Stage‐Structured Interactions in Bluegill: Consequences of Abult Resource Variation

Ontogenetic shifts in diet and habitat by the bluegill sunfish (Lepomis macrochirus) separate its life history into two functionally distinct stages: juveniles, which are restricted by predators to feeding in vegetated littoral habitats, and adults, which feed on zooplankton (Daphnia) in the open water. Through comparative and experimental studies in natural lakes, we show how variation in Daphnia size and abundance has a strong impact on adult bluegill foraging gain and growth, and how the bluegill in turn influences Daphnia abundance and size. A two—life—stage model is used to predict how natural variation in Daphnia productivity among lakes should affect both adult and juvenile bluegill densities and growth rates. The model predicts that adult bluegill growth and density should be positively correlated among lakes, while juvenile growth and density should be negatively correlated in these same lakes. These predictions arise despite density dependence acting in each life—stage. A long—term data set on bluegill growth and abundance in a series of natural lakes supports these predictions. Finally, we consider what factors might drive variation in Daphnia abundance and size structure among lakes, and suggest that the volume (and/or productivity) of the hypolimnetic refuge available to Daphnia is a key factor. The strong interaction between adult bluegill and Daphnia, coupled with the bluegill's two—stage life history, provides an important mechanism by which the dynamics of littoral species (fish and their invertebrate resources) are coupled to the dynamics of open—water zooplankton.