Habitat Degradation and Settlement Behavior: Effects on Fish Settlement, Survival, and Recruitment

As coral reefs change in their composition, the dynamics of organisms that depend on them will likely also be affected. Traditionally, marine ecologists have recognized that settlement of fishes (and invertebrates) is tremendously variable in space and time, and changes in coral communities might therefore be expected to operate through effects on settlement. However, recent work demonstrates that density-dependent survival also varies spatially, with high quality sites offering greater recruitment than low quality sites at comparable settler densities. Furthermore, variation in settlement may often be correlated with site quality. As a result, understanding the response of fish populations to changes in coral communities requires that we understand how site quality and settlement interact through larval behavior. Here, we explore the implications of declines in site quality (e.g., due to the loss of coral habitat) and larval behavior. Specifically, we explore effects of larval site selection (and its dependency on site quality) and larval redirection (the extent to which larvae that would have settled at degraded sites can go on to settle elsewhere). We use data from the sixbar wrasse (Thalassoma hardwicke) system in Mo'orea, French Polynesia. We show that under some patterns of larval behavior, settlement, recruitment and survival can actually increase as habitat loss increases. These counter-intuitive results have important implications for informing monitoring studies of reef fish. The results also suggest the need for understanding the relationship between settlement rates and site quality, and for quantifying the extent of larval redirection.