Effect of Population Size Structure on Reproductive Investment of Male Bluegill

Abstract

Although a goal of fisheries management is to understand factors affecting the growth rate and size structure of exploited populations, the relation between growth rates and variation in reproductive behaviors in Lepomis species has received little attention. Allocation of energy to reproductive functions (e.g., gonadal maturation, gamete production, nesting and spawning activities, and brood defense) negatively affects growth rate and, ultimately, maximum body size. To assess how social factors influence energy allocation, we manipulated population size structure of male bluegills L. macrochirus in experimental ponds and evaluated individual reproductive behavior and testes development. We predicted that smaller parental males would invest less in reproduction in the presence of larger parental males than they would in their absence. Observations were consistent with this prediction; smaller parental males had smaller testes and nested less frequently in the presence of larger males than in their absence. Furthermore, when males of both size-groups occupied nests, larger males were more successful. Size-selective angling that removes larger males may, therefore, negatively affect the size structure of Lepomis populations by creating conditions under which smaller, and often younger fish, mature sexually and reproduce. Under those conditions, growth rates would slow at younger ages, and the maximum size attained by these fish would be reduced.