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Walleye Sander vitreus is a valued sportfish in eastern North America, including the upper New River of Virginia, where individuals can grow to a large size (>7 kg). After construction of dams, especially Claytor Dam in 1939, the population declined and non-native walleye were stocked. Stocking of non-native walleye was stopped in 1997, and molecular marker data showed that the presumptive native population had persisted. To restore the native stock, selection of broodstock candidates bearing native marker alleles and hatchery-based augmentation have been practiced over a 20-year period. We evaluated the success of the marker-assisted selection and hatchery-based augmentation program. Marker-assisted selection of native New River walleye began with mean frequencies of marker alleles at microsatellite loci Svi17 and Svi33 of ~30%, and continuing selection has driven marker allele frequencies to ~65–70%. Numbers of walleye collected in fall gillnet and spring electrofishing surveys were responsive to augmentations with hatchery fish 2–3 years earlier. Stocking was not practiced in 2012–2013, and a decrease in walleye catch rates was noted in 2016, suggesting that the native New River walleye population still depends upon hatchery-based augmentation. We recommend the development of a small panel of single nucleotide polymorphism markers for more rigorous selection of broodstock representative of the native walleye population.

Background Globally, temperate lakes are experiencing increases in surface water temperatures, extended periods of summer stratification, and decreases of both surface and deep water dissolved oxygen (DO). The distribution of fish is influenced by a variety of factors, but water temperature and dissolved oxygen are known to be particularly constraining such that with climate change, fish will likely feel the “squeeze” from above and below. Methods This study used acoustic telemetry to explore the effects of both thermal stratification and the deoxygenation of the hypolimnion on walleye ( Sander vitreus ) movements in a coastal embayment in Lake Ontario. Using historical water quality monitoring data, we documented seasonal and annual fluctuations in availability of both ‘suitable’ (all temperatures, DO > 3 mg/L) and ‘optimum’ (temperatures 18–23 °C, DO > 5mg/L) abiotic habitat for walleye and determined how these changes influenced walleye movements over a three-year period. Results Hypoxia (< 3 mg/L DO) was present in Hamilton Harbour every summer that data were available (32 of the 42 years between 1976 and 2018), with a maximum of 68.4% of the harbour volume in 1990. We found that thermal stratification and a hypoxic hypolimnion greatly reduced the volume of suitable habitat during our telemetry study. The reduction of suitable habitat significantly reduced walleye movement distances, however as the summer progressed, this remaining suitable habitat warmed into their thermal optimum range which was found to increase walleye movement distances. Despite the seemingly poor conditions, tagged walleye remained in the harbour for most of the year, and were the fastest growing individuals compared to other sampled coastal subpopulations in Lake Ontario. Conclusions Although we documented a reduction in the quantity of non-hypoxic habitat available to walleye, the water temperature of the remaining habitat increased throughout the summer into the physiologically optimum range for walleye and increased in metabolic quality. Many abiotic factors influence how, where, and what habitat fish choose to use, and this study reveals the importance of considering both habitat quality (temperature and dissolved oxygen) and quantity when evaluating fish habitat use and behaviour.

Walleye is an important sportfish across eastern North America, is commercially fished in the Laurentian Great Lakes region, and has been introduced outside its native range. Thirty-eight Walleye populations within six watersheds across the Eastern Highlands and other portions of the native range were screened at eight microsatellite DNA loci to better understand evolutionary history and to inform fishery management and conservation efforts. Population genetic variation showed divergent assemblages of populations, respectively, living in the Mobile Bay, Mississippi River, Eastern Highlands (Tennessee, New, and Ohio Rivers), and Great Lakes drainages today. All estimates of effective numbers of breeding individuals were under 25, and all populations within all watersheds had ~15–20% inter-individual relatedness, likely attributable to the effects of both natural demographic processes and stocking. The extent of Eastern Highlands Walleye includes both the Ohio and Tennessee River basins.

Molecular techniques have been increasingly used in a conservation physiology framework to provide valuable information regarding the mechanisms underlying responses of wild organisms to environmental and anthropogenic stressors. In the present study, we developed a reference gill transcriptome for walleye (Sander vitreus), allowing us to pair a gene-suite approach (i.e. multiple genes across multiple cellular processes) with multivariate statistics to examine the physiological status of wild-caught walleye. For molecular analyses of wild fish, the gill is a useful target for conservation studies, not only because of its importance as an indicator of the physiological status of fish but also because it can be biopsied non-lethally. Walleye were non-lethally sampled following short- (~1.5 months) and long-term (~3.5 months) confinement in the Delta Marsh, which is located south of Lake Manitoba in Manitoba, Canada. Large-bodied walleye are confined in the Delta Marsh from late April to early August by exclusion screens used to protect the marsh from invasive common carp (Cyprinus carpio), exposing fish to potentially stressful water quality conditions. Principal components analysis revealed patterns of transcript abundance consistent with exposure of fish to increasingly high temperature and low oxygen conditions with longer holding in the marsh. For example, longer-term confinement in the marsh was associated with increases in the mRNA levels of heat shock proteins and a shift in the mRNA abundance of aerobic to anaerobic metabolic genes. Overall, the results of the present study suggest that walleye confined in the Delta Marsh may be exhibiting sub-lethal responses to high temperature and low oxygen conditions. These results provide valuable information for managers invested in mediating impacts to a local species of conservation concern. More broadly, we highlight the usefulness of pairing transcriptomic techniques with multivariate statistics to address potential confounding factors that can affect measured physiological responses of wild-caught fish.

