Bullhead Life History and the Effects of Removals in Northern Wisconsin

Abstract

Bullheads Ameiurus spp. are an environmentally tolerant omnivorous fish species that are found throughout much of North America and parts of Europe. Despite their prevalence, bullheads are an infrequently studied species making their biology, ecology, and life history poorly understood. Although limited information has been published on bullheads, evidence suggests that bullheads can dominate the fish biomass and have profound influences on the fish community in some north temperate USA lakes. This is particularly concerning because natural recruitment and abundances of popular coolwater sportfishes such as Walleye Sander vitreus and Yellow Perch Perca flavescens have been declining in many northern Wisconsin lakes over the last couple of decades. These observed declines, coupled with increasing bullhead densities in some northern Wisconsin lakes, have prompted fisheries biologists to conduct whole-lake bullhead removals to investigate potential interactions between bullheads and other popular fishes and to determine whether bullhead presence/density is an important factor structuring fish assemblage characteristics. The goals of my research were to: (1) summarize results of previous whole-lake bullhead removals in Northern Wisconsin; (2) summarize Black Bullhead Ameiurus melas life history characteristics from a north-temperate Wisconsin lake; and (3) design and perform a whole-lake bullhead removal biomanipulation experiment to test for responses and underlying mechanisms that bullhead removal have on the fish community in Howell Lake, Forest County, Wisconsin. Mole Lake tribal and Wisconsin DNR fisheries staff performed whole-lake bullhead removals on four lakes in Forest and Florence Counties, Wisconsin during 2008 to 2018. Removal of Black Bullheads and Yellow Bullheads Ameiurus natalis from these lakes resulted in substantial changes in the fish communities. Naturally reproduced age-0 Walleye catch per unit effort (CPUE) increased in Lake Metonga and Patten Lake following the removal of bullheads. Additionally, survival of stocked Walleye increased in Crane and Pickerel lakes following bullhead removals resulting in higher CPUE of age-0 and age-1 Walleye. Subsequently, adult Walleye abundance has increased or remained steady in all bullhead removal lakes. Significant declines in Bluegill Lepomis macrochirus and increases in Yellow Perch and Black Crappie Pomoxis nigromaculatus abundances were also observed following bullhead removals further illustrating a shift in fish community composition. These observations suggest that when at high densities, bullheads can play a significant role in structuring fish communities and removing bullheads can increase recruitment, survival, and abundance of sportfishes like Walleye and Yellow Perch; however, the mechanism(s) behind these changes remain unclear. To further understand the role of bullheads in north temperate lakes, I explored the population demographics, growth rates, life history characteristics, and seasonal diet preferences of Black Bullheads in a northern Wisconsin lake. Using common fish collection gears (fyke netting, electrofishing), fish aging protocols, fecundity assessments, and diet indices, my results suggested that Black Bullheads exhibited relatively fast growth rates, early ages at maturity, moderate fecundity, and an opportunistic and omnivorous diet. Due to these demographic and life history characteristics, Black Bullheads have the potential to dominate fish community biomass in their native and introduced range. Results from my study may inform the management of Black Bullhead when they occur as a native or invasive species. To further explore the ecological role of bullheads, I conducted a whole-lake bullhead removal in a northern Wisconsin lake. During 2019 - 2021, 779,208 bullheads (11,293 fish/ha) accounting for 3230.7 kg of biomass (46.8 kg/ha) were removed from Howell Lake, Forest County with 685 net/nights, 12 hours of electrofishing, and 19 hours of dipnet effort. During the study period, few definitive or directional changes in the aquatic ecosystem and fish community were observed. Fyke net and mini fyke net catches in 2019 yielded significantly higher CPUE of panfish, age-0 Largemouth Bass Micropterus salmoides CPUE increased in 2021, and adult Northern Pike Esox lucius density significantly declined from 2020 to 2021. Given these variable responses, it is difficult to conclude that these results are a direct effect of the bullhead removal. Future monitoring of Howell Lake will be imperative, as previous mass removals of adult fish have been shown to result in increased recruitment of all species because of a potential competitive release. Furthering our knowledge of the role of bullheads in fish communities through removals will provide insight into how to best manage for other sportfishes and how to best control bullheads in circumstances where appropriate.