Dynamics of Quality Bluegill Populations inTwo Michigan Lakes with Dense Vegetation

Abstract

Population characteristics of bluegill Lepomis macrochirus and associated species were studied intensively to determine why many large bluegills were present at two Michigan lakes—Dead Lake and Blueberry Pond—in which macrophytes covered 41–83% of the surface. Both lakes contained unusually high proportions and densities of bluegills greater than 203 mm in total length due to high survival of adults, low fishing mortality, a favorable growth pattern, and low recruitment to age 2 or 3. Growth was rapid from 75 to 200 mm, then growth slowed and condition deteriorated among older fish. Rapid growth was stimulated by consistently low recruitment and consumption of limnetic Daphnia spp. and littoral benthos for food. Low recruitment was partially due to predation; a diet study at one lake estimated 303,300 juvenile bluegills per year were consumed by piscivores, mostly largemouth bass Micropterus salmoides. Each lake contained dense populations of slow-growing piscivores, but these fish constituted less than 20% of the total fish biomass. Observations indicated that few adult bluegills attempted to spawn in Blueberry Pond, and this lack of spawning was probably more important than predation in controlling bluegill abundance. A supplemental pond study demonstrated that young and old adults of varying condition had normal reproductive potential. The triggering of spawning behavior may be linked to adult density, and perhaps adult growth, by behavioral or bioenergetic mechanisms. The implication for fisheries management is that weedy lakes need not be dominated by small, stunted bluegills but are capable of producing large bluegills if fishing harvest is restricted and a favorable food chain is present.