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ABSTRACT Fish stocking is a common management practice used to mitigate the effects of overfishing and other threats to wild fish populations. It is mainly employed in freshwater ecosystems with complex and variable strategies (different timings, fish lengths and origins), making its effectiveness challenging to assess. To assess and evaluate the prevalence of artificially stocked individuals in a population with a complex management history (pikeperch—Stizostedion lucioperca—from Lipno Reservoir, Czechia), laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) was used to measure the micro‐elemental composition at the core of otoliths to verify the natal origin of fish (wild and stocked individuals) and confirm the origin of fish sources (facilities). Of the ten elements investigated, nine (Ba/Ca, Cu/Ca, K/Ca, Li/Ca, Mg/Ca, Mn/Ca, Na/Ca, Rb/Ca and Sr/Ca) exhibited significant inter‐origin variation, whereas Zn/Ca failed to provide a meaningful discriminatory signal. A machine learning classification algorithm, trained with data from 100 fish of known origin, was used to classify 70 fish of unknown origin. The five most relevant variables for correctly classifying fish origins were Rb/Ca, Li/Ca, Sr/Ca, Ba/Ca and Mg/Ca. The model achieved an overall accuracy of 99%, with 97% accuracy for wild fish and 100% for stocked fish. For identifying exact fish sources, overall accuracy for stocking facilities was approximately 89%, with individual facility detection accuracy ranging from 28% to 100%. The model determined that most fish of unknown origin were autochthonous (57 individuals, 81.4%), with a smaller proportion identified as stocked fish (13 individuals, 18.6%). Given the relatively small contribution of stocked fish to the pikeperch population, the efficiency of the stocking programme should be further scrutinised to avoid ineffective management strategies. More broadly, these results highlight that otolith microchemistry combined with machine learning provides a robust framework for evaluating stocking efficiency and supporting evidence‐based fisheries management.

Large, dominant fish species that are the basis of many fisheries may be naturally so successful due partly to \"cultivation effects,\" where adults crop down forage species that are potential competitors/predators of their own juveniles. Such effects imply a converse impact when adult abundance is severely reduced by fishing: increases in forage species may then cause lagged, apparently depensatory decreases in juvenile survival. Depensatory effects can then delay or prevent stock rebuilding. Cultivation effects are apparently common in freshwater communities and may also explain low recruitment success following severe declines of some major marine stocks such as Newfoundland Atlantic cod (Gadus morhua). Risk of depensatory effects should be a major target of recruitment research, and management policies should aim for considerably higher spawning abundances than has previously been assumed necessary based on recruitment data collected during adult stock declines associated with fishery development.

Data from nine reservoirs in the Czech Republic were used to investigate density and size distribution patterns for dominant fish species of the juvenile pelagic community at night. Clear trends of increasing density along the longitudinal gradient for bream, roach, bleak, and pikeperch were observed in long (>5 km) reservoirs but were absent in short reservoirs. Increasing body size towards the tributary was observed for bream, perch, pikeperch, and ruffe. Cyprinids showed a clear affinity to the surface layer (0–3 m) of long reservoirs, while pikeperch and ruffe juveniles occurred homogeneously at depths down to six meters. Perch juveniles had the highest densities especially in 3–6 m water layer. Size did not change with sampling depth for any species in the reservoirs studied. Our data showed gentle spatial segregation of different juvenile fish species. Although the tributary area is usually the most productive, it is not able to support all juveniles leading to inter- and intra-specific competition and the relegation of juveniles of some species to less productive habitat. This trend was more apparent in long reservoirs with strong trophic gradients. The gradient of nutrients seems to be an important factor driving fish distribution along longitudinal profile of reservoirs.

Sexual size dimorphism of walleye (Stizostedion vitreum vitreum) is achieved by females having higher growth rates than males before and after maturation, resulting in females with greater asymptotic sizes. Mercury (Hg) concentrations in epaxial muscle by age and weight for 31 populations of walleye were used to evaluate the relative importance of differences in consumption and activity in generating sexual size dimorphism. Growth efficiency by sex, age, and maturity is estimated by a ratio of annual increments in weight (g) to annual increments of Hg (mg), using the pooled changes in weight and Hg loadings of males and females from all lakes. The higher growth rates of females arise from greater consumption and higher growth efficiency. Growth efficiency of both sexes is similar before maturity, but the growth efficiency of mature males is substantially lower than that of either immature males or mature females. We propose that the inferior growth efficiency of males is a function of the greater activity of males, particularly during the spawning season when scramble competition for fertilization is likely to produce substantial increases in male fitness as a result of increased efforts to find and spawn with females.

