Tyler D. Ahrenstorff’s research while affiliated with Minnesota Department of Natural Resources and other places

Cisco (Otoonapii in Ojibwe; Coregonus artedi Lesueur, 1818), is a widely distributed stenothermic freshwater fish whose embryos typically incubate under ice and in the dark. We used Cisco as a model organism for testing the potential of UV‐induced escape hatching behaviour. Owing to reduced ice cover and increased water transparency in north temperate lakes, these experiments provide insights into the resilience of coregonine embryos if exposed to ultraviolet radiation (UV‐B; 280–320 nm). Eyed Cisco embryos were exposed to artificially sourced UV‐B through a series of experiments that measured the hatching rate and fitness (heart rate and pigmentation pattern) 2 days after hatching and under cold [6.6°C] and warm [8.6°C] conditions. These experiments supported an extension of the escape hatching behaviour hypothesis, whereby UV‐B exposure induced earlier (ca 30 days) and more punctuated hatching of Cisco embryos, independent of an increase in water temperature. UV‐B exposure produced more larvae with irregular pigmentation patterns and reduced heart rates (by about 20%) – both of which could be indicative of reduced fitness. UV‐induced escape hatching adusts the fundamental framework in which we characterise fish embryo resilience to increased UV‐B exposure and the potential consequences of reduced ice cover. Earlier hatching from UV‐B exposure could increase the recruitment bottleneck of these fish by reducing survivorship of the post‐hatched larvae.

Muskellunge, Northern Pike, Walleye, and Largemouth Bass are popular sport fish that often co-occur in aquatic ecosystems, yet little is known about what, and how much, food these populations consume. We combined lake-specific population estimates and bioenergetic models (that used fish diets, growth, and water temperatures) to examine consumption patterns of these top predators in three lakes. Population estimates between lakes were remarkably consistent where Largemouth Bass were the most abundant (47-50% of the total predator population), followed by Northern Pike (31-34%), Walleye (17-18%), and Muskellunge (1-4%). Of all the food eaten by these top predators in each lake, Northern Pike populations consumed the most (38-50%), followed by Largemouth Bass (23-29%), Walleye (15-23%) and Muskellunge (6-16%). The higher consumption by Northern Pike and Muskellunge populations relative to their population size was driven by the fact that larger fish eat more food on an individual basis. Notably, Muskellunge consumed many different prey resources, such as White Sucker, Northern Pike, and bullheads, compared to the other predators. Walleye and Northern Pike consumed primarily Yellow Perch and Bluegill while Largemouth Bass ate mostly crayfish. These results will directly benefit MN DNR managers, policy makers, and the public by clarifying the ecological role of top-level predators and providing integral information that helps form the basis for management decisions.

Largemouth Bass (Micropterus nigricans), Muskellunge (Esox masquinongy), Northern Pike (Esox lucius), and Walleye (Sander vitreus) co-exist in many lakes. Understanding diet overlap and potential for resource competition among these species can inform management and stocking decisions. Previous studies have examined diets in these species but seasonal and interannual variability in diets are less known. We assessed diets in all four species (n = 457 diets) in spring, summer, and fall in both 2019 and 2023 in Bald Eagle Lake, MN, to address this data need. Prey in diets were expressed as percent of total mass in diets, and we used PERMANOVA to test for significant differences among species, seasons, and years. Tests for time effects showed diets of Walleye, Northern Pike and Largemouth Bass changed significantly between years, Largemouth Bass was the only species to differ among seasons, while Muskellunge showed no year or season effects. Tests for species effects showed all pairwise-tests were significant in spring and fall (excluding Northern Pike-Walleye in spring), while no summer pairwise tests were significant. Interannual differences were driven by offsetting changes in consumption of Yellow Perch and Black Crappie between years. Seasonal differences between species showed Muskellunge had the most generalized diet, Largemouth Bass consumed more invertebrates in spring and more sunfish in fall, while Walleye consumed more Yellow Perch in fall relative to Northern Pike. Similar work is needed on additional lakes, but our results indicate that temporal variability should be accounted for in diet studies of these fish.

