Combined effects of condition and density on post-settlement survival and growth of a marine fish

For species with complex life cycles, variation in growth or condition during early life stages may affect survival in later stages or during important life-stage transitions. These effects on survival may be an important source of recruitment variability. However, survival during early stages is often density-dependent and it is unclear how the effects of early condition on recruitment compare to the effects of density alone. In this study, I investigated both the independent and interactive effects of condition and density on post-settlement survival and growth of bicolor damselfish (Stegastes partitus). In a field experiment, I crossed two levels of population density with two levels of condition (established by laboratory feeding of recent settlers) and measured growth and survivorship over a 30-day period. Low condition and high density both decreased survivorship. However, the effects of density were stronger and the effects of condition and density on survivorship were independent. In contrast, condition and density had interactive effects on growth in that density reduced growth only in the high-condition treatments. In a second experiment, I examined how condition and density affected intraspecific aggression and shelter use, two behaviors that may influence growth and survival. Shelter use was unaffected by density and condition. The rate of intraspecific chases was greater and increased more strongly with density when fish were in higher condition. Average growth rate of fish decreased with increasing rate of intraspecific chases, suggesting that the difference in aggression between condition levels was the mechanism leading to differences in density-dependent growth. These results suggest that variation in condition at settlement can significantly contribute to variation in population abundance, despite density dependence in survival. However, because behaviors that affect density-dependent growth can be altered by condition at settlement, the degree to which population biomass is regulated may depend on initial condition.