Largemouth Bass in Wisconsin: Dietary Interactions with Walleyes and Factors Regulating First-Year Growth and Diet
Kelling, Craig J.
University of Wisconsin-Stevens Point, College of Natural Resources
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Largemouth bass Micropterus salmoides abundance has increased in many northern Wisconsin lakes over the last decade causing concern among anglers and fishery managers regarding the potential effects of bass on walleye Sander vitreus populations. Largemouth bass could negatively influence walleye populations through predation or competition for prey resources. Factors regulating largemouth abundance are not fully known at this time, although previous studies have suggested environmental conditions during the first year of life strongly influence bass recruitment. Environmental conditions in previous years may have promoted early hatching of largemouth bass in Wisconsin lakes, which could have resulted in higher first-year growth and survival. However, the early life history of largemouth bass has not been well studied in north temperate lakes. My objectives were to determine if: (1) diet overlap and predation occurred between adult walleye and largemouth bass in four northern Wisconsin lakes; (2) hatch timing influenced growth of age-0 largemouth bass and (3) the extent of piscivory varied in relation to total length of age-0 largemouth bass in lakes throughout Wisconsin. During June-September of 2012 and 2013, adult largemouth bass and walleyes were collected via nighttime AC boat electrofishing from four lakes in northern Wisconsin. Prey items were collected from a total of 945 largemouth bass and 641 walleye via gastric lavage. Prey items were visually identified to order for invertebrates and to genus for identifiable fish. All unidentifiable fish were identified to species using DNA barcoding procedures. The diets of largemouth bass were largely comprised of yellow perch Perca flavescens, Lepomis spp., and crayfish, whereas walleye diets were largely comprised of yellow perch and Lepomis spp. Largemouth bass predation on walleyes was only observed in Little John Lake where a single walleye was observed in the diet of one largemouth bass in September 2013, comprising 2% of the total diet composition for all largemouth bass by wet weight. Pianka’s index of niche overlap was used to determine the extent of dietary overlap between largemouth bass and walleye. Results of this analysis indicated moderate to high diet overlap between largemouth bass and walleye throughout much of the study period. These results suggest that largemouth bass predation is probably not a primary factor affecting walleye abundance in the four study lakes selected, but high diet overlap values observed suggest the potential for competition between the two species. If competition for prey is occurring between the two species, recent management efforts to reduce largemouth bass abundance could result in higher walleye abundance. During June-September of 2012 and 2013, age-0 largemouth bass were collected from 11 lakes using a mesh beach seine. Sagittal otoliths were removed from 1,626 age-0 largemouth bass to estimate hatch dates from counts of daily growth increments. For each lake, age-0 largemouth bass were grouped into early-, intermediate-, and late-hatched sub-cohorts using the 33rd and 66th percentiles of the hatch date distribution range for fish collected throughout the study period. Analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) tests was used to determine if mean TL varied among age-0 sub-cohorts within each lake. For this analysis, only age-0 largemouth bass collected within a 30-d window during July and August of each year were used to account for variation in sampling intensity and timing among lakes. Daily growth rates were compared among sub-cohorts within each lake using analysis of covariance (ANCOVA) and loge TL-age relationships. Results of these analyses suggest that early-hatched sub-cohorts of largemouth bass grew faster and attained greater TLs compared to late-hatched sub-cohorts. However, these differences were not always statistically significant, which suggests that factors including weather, prey availability, and temperature regimes may also affect growth of age-0 largemouth bass. Additionally, logistic regression models were used to determine if the probability of age-0 largemouth bass exhibiting piscivory varied in relation to TL. Results of these analyses suggest that TL explained significant variation in the percentage of age-0 largemouth bass exhibiting piscivory in some, but not all lakes, suggesting that other factors such as prey availability also affect the extent of piscivory in age-0 cohorts of largemouth bass.