The effects of prey availability and predation risk on habitat selection by juvenile bluegill sunfish

We examined the effects of variation in prey abundance on foraging site selection by juvenile bluegill sunfish, in the absence and presence of a predator. Groups of bluegills chose between two plots of artificial vegetation differing in plant stem density (i.e., 100 and 500 stems/m²). Two prey distribution treatments were tested. In the first treatment, the 500 stem plot had sufficient prey to satiate all group members, whereas the 100 stem plot had one-fifth this amount. In the second treatment, we reversed this prey distribution. We tested bluegills before, during, and after exposure to a potential predator (largemouth bass, Micropterus salmoides). In all trials the number of bluegills choosing, and time spent foraging in, each plot of vegetation was recorded. We also recorded bluegill foraging rate in, and the proportion of prey consumed from, each plot.

With no predator present (i.e., before and 24 h after exposure to a predator) bluegills preferred to forage, and achieved a higher foraging rate, in the vegetation plot with the greatest initial number of prey. The presence of a predator had no significant effect on patch choice or foraging rate when the most rewarding plot of vegetation also offered a refuge from predation (i.e., 500 stems/m²). In contrast, when the 100 stem plot contained the greatest number of prey, bluegills reacted to the presence of a predator by increasing time spent in the 500 stem plot while simultaneously increasing their foraging rate when in the 100 stem patch. Therefore, bluegills altered their foraging activities to reduce time spent exposed to predation.