Effects of Three Years of Minimum and Slot Length Limit Regulations for Largemouth Bass

Abstract

n spring 1982 length limit regulations for largemouth bass (Micropterus salmoides) were enforced on four lakes (16 to 338 hectares) in southeast Wisconsin in an attempt to improve the quality of angling. It was suspected that overharvest of largemouth bass had resulted in too little predation on the bluegill (Lepomis macrochirus) populations, resulting in an overabundance of slow-growing bluegills. A minimum length limit of 406 mm (16 inches) was introduced on Pretty and Browns lakes, and a slot length limit of 305-406 mm (12-16 inches) was established on Beulah and Rockland lakes. Night electrofishing, fyke netting, and creel surveys were used to evaluate largemouth bass and bluegill community structure and angling characteristics before (1980 to 1982) and after (1983 to 1985) the regulation changes. The regulations were effective in reducing but not eliminating, harvest of largemouth bass of protected sizes. An average of 29% of the total harvest was bass of protected sizes in the three years that the regulation was in effect. I do not know the degree to which this noncompliance affected the results of this study, but it could be substantial. Abundance (catch per effort) of protected largemouth bass did not change significantly in any lake except Browns, where it decreased. Schnabel population estimates of largemouth bass in one lake, Rockland, also did not indicate increased abundance of bass. Catch per effort of bluegills of quality size(~ 150 mm) increased in Pretty Lake, the only lake in which the change was significant. Proportional stock density of both largemouth bass and bluegills did not fall within the recommended ranges in any year. Mortality rates of largemouth bass decreased significantly in Pretty and Browns lakes, and bluegill mortality rates increased significantly in Beulah Lake. Change in growth rates and Relative Weight (Wr) of both largemouth bass and bluegill were minor and inconsistent, and did not coincide with a change in abundance of either species. Probably more time is necessary before mortality and Wr will reflect post-regulation conditions only. Fishing pressure decreased significantly for Rockland and Browns lakes. Catch and harvest rates and mean length of bluegills harvested increased significantly in all lakes except the harvest rate in Beulah Lake did not change. Catch rate of bass increased significantly in Pretty Lake and harvest rate decreased significantly in Browns Lake. Significant changes in the mean length of bass harvested occurred only in Browns and Rockland lakes, where it increased, and in Beulah Lake, where it decreased. Foods other than bluegills formed an average of 76 percent of the volume in stomachs of largemouth bass in October, 1984.