Ontogenetic Habitat Shifts in Bluegill: The Foraging Rate‐Predation Risk Trade‐off

The bluegill sunfish (Lepomis macrochirus) undergoes several habitat shifts between the littoral and pelagic zones of small lakes during its life history. After hatching in the littoral zone, bluegill fry migrate to the pelagic zone to feed on zooplankton. In this study the fry then returned to the littoral zone in four different lakes at a relatively constant size of °12.5 mm standard length. Several years are spent feeding in the littoral zone vegetation before the bluegill again shifts to feeding on zooplankton, first in the water column above littoral vegetation and subsequently in the true pelagic zone. This shift to feeding on zooplankton occurred at a specific body size within a lake, but the size ranged from 50 to 83 mm among five different lakes. The size at which the shift occurred was directly correlated with the density of the major predator of the bluegill in these lakes, the largemouth bass (Micropterus salmoides). We demonstrate that the bluegill is faced with a growth (or feeding) rate—predation risk trade—off in making these habitat shifts. Analysis of stomach contents, the growth of small fish caged in the pelagic zone, and predictions of foraging rates in both habitats all indicate that the pelagic zone is the more profitable habitat for all size classes of the bluegill. Through a series of pond experiments we further show that risk of predation by largemouth bass was 40—80 times as great for small bluegills in the open water as for those in the vegetation. We interpret the patterns of habitat use by the bluegill in terms of a model that explicitly weights the costs (predation) and benefits (growth) of making a size—specific habitat shift to the pelagic. Finally, we discussed evidence that the bluegill can facultatively respond to changes in feeding rates and predation risk, and the consequences of such ontogenetic habitat shifts to community dynamics.