Competition Among Refuging Sunfishes and Effects of Fish Density on Littoral Zone Invertebrates

In small lakes, juvenile sunfishes (Centrarchidae) commonly occupy vegetated habitats, reducing the risk of predation by piscivorous fish. Eight species of sunfish coexist in Lawrence Lake, Michigan, and the bluegill greatly dominates the fish fauna in terms of number sand biomass. I hypothesized that the bluegill's use of the vegetation as a predation refuge could have a significant negative effect on the growth rates of other species occupying this habitat. A "target—neighbor" experimental design was used to test this hypothesis. Twelve 3—m² cages were arrayed linearly along the vegetated littoral zone of Lawrence Lake in 1986. Two juvenile pumpkinseeds (the "target" species) were placed into each cage, together with either 0, 2, 3, 6, 8, or 10 juvenile bluegills. Bluegill densities were randomly assigned to cages; average bluegill density in Lawrence Lake is equivalent to °5 fish per cage. Growth in mass over a 50—d period was used as a measure of competitive effect. Benthic invertebrates were also sampled from each cage to determine whether fish density significantly affected invertebrate size and abundance and whether invertebrate availability influenced sunfish growth. Growth of both pumpkinseeds and bluegills declined linearly as a function of final bluegill density in the cages, indicating that the species were competing while occupying the vegetation refuge. Growth was positively related to the density of large invertebrate prey remaining at the end of the experiment. Using an optimal foraging model, I estimated the net energy return available to bluegills in each cage. Predicted net energy gains (in joules per second of foraging time) were also positively related to bluegill growth, indicating that prey availability directly influenced growth rates. Mean invertebrate size decreased significantly as fish density increased, owing to a reduction in the number of large invertebrates; the number of small prey showed no relationship to fish density. These results are consistent with a hypothesis of exploitative competition between juvenile sunfishes and indicate that the behavioral avoidance of predators by small fish can have significant effects on invertebrate size and abundance in littoral habitats.