Male Reproductive Competition and Sex-Specific Growth Patterns in Bluegill

Abstract

A comparative study of populations of bluegill Lepomis macrochirus from 12 southern Wisconsin lakes was conducted to examine the social and reproductive factors associated with variation in growth rates, and to determine whether changes in growth rates and the relative abundance of alternative male mating types could be attributed, in part, to adaptive changes in bluegill life history parameters. Actively spawning colonies were encircled with 46-m-long seines to collect nesting parental males, reproductive females, and precocial males (cuckolders). Scale samples were used to back calculate size and estimate age of first maturation. Morphometric image analysis was used to quantify the degree of sexual dimorphism within populations. Adult parental males were more likely to exhibit reduced growth rates after maturation than females. In addition, adult parental males from lakes containing stunted populations matured at younger ages than males from lakes with populations that were not stunted. This shift in the age of maturation was correlated with a reduction in sexual dimorphism and the intensity of male competition for nesting sites. The abundance of alternative male mating types was inversely related to the intensity of male sexual selection, with cuckolder abundance reaching nearly 60% of reproductive males within colonies from the most stunted populations. These changes in sex-specific life history patterns are in accord with current theory on the evolution of life history patterns. At least two factors may be responsible for the observed shifts. Increased availability of nesting habitat in shallow littoral lakes may reduce competition among males by allowing less competitive males to acquire territories. Alternatively, sex-specific angling practices that remove large males from nesting colonies may allow younger, less dominant males to move into colonies and successfully spawn. These findings suggest that removal of nesting males will have the greatest effect on male growth in lakes with small amounts of pelagic habitat and where bluegill nest in numerous small colonies.