Illinois Natural History Survey - University of Illinois

Quality Management of Bluegill Populations: Understanding Factors Affecting Population Size Structure

The bluegill (Lepomis macrochirus) is an important target sport fish throughout North America, with annual harvest rates for this species often surpassing all other sport fish combined. In Illinois reservoirs, fisheries managers and ecologists are faced with conservation issues that may be associated with high rates of harvest; some bluegill populations contain only small fish. These "stunted" populations produce males that rarely exceed six inches in total length, which is in sharp contrast to "quality" bluegill populations that consistently produce males eight inches or greater. Management efforts to increase the size structure of bluegill populations are often unsuccessful because managers have failed to address factors that determine the ultimate size bluegill attain in these populations. The focus of our research involves investigating these factors and determining appropriate management strategies to alleviate stunted bluegill populations.

Four components of growth determine the ultimate size of a fish: juvenile growth rates, age-at-maturation, adult growth rates, and longevity. Factors affecting any one of these four components can cause changes in the size a fish attains. Traditional management practices have focused on density-dependent factors related to growth rates (i.e., too many small bluegill and limited food resources causes decreased growth and smaller ultimate sizes). Although density-dependent processes may affect bluegill during each life stage, management efforts to alleviate stunting based on this mechanism alone fail to address key principles of bluegill life history. Data comparing life histories among populations of bluegill across Illinois indicate that age-at-maturation is a critical determining factor.

A blue gill, Lepomis macrochirus.

Bluegill are colonial spawners, and males compete for the best nest sites within these colonies. Because nests that are centrally located within colonies provide the best protection against egg predators (that must enter the colony from the periphery), females spawn preferentially with males in these central nests. Male-male competition for these premium locations is intense, and larger males are better competitors. To be competitive with large, dominant males, smaller, less competitive males are forced to delay maturation and grow for an additional year. If, however, large males are removed from nests via recreational angling practices (which often target these large, bright, aggressive males), younger, smaller males stop growing to become reproductively mature and successfully compete for spawning opportunities. This change in life history results in stunted populations; not because of slow growth, but because of early maturation.

Experimental management efforts on 32 state impoundments are focusing on identifying management practices that can best increase age-at-maturation of stunted populations. These management initiatives combine traditional approaches (thinning juvenile bluegill populations) with components designed to maximize the number of larger adults in an effort to suppress early maturation of marginally competitive juveniles. Specifically, our current study tests harvest regulations (8-inch minimum size limit/10 fish daily bag) and predator (largemouth bass) additions, separately and in combination, in a variety of reservoirs throughout the state. This investigation will test the effectiveness of management strategies aimed at addressing components of bluegill reproductive ecology not previously considered in traditional management practices. We believe that this comprehensive approach, which addresses age-at-maturation, will achieve better success at increasing size-structure of bluegill populations than traditional approaches based solely on growth rates.

Derek Aday, John Hoxmeier, Julie Claussen, David Wahl, and David Philipp, Center for Aquatic Ecology

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