Regulation of growth and mortality in larval bluegills: implications for juvenile recruitment

Factors related to growth and survival of 10-d cohorts of larval bluegills Lepomis macrochirus were examined in two small impoundments via daily growt h increments on otoliths. Successful spawning occurred from April through S eptember, producing peak larval densities greater than 360 fish/m(3). Larva l bluegill density was negatively correlated with zooplankton density in bo th ponds, suggesting that larvae reduced zooplankton abundance via predatio n. Several variables were related to larval growth and mortality. Cohorts w hose larval densities were low tended to have faster growth and higher surv ival than cohorts with high larval densities. The mechanism responsible for this density-dependent relation was not clear in all instances; however, s ome evidence suggested that control occurred via intraspecific competition for zooplankton. Relative survival of larval bluegills to the juvenile stag e was negatively correlated with limnetic larval fish density; however, no relations were detected between larval survival and juvenile recruitment ac ross cohorts, suggesting that juvenile recruitment was set at larval sizes larger than those we examined. Larval growth was negatively correlated with larval mortality in one pond. Evidence for size-selective mortality existe d for late-spawned fish, with selection favoring small-bodied larvae. We su ggest that growth and mortality of larval bluegills are influenced by larva l abundance and intraspecific competition for zooplankton when resources ar e limiting; however, recruitment of juveniles did not appear to be set duri ng the larval stage.