We examined how variation in sperm quality influences a male's success at fertilizing ova (male fertilization success) in a wild population of walleye (Sander vitreus). To do this, we conducted controlled fertilization trials using milt and eggs (ova) from wild-spawning fish and measured male fertilization success (percentage of ova fertilized) by examining eggs after 24 h of incubation. We found that both the number of sperm and sperm swimming speed (at 10 s after activation) were significantly related to fertilization success. There was, with respect to fertilization success, a relatively large return on male investment in the number of sperm, but this return diminished as the percentage of ova fertilized increased above 50%. This is in agreement with theoretical predictions based on external fertilization dynamics. When the number of sperm used in the experimental trials was kept constant, variation in sperm swimming speed (at 10 s after activation) explained approximately 90% of the variation in a male's fertilization success. These findings demonstrate that the variation in sperm quality found in wild spawning populations has the potential to dramatically influence male reproductive success.

Young-of-year piscivores typically undergo ontogenetic diet shifts from planktivory to benthivory to piscivory. These shifts are often the result of changes in predator foraging abilities, but little is known about the influence of relative prey availability. As a result, we examined diet shifts across a range of sizes (20-150 mm) of a young-of-year piscivore, walleye (Sander vitreus), in feeding experiments in which zooplankton, benthic invertebrates, and fish were made available at different density combinations. Consumption of each prey type changed with walleye size and prey densities. Small juveniles (20 mm) selected zooplankton and fish, whereas larger walleye (40-100 mm) selected benthic invertebrates and fish. Relative prey densities influenced consumption; increased densities of more profitable prey types resulted in reduced consumption of less profitable prey. Walleye larger than 100 mm selected only fish. Foraging efficiencies also varied with size and prey types; small walleye (20 mm) were less likely to pursue benthic invertebrates and retain captured fish. These ontogenetic changes in foraging patterns are linked to prey profitability, have growth consequences for juvenile walleye, and have implications for understanding diet shifts of other juvenile fish.

Discerning population genetic structure is challenging for highly vagile open water animals, as contemporary gene flow may obscure historic phylogeographic patterns. We examined genetic variation among all 10 major river and reef spawning groups of walleye (Sander vitreus vitreus) in Lake Erie for evidence of isolation by distance, segregation by physiographic partitions, and natal site fidelity using 10 nuclear DNA microsatellite loci. Results revealed that although most spawning groups were distinguishable, relationships did not correspond with physiographic basins or distances among localities. Bayesian analyses showed connectivity among some southern shore spawning groups, which included the largest-sized groups. Significant genetic divergence was discerned among walleye spawning in the river systems of eastern Lake Erie, as well as in two sites in western Lake Erie, along with marked isolation from Lake St. Clair. Population structure of Lake Erie walleye thus appears to reflect the interaction of two different intrinsic factors: isolation due to natal site fidelity that maintains patterns of divergence, and connectivity due to individuals that stray from their natal sites to spawn.

We examined the importance of prey availability, predation, and abiotic factors in determining growth and survival of age-0 walleye (Sander vitreus) across 15 Illinois reservoirs during 7 years. Multiple life stages were examined by stocking walleye at three different size groups: larval (6 mm total length (TL)), small (46 mm TL), and large (100 mm TL). Factors affecting growth and survival of walleye varied depending on walleye size. Growth of small and large walleye increased with benthic invertebrate density. Temperature had a positive effect on larval and small walleye growth but a negative effect on large walleye growth. Prey availability was an important factor for walleye survival across all size groups, whereas temperature affected only larval and large walleye. Juvenile centrarchid density had a negative effect on larval walleye survival, presumably caused by predation. Our best predictive models explained substantial variation in survival for larval (97%), small (57%), and large (83%) walleye. We also explained a high proportion of variation in growth of large (98%), small (55%), and larval (52%) walleye. Our study demonstrates the importance of examining multiple life stages to predict growth and survival and leads to a better understanding of walleye recruitment and recommendations for stocking strategies.

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.