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

Abstract

Population structure and other vital statistics were determined for warmwater fish populations in two adjacent lakes in central Wisconsin for two successive 3-year periods. Allen Lake (7.8 ha) containing bluegill (Lepomis macrochirus), pumpkinseed (Lepomis gibbosus), yellow perch (Perca flavescens), and largemouth bass (Micropterus salmoides) was open to angling during, and for years before this investigation. Mid Lake (4.7 ha), with the same species plus northern pike (Esox luciush was closed to angling for 20 years before angling was permitted at the beginning of the second 3-year period. Before angling in Mid Lake a large proportion of the populations was large, old fish, and total annual mortality rates, which were natural mortality rates, were low. The exploited populations in Allen Lake were mainly composed of 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 to smaller sizes, population density and standing stocks decreased for larger fish and increased for smaller fish, mean age and life spans decreased, mortality rates increased, and proportional stuck density generally declined from values above to values below recommended ranges. The changes in population structure were most pronounced for yellow perch followed in order by pumpkinseed, bluegill, largemouth bass, and northern pike. Estimated fishing effort was 83 hrs/ha in May 1976, the first month that Mid La.ke was opened to angling, and it was 36 hrs/ha in May 1979. Estimated exploitation rates in May 1976 for fish of sizes acceptable to anglers were 86, 74, 35, 53, and 46%, respectively for the species just mentioned, and most of these caught were taken in the first two days. Growth rates in Mid Lake did not change within the period of this study, but bluegill in Allen Lake were growing slowly or "stunted.” Electrofishing provided more representative samples of lengths and ages of fish present than fyke nets, hoop nets, or fish traps.