Pelagic prey fish have often been linked to faster growth and improved condition in predator fish populations. Cisco Coregonus artedi are one such cold-water pelagic prey species frequently observed in the diets of predators such as Walleye Sander vitreus, Northern Pike Esox lucius, and Lake Trout Salvelinus namaycush. Correlations between the presence of Cisco and improved growth in Muskellunge E. masquinongy populations exist, despite Cisco rarely being observed in Muskellunge stomach contents. In this study, we aimed to quantify Muskellunge diets in lakes with and without Cisco to test for potential differences related to the availability of this prey species. Stomach contents were collected from Muskellunge via gastric lavage in eight Minnesota lakes with varying Cisco population levels, and diet compositions were compared among lakes using permutational multivariate ANOVA (PERMANOVA). Although Cisco were observed in greater numbers than many previous investigations of Muskellunge diets, the overall contribution to the diet was minimal. Nevertheless, PERMANOVA results indicated Muskellunge diets were significantly different relative to Cisco abundance. Indicator species analysis identified centrarchids and ictalurids as important prey in lakes with low Cisco abundance, while Cisco were indicators of diets in lakes with high Cisco abundance. Furthermore, Yellow Perch Perca flavescens, Northern Pike, catostomids, and invertebrates were identified as prey taxa indicative of Muskellunge diets regardless of Cisco abundance. Model selection techniques were then used to identify physical and biological attributes of lakes that were correlated with consumption of indicator species. Our results indicate that while Cisco may not be frequently detected in diets of Muskellunge, they may still be an important prey resource in certain systems. However, the broad diets and importance of warm and cool water prey, such as catostomids, ictalurids, centrarchids, and Yellow Perch, across systems indicate Muskellunge diets are likely to adapt as prey fish communities continue to change.

Walleye (Sander vitreus; WAE) and yellow perch (Perca flavescens; YEP; collectively percids) are freshwater fishes threatened by multiple stressors, including aquatic invasive species. Zebra mussels (Dreissena polymorpha; ZM) and spiny water fleas (Bythotrephes cederströmii; SWF) are aquatic invasive species that reduce pelagic zooplankton biomass, an important food resource for both age-0 percids and the prey fish of adult percids. Both percid species are generalist consumers, and it is unknown how they respond to reduced pelagic energy resources associated with invasions. We examined pelagic and littoral energy use in percids from nine large north temperate lakes which vary in invasion status. We sampled adult and age-0 percids from each lake in 2017 or 2018 and analyzed muscle tissue for δ¹³C and δ¹⁵N isotope ratios. We characterized isotope baselines with littoral and pelagic invertebrates to allow cross-lake comparisons. We estimated the proportion pelagic reliance using Bayesian mixing models and determined the variance contribution of ZM and SWF presence to estimates using model comparison. Pelagic reliance of percids sampled from ZM-invaded lakes was consistently lower than uninvaded lakes, while pelagic reliance of percids sampled from SWF-invaded lakes was greater than uninvaded lakes, although neither effect was statistically distinguishable from zero. Model comparison indicated pelagic reliance of adult WAE, age-0 WAE, and age-0 YEP was influenced by ZM presence, while pelagic reliance of adult YEP was influenced by SWF presence. If percid populations persistently rely on pelagic resources, despite those resources being reduced, there may be negative consequences to growth, survival, or recruitment.

Muskellunge Esox masquinongy, Northern Pike E. lucius, Walleye Sander vitreus, and Largemouth Bass Micropterus salmoides are popular sport fish that often co‐occur in aquatic systems. Although numerous studies have investigated interactions among these species, the simultaneous evaluation of diet patterns and niche overlap among all four species has not been conducted. Our experimental design aimed to quantify diet overlap among Muskellunge and other piscivores, while lakes without Muskellunge were also sampled to compare the diets of other piscivores in their presence or absence. Diets of piscivores from 10 Minnesota lakes were collected via gastric lavage and quantified using an index of relative importance. Diets of individual species were compared among seasons and length categories, and among‐species comparisons were also conducted by season and relative to Muskellunge presence using permutational multivariate analysis of variance (PERMANOVA). Muskellunge consumed a wide range of prey, whereas Northern Pike and Walleye diets consisted primarily of Yellow Perch Perca flavescens and centrarchids. Largemouth Bass consumed more invertebrates, especially crayfish Faxonius spp. No species exhibited seasonal diet shifts, but diets were different among length categories for all species except Walleye. Although NMDS ordinations indicated shared prey use, PERMANOVA results indicated diets of Muskellunge and Largemouth Bass were most different from each other and other piscivores across all seasons. Conversely, Northern Pike and Walleye diets were similar regardless of season or Muskellunge presence. Finally, lake‐scale habitat variables were correlated with piscivore diets and Yellow Perch abundance was correlated with Walleye diets. Our results indicate that while Muskellunge, Northern Pike, Walleye, and Largemouth Bass can co‐exist in a variety of lakes, populations of important prey and habitat variables should be examined before management actions (e.g., stocking) are implemented to ensure adequate prey availability and competition among these piscivores is not increased to the detriment of existing fisheries.