The occurrence of cyanobacteria is particularly characteristic of shallow eutrophic waters, and they often form massive ‘blooms’ that can affect aquatic invertebrates and fish. However, even a low abundance of cyanobacteria can be hazardous to aquatic organisms, due to the production of toxic metabolites. The aim of this study was to investigate the relationship between cyanobacteria and their toxicity (biological activity) towards zooplankton and fish communities, when only low concentrations of cyanobacterial chlorophyll a (less than 20 μg L-1) are detected, i.e. in sub-bloom conditions. Measurements were performed in Sulejow Reservoir (Central Poland), a shallow, lowland, eutrophic reservoir, in which cyanobacterial blooms occur regularly. Fish were assessed using echo-sounding (distribution) and by gillnetting (species composition). Simultaneously, zooplankton, cyanobacteria and physico-chemical characteristics were studied at 14 stations situated along hydroacoustic transects. Parameters that characterized the cyanobacteria (cyanobacterial chlorophyll a concentration, the number of 16S rRNA and the mcyA gene copies and microcystin (MC) concentration) were consistently correlated (based on a principal component analysis), and the highest values were found in the downstream region of the study area. This ‘cyano-complex’ was also positively correlated with oxygen concentration, pH and phosphate levels, but was negatively correlated with temperature and the concentrations of nitrates and nitrites. In Sulejow Reservoir in 2013 the biomass of large zooplankton filter feeders decreased along with increasing MC concentration and fish densities, while small filter feeders did not present such relationships with regards to fish densities. Fish abundance tended to decrease at stations with a lower abundance of cyanobacteria and with growing toxic genotype copies and MC concentration.

Numerous models have been developed to predict the bioaccumulation of organic chemicals in fish. Although chemical dietary uptake can be modeled using assimilation efficiencies, bioaccumulation models fall into two distinct groups. The first group implicitly assumes that assimilation efficiencies describe the net chemical exchanges between fish and their food. These models describe chemical elimination as a lumped process that is independent of the fish's egestion rate or as a process that does not require an explicit fecal excretion term. The second group, however, explicitly assumes that assimilation efficiencies describe only actual chemical uptake and formulates chemical fecal and gill excretion as distinct, thermodynamically driven processes. After reviewing the derivations and assumptions of the algorithms that have been used to describe chemical dietary uptake of fish, their application, as implemented in 16 published bioaccumulation models, is analyzed for largemouth bass (Micropterus salmoides), walleye (Sander vitreus = Stizostedion vitreum), and rainbow trout (Oncorhynchus mykiss) that bioaccumulate an unspecified, poorly metabolized, hydrophobic chemical possessing a log KOW of 6.5 (i.e., a chemical similar to a pentachlorobiphenyl).

In this study, the concentrations of Se and Hg were determined in perch (Perca flavescens) and walleye (Stizosedion vitreum) muscle from nine lakes that varied in distance (4-204 km) from the metal smelters of Sudbury, Canada. Significant inverse relationships between Se and Hg in perch (r2 = 0.79, P < 0.05) and walleye tissue (r2 = 0.97, P < 0.01) were detected, which suggests a strong antagonistic effect of Se on Hg assimilation by these fish species. Concentration of Hg decreased exponentially with an increase of Se in fish muscle. Total dissolved Se concentrations of lake water declined with distance from smelters and were correlated to Se in perch (r2 = 0.75, P < 0.05) and walleye (r2 = 0.95, P < 0.01). Hg concentrations in the fish from lakes near the smelter were well below average values in fish in boreal shield lakes of this region.

