability to mark many individuals with relatively low time commitment and expense. However, long-term retention is not well understood, and external marks can quickly degrade in sunlight. Our objective was to assess retention of internal calcein marks on the otoliths of Brook Trout Salvelinus fontinalis 926 d after initial marking and to determine whether exposure to sunlight affected this retention.Methods Juvenile Brook Trout were immersed in a calcein solution for initial marking before being placed into one of two outdoor raceways. One raceway was exposed to sunlight for the duration of the study, while the other was covered with a black shade cloth. After 2.5 years of grow out in the raceways, otoliths were removed and polished. Multiple observers then performed a blind examination to assess the presence or absence of calcein marks on otoliths. We calculated inter-reader agreement to assess reliability among readers, and we examined the effect of sunlight exposure on the likelihood of correct identification using a generalized linear mixed model.Results About 72% of otoliths had fluorescent marks, and there was no difference in the probability of identifying marks in fish that were exposed to sunlight and those not exposed to sunlight. Further, agreement was high among the three independent readers.Conclusions Collectively, our results suggest that calcein marks on internal structures may be useful for marking fishes over a length of time necessary to answer fisheries management questions. Calcein marking of juvenile Brook Trout can be easily performed in hatchery settings on large numbers of fish at once. These internal marks may be retained for over 2 years and can help managers to differentiate between different groups of stocked fish.

Fish assemblages, defined by taxonomy or functional traits, -respond to regional and local habitat variation. Our hypothesis was that fish assemblages could be best predicted using reach-scale (RS) hydrology variables over valley-scale (VS) hydrogeomorphology variables for US and Mongolian rivers. We further predicted that fish traits were predicted better by RS than VS variables. We evaluated the FS and VS hydrogeomorphologies of rivers in the United States and Mongolia in each of three ecoregions: grassland, forest and endorheic. Fishes were collected using a backpack electrofisher, following standard protocols. Constrained ordination analyses were conducted at three scales: among continents, by continents and by individual ecoregions within continents. We found no significant difference in mean variation explained by VS versus RS or by taxonomy versus traits. Ecoregions differed in factors contributing to fish assemblage patterns, likely a result of differences in hydrogeomorphology, hydrological connectivity and historical influences. We found that fish assemblages were structured by hydrogeomorphic processes occurring at VS and RS, and that variables predicting fish assemblages varied with scale and whether fishes were classified by taxonomy or traits. Although anthropogenic impacts were substantially higher for western US rivers than for Mongolian rivers, we were unable to detect strong differences in our ability to predict fish assemblage variation from RS and VS habitat variables.

OBJECTIVE:We sought to evaluate accurate and reproducible detection of Myxobolus cerebralis (Mc), the causative agent of whirling disease, by using nested polymerase chain reaction (nPCR) and three previously established real-time quantitative PCR (qPCR) assays: K18S (Kelley 18S), C18S (Cavender 18S), and Hsp70 (heat shock protein 70). We used a "fit for purpose" approach combined with intra- and interlaboratory testing to identify a molecular testing method that would be equivalent to the currently accepted nPCR procedure for Mc. METHODS:Assay performance was compared using a combination of intra- and interlaboratory testing that used synthetic gBlocks along with naturally and experimentally infected fish tissue. North American isolates representing geographically distinct locations were also tested using all three assays. RESULT:The K18S and C18S assays exhibited high assay sensitivity, intra- and interlaboratory repeatability of sample replicates, and reproducible identification of all test samples across multiple laboratories. In contrast, the Hsp70 assay failed to detect several positive samples at low DNA concentrations during intra- and interlaboratory testing. The K18S assay was the only procedure that demonstrated perfect detection accuracy when testing geographically distinct Mc isolates. Results demonstrated the K18S assay is robust under variable test conditions, is more accurate than the C18S and Hsp70 assays, and provides detection capabilities equivalent to those of the currently accepted nPCR confirmation assay "gold standard" that is described in the American Fisheries Society-Fish Health Section (AFS-FHS) Blue Book. CONCLUSION:The "fit for purpose" approach and preliminary completion of the World Organization for Animal Health validation pathway demonstrate that the K18S assay provides an alternate method for Mc testing. This work provides the foundation for acceptance of the K18S assay into the AFS-FHS Blue Book as a standardized test procedure for Mc.

