Diet overlap and habitat segregation between redbreast sunfish and age-o smallmouth bass in the North Anna River, Virginia

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Virginia Polytechnic Institute and State University


I assessed the potential for competition between redbreast sunfish and age-0 smallmouth bass in a regulated Virginia River. I sampled stomachs of sunfish and bass throughout spring and summer. Sunfish were divided into small, medium, and large size-classes. Diets of bass shifted from predominantly Chironomidae (Insecta: Diptera) in June to Baetidae (Insecta: Ephemeroptera) in July and August. Fish also became increasingly important diet components of bass when assessed volumetrically. Small and medium sunfish ate predominantly Chironomidae in all sampling periods. Large sunfish ate mostly terrestrial-based invertebrates, Chironomidae, and Baetidae.

Diet overlap of bass with small and medium sunfish was high in June. Bass diets differed from all sunfish in July. In August, high overlap occurred between bass and large sunfish because of their common consumption of Baetidae.

Food acquisition was not a serious problem for either species during the summer. Empty stomachs were rare, and total volume of diet items in stomachs remained fairly consistent. Densities of aquatic-based food resources measured in pools and riffle/run habitats fluctuated, but were never scarce. Bass and sunfish foraging behaviors were different, particularly in late summer.

Microhabitat used by bass and sunfish was assessed by snorkeling and measuring eight variables in early and late summer. Multivariate analyses revealed that distance from shore was the most important habitat variable separating these species. Bass were found farther from shore than sunfish. In late summer large sunfish were also located far from shore, but bass were associated with bedrock and boulders while large sunfish were associated with silt and sand.

Manipulative enclosure/exclosure experiments revealed that the most important mechanism responsible for habitat use and foraging patterns of bass and sunfish was habitat-specific foraging success (pool vs. riffle/run habitats). Bass foraged at least four times more effectively in riffle/run habitat while sunfish foraged about twice as effectively in pools. Sunfish did not depress the food resources in pools enough to reduce the amount of prey consumed by bass. Competition (both exploitative and interference) was much less important in determining diet and habitat use of bass and sunfish than the distribution of food resources.