Optimal Foraging and Growth in Bluegills

Two models of foraging behaviour (optimal prey selection and random prey selection) were used to calculate the potential net energy intake of bluegill sunfish (Lepomis macrochirus) in a small Michigan lake. These predictions were then compared to the actual growth exhibited by bluegills in the lake. Predictions of net energy gain derived from optimal foraging criteria were significantly correlated with the seasonal gain in mass by the fish; both energy gain and growth were positively related to bluegill length. Predictions of net energy intake based upon non-selective foraging (i.e. prey eaten as encountered) were not significantly correlated with bluegill growth. Comparing the net energy intake of bluegills feeding optimally versus non-selectively demonstrates that optimal prey selection increases average energy gain by 4 to 10 fold. This result illustrates the strong evolutionary advantage afforded to foragers that maximize net energy intake in a natural environment. Finally, the potential usefulness of optimal foraging models to the study of species and/or size-class interactions is discussed and a heuristic example pertaining to the development of "stunted" populations in fishes is provided.