We used bomb calorimetry to quantify the energy density of three Prosopium fish species endemic to Bear Lake, Utah- Idaho, that we collected in 2020-2021: Bear Lake Whitefish Prosopium abyssicola, Bonneville Whitefish Prosopium spilonotus, and Bonneville Cisco Prosopium gemmifer. We found that mean & PLUSMN; standard deviation wet weight energy densities were 6,312 & PLUSMN; 760 J/g for Bear Lake Whitefish; 5,301 & PLUSMN; 778 J/g for Bonneville Whitefish; and 4,743 & PLUSMN; 443 J/g for Bonneville Cisco. We built linear mixed models and found relationships between energy density and dry matter ratio (i.e., ratio of dried weight to wet weight of a fish) for all three species, suggesting that the energy density of future samples collected in Bear Lake could potentially be determined from comparisons between the dried and wet weight of fishes belonging to these species. Our results are useful for future bioenergetics modeling with these three Bear Lake endemic species and potentially with others species in related genera that share similar feeding, behavior, and life-history traits.

ObjectiveJuvenile tiger muskellunge (Northern Pike Esox lucius x Muskellunge E. masquinongy) are cannibalistic, which can lead to a substantial reduction in yields within an intensive culture environment. Utah hatcheries have traditionally raised the female Northern Pike x male Muskellunge cross, and hatchery managers are reluctant to raise the female Muskellunge x male Northern Pike due to a perceived difference (increase) in cannibalism rates when rearing this cross.MethodsWe raised both crosses in replicated microcosms to evaluate the differential cannibalism between them. All rearing conditions were consistent among treatments and replicates. We evaluated weekly cannibalism using generalized linear regression models.ResultAfter 6 weeks, we found that the female Muskellunge x male Northern Pike cross offspring were significantly more cannibalistic than juveniles from the female Northern Pike x male Muskellunge cross, with total cannibalism across the 6-week period estimated at 27% and 5.6%, respectively.ConclusionWe documented a previously unreported difference in tiger muskellunge cross cannibalism rates. Choosing the female Northern Pike x male Muskellunge cross could substantially increase survival of juvenile tiger muskellunge in aquaculture facilities. Some fish species are cannibalistic when raised in crowded aquaculture environments. Although this is understood by hatchery professionals, questions remain about best strategies to reduce cannibalism and maximize yield. We compared cannibalism rates of two tiger muskellunge crosses (reciprocal parentage) and found that the female Northern Pike cross was significantly less cannibalistic than the female Muskellunge cross. Raising the female Northern Pike cross instead of the reciprocal could substantially increase yields and the efficiency of hatchery programs that raise this hybrid.Impact statement

Quantifying the trophic basis of production for freshwater metazoa at broad spatial scales is key to understanding ecosystem function and has been a research priority for decades. However, previous lotic food web studies have been limited by geographic coverage or methodological constraints. We used compound-specific stable carbon isotope analysis of amino acids (AAs) to estimate basal resource contributions to fish consumers in streams spanning grassland, montane and semi-arid ecoregions of the temperate steppe biome on two continents. Across a range of stream sizes and light regimes, we found consistent trophic importance of aquatic resources. Essential AAs of heterotrophic microbial origin generally provided secondary support for fishes, while terrestrial carbon did not seem to provide significant, direct support. These findings provide strong evidence for the dominant contribution of carbon to higher-order consumers by aquatic autochthonous resources (primarily) and heterotrophic microbial communities (secondarily) in temperate steppe streams.

