Optimal foraging as the criteria of prey selection by two centrarchid fishes

The nature of prey selection by two centrarchids (white crappie and bluegil l) is presented as a model incorporating optimal foraging strategies. The v isual field of the foraging fish as represented by the reactive distance is analysed in detail to estimate the number of prey encounters per search bo ut. The predicted reactive distances are compared with experimental data. T he energetic cost associated with fish foraging behaviour is calculated bas ed on the sequence of events that takes place for each prey consumed. Compa risons of the relative abundance of prey species and size categories in the stomach to the lake environment indicated that both white crappie and blue gill (length > 100 mm) strongly select prey utilising an energy optimizatio n strategy. In most cases, the fish exclusively selected large Daphnia igno ring evasive prey types (Cyclops, Diaptomids) and small cladocera. This sel ectivity is the result of fish actively avoiding prey with high evasion cap abilities even though they appear to be high in energetic content and havin g translated this into optimal selectivity through capture success rates. T he energy consideration and visual system, apart from the forager's ability to capture prey, are the major determinants of prey selectivity for large- sized bluegill and white crappie still at planktivorous stages.