HUSKEY, S.H.*; TURINGAN, R.G.: Variation in prey-resource utilization and oral jaw gape between two populations of largemouth bass, Micropterus salmoides.

Ontogenetic changes in diet and jaw gape were compared between two populations of largemouth bass, Micropterus salmoides to test the hypothesis that ecomorphology varies among broadly distributed fish populations. Two hundred seventy-two temperate (southwest Michigan) M. salmoides and 265 subtropical (east-central Florida) conspecifics were analyzed for food habits and oral jaw gape height and width. Percent volumetric contributions of four functional prey categories (plankton, insect, crustacean, and fish) were compared among fish-size classes to determine if stage-specific differences in prey consumption existed between populations. Subtropical M. salmoides shifted from feeding on plankton and macroinvertebrates to piscivory by 20mm standard length (SL); and stopped consuming plankton by 29mm SL. Temperate largemouth bass did not become piscivores until 37mm SL and continued utilizing plankton up to 69mm SL. Following the onset of piscivory, 100-260mm SL subtropical bass began utilizing more crustaceans than fish. In contrast, temperate M. salmoides consistently fed on fish following the onset of piscivory. Variations in food habits were associated with differences in gape size between temperate and subtropical populations. Temperate largemouth bass had significantly larger gape height and width than subtropical bass. Although piscivory is a well-known feature of M. salmoides, the ontogeny of piscivory may vary between locations. We hypothesize that intraspecific differences in jaw gape may be related to variations in prey-resource utilization.