Competition and Habitat Shift in Two Sunfishes (Centrarchidae)

Abstract

The bluegill sunfish (Lepomis marcrochirus) in small ponds feeds on relatively large prey associated with the vegetation. However, in the presence of the green sunfish (L. cyanellus) it shifts to feeding on smaller, less preferred prey in the open water column. The mechanisms responsible for this habitat shift were examined by experimentally confining both species, alone and together, in homogeneous patches of the preferred habitat (vegetation). When confirmed together in the vegetation the green sunfish exhibited higher survivorship, growth rates, and amount of food in the stomachs that the bluegill. The bluegill fed on smaller items and consumed benthic prey than did the green sunfish. The presence of the congener did not alter the food habits of either species or the growth rates of the green sunfish in relation to species stocked alone. Presence of the congener did affect the growth rates of the bluegill. Overlap in the diet was 70% when these species were confined to the vegetation as compared to 44% in an earlier study where habitat separation was permitted. The green sunfish is more of a sit—and—wait predator and is able to utilize a wider food size spectrum than the bluegill. This results in a strong asymmetry in the competition function favoring the green sunfish in the vegetation. However, in the open water column the distribution of food sizes is truncated and this provides a competitive refuge for the bluegill which handles small foods more efficiently. The bluegill appears to be more flexible in its habitat use while the green sunfish is more aggressive and limited in the habitats utilized. Comparisons with studies of habitat shifts in salmonids suggest that the competitive mechanisms outlined are of general relevance in fish communities. Considerations of the relation between habitat structure, the correlated distribution of food sizes, and species morphology provides a framework for specifying the occurence of habitat shifts and which species of the interactive set will shift.