Walleyes Sander vitreus are the most popular fish among South Dakota anglers, but smallmouth bass Micropterus dolomieu were introduced to provide new angling opportunities. Some walleye anglers have reported reductions in the quality of walleye fisheries since the introduction of smallmouth bass and attribute this to the consumption of young walleyes by smallmouth bass and competition for shared prey resources. We quantified the diets of walleyes and smallmouth bass in the lower reaches of Lake Sharpe (a Missouri River reservoir), calculated the diet overlap between the two predators, and determined whether they partitioned shared prey based on size. We also quantified walleye diets in the upper reach of the reservoir, which has a different prey base and allowed us to compare the growth rates of walleyes within Lake Sharpe. Age‐0 gizzard shad Dorosoma cepedianum composed a substantial proportion of the diets of both predators, regardless of location, for most of the growing season; the patterns in shad vulnerability appeared to drive the observed patterns in diet overlap. Smallmouth bass appeared to consume a smaller size range of gizzard shad than did walleyes, which consumed a wide range. Smallmouth bass consumed Sander spp. in some months, but in very low quantities. Given that global climate change is expected to alter the population and community dynamics in Great Plains reservoirs, we also used a bioenergetics approach to predict the potential effects of limiting prey availability (specifically, the absence of gizzard shad and rainbow smelt Osmerus mordax) and increased water temperatures (as projected from global climate change models) on walleye and smallmouth bass growth. The models indicated that the absence of rainbow smelt from the diets of walleyes in upper Lake Sharpe would reduce growth but that the absence of gizzard shad would have a more marked negative effect on both predators at both locations. The models also indicated that higher water temperatures would have an even greater negative influence on walleye growth; however, smallmouth bass growth was predicted to increase with higher temperatures. Fisheries managers should consider strategies to enhance the prey base or mitigate the effects of increased water temperatures that may occur in the future as a result of global climate change. Such proactive actions may alleviate potential future competition between walleyes and smallmouth bass resulting from changes in the fish community.
The American Fisheries Society (AFS) has recommended standard gears and methods for collecting North American freshwater fish data, but selectivity of these gears, including gill nets, is poorly described for most species. We calculated contact selectivity for species commonly collected with AFS standard gill nets in lakes and reservoirs of North America, including Black Bullhead Ameiurus melas, Channel Catfish Ictalurus punctatus, Walleye Sander vitreus, and Yellow Perch Perca flavescens. The normal selectivity curve provided the best fit to empirical catch data for all species. Inclusion of a tangle factor improved model fit for Black Bullhead, Channel Catfish, and Walleye, indicating that tangling is an important means of capture for these species. Channel Catfish were more susceptible to tangling than the closely related Black Bullhead, likely because larger Channel Catfish are more vulnerable to tangling by spines in the smallest meshes, whereas Black Bullhead in the same meshes were wedged. Failure to include a tangle factor for Channel Catfish and Walleye would have underestimated peak length of captured fish by 80 and 40 mm, respectively. Tangling was not an important factor in Yellow Perch capture. Total selectivity curves for each species captured with the AFS standard gill net were calculated, and relative selectivity values are provided as correction factors for size-selectivity bias. Our correction factors and modeled selectivity curves for Channel Catfish, Walleye, and Yellow Perch corroborate previous studies that modeled selectivity of the AFS standard gill net for Channel Catfish, Walleye, and Yellow Perch, but our study provided novel information on Black Bullhead. Improved selectivity information for the AFS standard gill net may help to promote further voluntary adoption of AFS standard gears and methods.
The recently developed empirical percentile (EmP) method, a technique for deriving standard weight (W s ) equations, putatively reduces the length-related biases that often plague such equations. To determine whether the EmP method is superior to the regression line-percentile (RLP) method in reducing length-related biases, we developed new W s equations by applying both methods to two morphologically distinct species, walleye Sander vitreus and black crappie Pomoxis nigromaculatus. We also investigated diagnostic approaches to provide quality control for weight-length data. We evaluated the new W s equations with filtered independent data to determine which equation reduced length bias the most. We suggest a protocol for evaluating length-related bias using an independent data set. Our results showed that for randomly selected walleye populations, the RLP method did not have any length-related biases when relative weight (W r ) was plotted as a function of length. However, the W r values calculated from the EmP W s equations were length biased when the latter were applied to those same populations. Both methods generated W s equations that were length biased when W r was plotted as a function of length for black crappies. Further, the absolute difference in W r between the RLP and EmP methods indicates that there is little difference between the methods as far as their relevance to management is concerned.. Based on these results, we believe that revising existing W s equations using the EmP method is unnecessary and that the RLP technique should remain the standard for developing W s equations pending the development of an approach that clearly eliminates methodological length bias.
Freshwater fisheries provide human benefits (e.g., food, recreation) but are increasingly threatened by climate change, invasive species, and other stressors. Our purpose was to survey fisheries administrators from state fisheries agencies and Agricultural Experiment Stations (AESs) about their perceptions of, and resource investment toward, threats to freshwater fisheries in the United States. Our rationale for studying these two types of fisheries administrators simultaneously was to inform state fisheries professionals about the fisheries relevance of AESs, elevate the profile of fisheries within AESs, and promote mutually beneficial state agency–AES partnerships. Survey respondents generally agreed that recreational, socioeconomic, and ecological services of fisheries were more important than nutritional and commercial benefits. The greatest perceived fisheries threats were water quality/quantity impairment, land‐use change, and invasive species—but, interestingly, not climate change. State fisheries agencies invested more personnel and finances into issues rated as less important but more controllable (e.g., fish production, habitat management) than issues rated as more important but larger in scale and more difficult to control (e.g., water quality/quantity, invasive species). Our research underscores the importance of ensuring that state agencies can address long‐term, socio‐ecologically critical management issues (e.g., climate change) amid budgetary constraints. We call for state agencies to collaborate with new partners (e.g., AESs) to mitigate fisheries threats by expanding fisheries management to more fully encompass terrestrial and human systems; promoting receptiveness to novel research/management ideas; actively predicting, monitoring, and planning for future stressors; and enhancing fisheries’ social–ecological resilience.
Recruitment may vary substantially in fish populations, which can drive not only adult population characteristics but also the dynamics of fishes dependent on the species of interest and recreational fisheries for these species. However, spatiotemporal trends in population fluctuations and potential drivers of recruitment variability are poorly understood. Therefore, we used a long-term (2000–2014) data set to estimate the extent of spatial synchrony in larval abundance and factors influencing variability in recruitment of yellow perch (Perca flavescens). Contrary to the prevailing paradigm that spatial synchrony in population fluctuations (i.e., recruitment) is typically absent or occurs at small spatial scales (<50 km) for freshwater species, abundance of larval yellow perch was synchronous among spatially segregated systems across a geographic scale of at least 180 km. Additionally, variation in larval yellow perch density was influenced by spatially-correlated climatic and hydrological variables (indicative of the Moran Effect). Results ultimately broaden the scale at which factors were previously thought to influence recruitment of freshwater fishes and provide important insight to patterns and processes that structure yellow perch populations.
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