Muskellunge Esox masquinongy, Northern Pike E. lucius, Walleye Sander vitreus, and Largemouth Bass Micropterus salmoides are popular sport fish throughout Minnesota that often co-occur in aquatic systems, either naturally or through stocking. Although numerous studies have investigated interactions among these species, the simultaneous evaluation of diet patterns and niche overlap among all four species has not been conducted. Our experimental design aimed to quantify diet overlap among Muskellunge and other piscivores, while lakes without Muskellunge were also sampled to compare the diets of other piscivores in their presence or absence. We used gastric lavage to examine the stomach contents of this suite of piscivores in 12 Minnesota lakes. Diets were quantified for each species by lake, and niche overlap was calculated using Pianka’s index. Across lakes, Muskellunge consumed a wide range of prey, whereas Northern Pike and Walleye diets consisted primarily of Yellow Perch Perca flavescens and centrarchids in most lakes. Largemouth Bass consumed more invertebrates, especially crayfish Faxonius spp., but centrarchids were also important prey in some systems. Diet overlap tended to be highest between Northern Pike and Walleye across systems, while Muskellunge generally had low levels of diet overlap with other predators. Patterns in predator diets across the state were also explored using multivariate analyses. While ordinations indicated shared prey use among all species, permutational multivariate analysis of variance indicated diets were significantly different between species with one exception, as Northern Pike and Walleye diets were statistically similar. Furthermore, predator diets were correlated with several lake-scale habitat variables. These results provide additional evidence that Muskellunge can co-exist with other predators in a variety of lakes with varying physical and biological properties while offering additional insight to resource managers regarding likely diet patterns and potential for competition among species.

Muskellunge Esox masquinongy are the largest members of the family Esocidae found in Minnesota and are managed for trophy angling opportunities with large minimum size requirements, limited harvest, and stocking to support existing populations or create new angling opportunities. While Muskellunge impacts at the community level appear minimal based on available literature, relatively little is known about Muskellunge diets, particularly in Minnesota. In this study, we used gastric lavage to examine gut contents of Muskellunge, Northern Pike E. lucius, Walleye Sander vitreus, and Largemouth Bass Micropterus salmoides. Diets were quantified using an index of relative importance (IRI) and diet overlap among species was determined using Pianka’s index of niche overlap. Our experimental design focuses on how the presence or absence of Cisco Coregonus artedi impacts diet and overlap, while lakes without Muskellunge were also sampled to compare diets of other piscivores in their presence or absence. Yellow Perch Perca flavescens and various Centrarchids were important prey items across all lakes for Muskellunge, Northern Pike, and Walleye, while crayfish Orconectes spp. and other aquatic invertebrates were critical for Largemouth Bass. Pianka’s index of niche overlap indicates that Muskellunge had low levels of dietary overlap with other predators, while Northern Pike and Walleye had relatively high levels of dietary overlap. Additionally, diet overlap tended to be lower among all species when Cisco were present, even though direct predation on Cisco was rarely observed. These results corroborate existing research on diets of Muskellunge, Northern Pike, Walleye, and Largemouth Bass in their native range.

Small nearshore fishes are an important part of lacustrine and functional diversity and link pelagic and benthic habitats by serving as prey for larger nearshore and offshore fishes. However, the trophic complexity of these small nearshore fishes is often unrecognized and detailed studies of their role in food webs are lacking. Here, we examined niche space patterns of small nearshore fish species using Bayesian analyses of carbon and nitrogen stable isotope data in nine freshwater lakes that are among the largest lakes in Minnesota. We found considerable variability in niche areas within species and high variability in niche overlap across species. At the assemblage level, niche overlap (average diet overlap of all species pairs at a lake) decreased as whole-lake species richness increased, possibly indicating a greater degree of resource specialization in more speciose lakes. Overall fish niche space was weakly but significantly related to niche space of their invertebrate prey. Although nearshore benthic resources contributed to fish diets in all lakes, all fish species also had non-negligible and variable contributions from pelagic zooplankton. This inter- and intraspecific variability in trophic niche space likely contributes to the multi-level trophic complexity, functional diversity, and potentially food web resilience to ecosystem changes.