Predation and Resource Partitioning in Two Sunfishes (Centrarchidae)

Abstract

This study examines the diet and habitat use of two congeneric sunfishes in light of the effects of size—specific predation risk and foraging efficiency. Large bluegills (Lepomis macrochirus) and pumpkinseeds (L. gibbosus) (> 75 mm SL) exhibited distinct diet and habitat separation in three small Michigan lakes. Bluegills foraged primarily on open—water zooplankton (Daphnia), while pumpkinseeds specialized on vegetation—dwelling gastropods. These differences in resource use by large fish were directly related to differences in their functional morphology and foraging ability. In laboratory feeding experiments, pumpkinseeds had significantly lower prey handling times of gastropods than did bluegills. The bluegill, on the other hand, was the more efficient of the two species when feeding on zooplankton (Daphnia). In contrast to the large fish, the diets and habitat use of small bluegills and pumpkinseeds (@<75 mm SL) were quite similar. Small fish of both species foraged predominantly in the vegetation, and 76—91% of their average seasonal diet was vegetation—dwelling prey (nongastropods). This increase in shared resources among small fish was due to two factors: (1) piscivorous fish restrict small size—classes to the vegetation where they are less vulnerable, and (2) small pumpkinseeds are unable to feed effectively on snails, the adult resource in the vegetation. Laboratory comparisons of the foraging rate of small bluegills and pumpkinseeds on vegetation—dwelling prey, and field measures of the growth rate of each species while occupying the vegetation, indicate that the two species are nearly equal competitors in the juvenile habitat. The two species, however, were not in equal abundance across the study lakes; the ratio of bluegills to pumpkinseeds ranged from 25:1 to 1:1. The pattern in species abundance was directly related to the availability of "adult" resources (snails and zooplankton) in each of the lakes, suggesting that the volume of the competitive refuge experienced by each species at larger sizes is the predominant factor controlling their relative numbers.