Bluegill Sunfish (Lepomis macrochirus) Foraging Behavior under Temporally Varying Food Conditions

Abstract

Risk sensitivity (foraging responses to variances in food availability) has been observed in numerous taxa, including fish, under conditions of extreme temporal variance (i.e., between patch visits, no matter how short the visit or the time between visits). Because fish can generally survive for weeks without food, they are expected to and have been observed to be risk-averse (i.e., prefer patches with low variance in food availability). We conducted patch choice experiments with individual bluegill sunfish, using an automated shuttlebox system, to test their sensitivity to variance in food availability under temporal variation that was less extreme and perhaps more realistic (i.e., 40-min feeding periods) than previous risk-sensitivity experiments. Because we expected bluegills to be riskaverse, we offered one patch with no variance in food availability whereas the other patch exhibited variable food as sampled from a normal distribution of food availability-mean food availability was equal between patches. Simultaneously, we explored food availability/temperature combinations above and below expected maintenance ration to determine whether fish response depended on energy balance (i.e., growth potential) as estimated from a bioenergetics model for bluegill. Even when energy balance was negative, bluegills appeared riskneutral (i.e., they did not respond to variance). In the second set of experiments, bluegills were tested for risk sensitivity with more extreme variance in food availability; the temporal distribution again was based on a 40-min period. Bluegills were again risk-neutral. Bluegills were able to effectively sample and track changing resources at least over 40-min periods. Overall variance in food availability over 40-min feeding periods appears not to be a major factor in patch choice for bluegills. Bluegill patch use depends on encounter rates with food items within a patch resulting in a foraging pattern based on mean food availability within a patch.