Effects of Angling on a Previously Fished and an Unfished Warmwater Fish Community in Two Wisconsin Lakes

Abstract

Population structure and other vital statistics were determined for warmwater fish populations in two adjacent lakes for two successive 3-year periods. Allen Lake (7.8 hectares), containing bluegills Lepomis macrochirus, pumpkinseeds Lepomis gibbosus, yellow perch Perca flavescens, largemouth bass Micropterus salmoides, and northern pike Esox lucius, was open to angling during, and for about 10 years before this investigation. Mid Lake (4.7 hectares), with the same species, was closed to angling until fishing was permitted throughout the second 3-year period. Before angling began in Mid Lake, large proporations of the populations were of large, old fish, and total annual mortality rates (which were natural mortality rates) were low. The fished populations in Allen Lake comprised mainly small or intermediate size fish throughout the study. After angling in Mid Lake the fish populations and their vital statistics became like those in Allen Lake: Length- and age-frequency distributions shifted toward smaller sizes and younger ages, mean age and life spans decreased, mortality rates increased, and proportional stock density declined from values above to values below recommended ranges. The changes in population structure were most pronounced for yellow perch, followed in order by pumpkinseeds, bluegills, largemouth bass, and northern pike. Estimated fishing effort was 231 hours/hectare in May 1976, the first month that Mid Lake was opened to angling, and declined to 62 hours/hectare in May 1979; harvest and catch rate also declined. Estimated exploitation rates (%) in May 1976 for fish of sizes acceptable to anglers were 86 for yellow perch, 74 for pumpkinseed, 35 for bluegill, 53 for largemouth bass, and 46 for northern pike; most fish were taken in the first 2 days. Growth rates in Mid Lake did not change within the period of this study, but bluegills and pumpkinseeds in Allen Lake were growing more slowly than those in Mid Lake before the advent of angling.