Spatial and temporal variation in prey selection of brown trout in a cold Arkansas tailwater

Abstract –  We examined prey utilisation patterns of brown trout ( Salmo trutta Linnaeus) as related to available benthos in a cold tailwater (Little Red River, Arkansas) having low biodiversity of benthic macroinvertebrates and low availability of fish as prey. Benthic macroinvertebrates were sampled monthly in upstream and downstream sites over a 1-year period. Stomach content analysis was also performed on 30 brown trout (10 each of three size classes) collected monthly from each site. Benthic and diet diversities were low at both sites, as isopods ( Lirceus ) accounted for 67% and 51% of the numerical abundance upstream and downstream, respectively, and 80% and 70% of all prey consumed by upstream and downstream brown trout. Physid snails (upstream) and chironomid larvae (downstream) accounted for most of the remaining prey taxa consumed. Piscivory and consumption of terrestrial invertebrates were rare. There was no clear relation between diet diversity and trout size. Densities of benthic macroinvertebrates and prey consumption were significantly greater upstream than downstream. Benthic macroinvertebrate assemblages and prey consumption were seasonally constant at both sites. Generally, prey consumption was consistent with availability of each prey in the benthos; the lack of benthic diversity and overwhelming abundance of one taxon ( Lirceus) most likely contributed to food selection patterns. Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna because of the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major limitation to brown trout growth.