The southern Saskatchewan mid-Miocene Wood Mountain Formation has been the basis of studies describing its mammalian and herpetofaunas; here we present the description of the fish material from the formation. The fauna, of probable Barstovian age, would have been isolated from the western side of the continent, in an area draining towards the northeast. Sixteen taxa are recognized to specific orders or lower taxonomic levels, although this number is approximate because some identifications are tentative. The fauna includes a lepisosteid (Lepisosteus), an amiine, a probable hiodontiform osteoglossomorph, a hiodontid (Hiodon), at least two unidentified cypriniforms and a leuciscine, three ictalurids (Ameiurus, Ictalurus, and cf. Noturus), an esocid Esox sp. more closely related to pikes than to pikerels, a possible moronid perciform, a centrarchid (cf. Pomoxis), two percids (including Stizostedion sp.), and an unidentified teleost. This fauna includes the earliest North American percids, the last occurrence of lepisosteid and amiine fossils in Canada west of the Great Lakes, and may constitute the earliest evidence of North American moronids. The assemblage is typical of well-oxygenated, lowland fluvial environments, and indicates a wide variety of substrates and flow strengths, as well as the presence of aquatic vegetation in the vicinity. Additionally, the fauna implies the nearby presence of turbid and deeper environments, suggesting that the area was an environmentally varied floodplain during deposition. The paleoenvironmental implications of this ichthyofauna are compatible with those of the herpetofauna: warm temperate to subtropical, with temperatures similar to those of northern Mississippi or southern Tennessee.

Double-crested Cormorants (Phalacrocorax auritus) increased dramatically in North America during the 1990s, providing the opportunity to study the effects of an increase of a top predator on an existing predator-prey system. In Oneida Lake, New York, USA, Double-crested Cormorants were first observed nesting in 1984 and had increased to over 360 nesting pairs by 2000. Concomitant with this increase in piscivorous birds was a decrease in the adult walleye (Stizostedion vitreum) and yellow perch (Perca flavescens) populations. Analysis of a 40-yr data series shows higher mortality of subadults (age 1-2 yr perch and age 1-3 yr walleye) for both species in the 1990s compared to the previous three decades. Cormorant diet was investigated from 1995 to 2000 using a combination of cast pellets, regurgitants, and stomach analysis. Walleye and yellow perch were a major portion of the cormorant diet during these years (40-82% by number). The number of subadult walleye and yellow perch consumed by cormorants suggests that the increase in subadult mortality can be explained by predation from cormorants. Mean mortality rates of adult percids attributed to cormorant predation were 1.1% per year for walleye and 7.7% per year for yellow perch. Our analysis suggests that predation by cormorants on subadult percids is a major factor contributing to the decline in both the walleye and the yellow perch populations in Oneida Lake. Other ecosystem changes (zebra mussels, lower nutrient loading, decrease in alternate prey) are not likely explanations because the potential mechanisms involved are not consistent with auxiliary data from the lake and would not affect subadult mortality. The likely impact of bird predation on percid populations in Oneida Lake occurs because cormorants feed on larger fish that are beyond the size range where compensatory mechanisms are important.

Life history theory predicts that organisms inhabiting environments with relatively poor conditions for the growth and survival of their offspring should produce fewer and larger offspring. We examined egg size (an index of offspring size) of an iteroparous, broadcast-spawning, freshwater fish-the walleye (Stizostedion vitreum)-from 34 populations across 26° of latitude in order to determine whether egg size varied with respect to environmental indices of juvenile habitat quality. Variation among populations (among environments) was compared to variation within populations (among females). Within populations, egg size generally increased with maternal size and age. Slopes of these relationships were much more variable among populations (CV > 100%) than interannually within populations (CV < 50%). Egg size vs. female size/age correlations were stronger in populations closer to the northern and southern limits of the walleye range. Egg size was also related to maternal growth history, but the effects of recent growth (previous year) were inconsistent. Egg size varied much less than fecundity among females of the same population. For a standard size/age of female, predicted egg size was more variable among populations (CV > 10%) than interannually within populations (CV < 5%), but only slightly more variable than among females within populations (mean CV = 8.5%). Nevertheless, among-population variability in egg size was related to environmental conditions. Mean egg size decreased with increasing latitude/decreasing mean annual temperature, contrary to our predictions. However, as predicted, egg size decreased with increasing lake productivity following adjustment for the latitudinal/temperature effect. These results suggest that egg size in fishes may be influenced by multiple environmental factors across populations, as well as by maternal effects within populations.