Individual Variation and Competition in the Everglades Pygmy Sunfish

Abstract

SUMMARY (1) Populations of Everglades pygmy sunfish (Elassoma evergladei, family Centrachidae) were manipulated in both the laboratory and the field to determine how individuals responded to changes in the intensity of competition. (2) In laboratory experiments, individually identifiable fish were raised at different densities (sixteen, eight, four, and one fish per tank) and each individual's competitive ability was measured by its growth rate and reproductive condition. For females, estimates of reproductive potential were ovary weight and egg number. For males, reproductive ability was estimated by recording the frequency of occurrence of bobbing, a sexual behaviour. In field experiments fish were raised at the same densities but in screened enclosures to evaluate the realism of the laboratory experiments. (3) Individual fish differed in their growth rates, and these differences were magnified under competitive conditions. Increased density also increased the variation among females within a population with respect to ovary size and egg number. (4) Increased population density markedly decreased the growth rate of the 'average' fish in the population, but only slightly the growth of the 'best' competitors. In each density there were always some fish who grew as quickly as the 'average' fish of a population half as large. (5) Increases in population size also produced non-linear reductions in estimates of both male and female potential reproductive success. For females these reductions could be attributed in part to density-induced reductions in growth rate. But density also had a direct effect on female reproductive success: for females of similar size or growing at a similar rate, those raised at higher densities had larger ovaries and more eggs.