Silver carp (Hypophthalmichthys molitrix) dispersed to the Wabash River, Indiana in the mid-1990s. These fish compete with native, filter-feeding fish species in other central US rivers but relatively little information is available for impacts in the Wabash River. We collected fishes by boat electrofisher on the Wabash River as part of a long-term study. Abundances of six Wabash River species varied temporally, and three had stronger relationships with year than the other species: common carp (Cyprinus carpio) decreased before and after silver carp introduction, freshwater drum (Aplodinotus grunniens) increased prior to silver carp introduction, and gizzard shad (Dorosoma cepedianum) decreased before silver carp introduction. Silver carp abundance was significantly correlated only with abundance of gizzard shad. Nonmetric multidimensional scaling analysis of long-term Wabash River fish assemblages resulted in distinctive temporal and spatial patterns. Silver carp abundance increased from zero in the early 1990s to 28 individuals/km in recent years. Gizzard shad declined to low abundance simultaneous to silver carp increases, yet we did not detect significant patterns for silver carp with other Wabash River fishes.

Abstract Stream fishes are restricted to specific environments with appropriate habitats for feeding and reproduction. Interactions between streams and surrounding landscapes influence the availability and type of fish habitat, nutrient concentrations, suspended solids, and substrate composition. Valley width and gradient are geomorphological variables that influence the frequency and intensity that a stream interacts with the surrounding landscape. For example, in constrained valleys, canyon walls are steeply sloped and valleys are narrow, limiting the movement of water into riparian zones. Wide valleys have long, flat floodplains that are inundated with high discharge. We tested for differences in fish assemblages with geomorphology variation among stream sites. We selected rivers in similar forested and endorheic ecoregion types of the United States and Mongolia. Sites where we collected were defined as geomorphologically unique river segments (i.e., functional process zones; FPZs) using an automated ArcGIS‐based tool. This tool extracts geomorphic variables at the valley and catchment scales and uses them to cluster stream segments based on their similarity. We collected a representative fish sample from replicates of FPZs. Then, we used constrained ordinations to determine whether river geomorphology could predict fish assemblage variation. Our constrained ordination approach using geomorphology to predict fish assemblages resulted in significance using fish taxonomy and traits in several watersheds. The watersheds where constrained ordinations were not successful were next analyzed with unconstrained ordinations to examine patterns among fish taxonomy and traits with geomorphology variables. Common geomorphology variables as predictors for taxonomic fish assemblages were river gradient, valley width, and valley slope. Significant geomorphology predictors of functional traits were valley width‐to‐floor width ratio, elevation, gradient, and channel sinuosity. These results provide evidence that fish assemblages respond similarly and strongly to geomorphic variables on two continents.

The importance of monitoring anthropogenic changes in a lotic system is not limited to chemical water quality monitoring. The addition of biological monitoring allows fish to be used as bioindicators because of their varying tolerance to pollution. For this study, we utilized long-term water quality and fish data to evaluate temporal changes brought on by passage of the Clean Water Act (1972). Non-metric multidimensional scaling (NMS) was used to describe changes in the fish community and also heavy metal concentrations of the West Fork White River in Muncie, Indiana, USA, over the past 33 years. A linear mixed effects model was used to evaluate the relationship between heavy metal concentrations and the fish community. The NMS results for both heavy metals and fish were separated into distinct decadal clusters. The shift in fish community data represented by NMS axis 1 was characterized by a drop in pollution-tolerant species and an increase in intolerant species. Decreases in heavy metal concentrations of chromium, zinc, and lead were also significant predictors of changes in the fish community. All NMS fish axis had a positive slope indicating an increase in intolerant species as heavy metal concentrations decreased. Our findings indicate that the water quality improvements documented in the West Fork White River have directly impacted its local fish community.

Freshwater drum (Aplodinotus grunniens) are native to wide, slow, meandering rivers of North America. Drastic habitat changes along with the expansion of metropolitan areas may have an impact on the growth rates or population structure of this species. To characterize these populations, we collected freshwater drum (n = 83) during the months of June and July 2013, using hook and line methods. We used analysis of variance to compare age, physical measurements, and volume of prey items consumed by fish, among sample sites. We used analysis of covariance to examine differences in growth among sample sites and sexes. Although we observed no significant variations in growth rates, there were differences in weight, relative weight, and prey among sites. Invasive Corbicula fluminea and Dreissena polymorpha were found in the guts of many drum and comprised all identifiable bivalve shells found in this study.