INTERACTIONS IN A STAGE-STRUCTURED SPECIES: IMPACTS OF ADULT BLUEGILL ON LARVAL GROWTH AND SURVIVAL

Jessica Erin Rettig

The bluegill sunfish (Lepomis macrochirus) is a stage-structured species with three ecologically distinct life history stages: larvae, juveniles, and adults. While several studies have explored the ecology of the adult stage and the juvenile stage, relatively little is known about the ecology of the larval life stage. Given this lack of knowledge, this dissertation seeks to explore the ecology of larval bluegill by examining three areas: 1) the distribution of larvae in lakes in S.W. Michigan, 2) the effect of variation in prey (zooplankton communities) on the growth and survival of larval bluegill, and 3) the potential for direct and indirect interactions between the adult life stage and the larval life stage.

In examining the distribution of larvae, I used protein electrophoresis to identify larvae caught in four lakes. I found that bluegill dominate the larval assemblage of some lakes but not others. In addition, the proportional contribution of different species to the larval assemblage can vary through time and among lakes, highlighting the dynamic nature of larval fish assemblages. Finally, larval assemblages may or may not reflect the distribution of adult species. This variation in larval assemblages and potential for larval assemblages not to reflect adult fish communities points to the difficulty in studying many aspects of larval ecology in lakes because the identity of larvae must be known before comparisons among species or lakes can be made.

I examined the effect of zooplankton variation in larval growth and survival using an enclosure experiment with two types of zooplankton communities, large-bodied and small-bodied. I found that larval survival was higher for larvae foraging on the small-bodied zooplankton community, but larval growth did not differ between the two communities. In addition, larval density played a strong role in influencing larval growth, with relative growth declining as larval density increased, resulting in significant density dependent effects on the final mass of larvae. While simple dichotomies of large versus small zooplankton may not explain the variation in larval growth seen in local lakes, differences in larval density among lakes may contribute to patterns of larval growth.

I used a field manipulation of adult and larval bluegill to examine potential interactions between adults and larvae. I found strong evidence that adult bluegill affect larval bluegill by foraging on the zooplankton community, a resource that both stages utilized. I also found that adults forage on aquatic insects, which are predators of larval bluegill, producing a positive indirect effect of adults on larvae. Finally, I found no indication that adults cannibalize larval bluegill. The presence of these interactions emphasizes the potential importance of interstage dynamics between adults and larvae beyond simple direct methods of birth and recruitment from stage to stage. Such interactions may play an important role in the population dynamics of bluegill in lakes.