Implication of changing coral communities: do larval traits or habitat features drive variation in density-dependent mortality and recruitment of juvenile reef fish?

Many lines of evidence suggest coral communities worldwide are changing rapidly. Responses of coral reef fish populations to such changes will, in part, depend upon how vital demographic rates of fish populations respond to changing coral communities. Our previous work on the reef fish Thalassoma hardwicke suggests the strength of density-dependent mortality (a measure of “site quality”) is heterogeneous in space and time. Here we examine environmental features that might account for spatiotemporal variation in quality, to facilitate more accurate predictions of how T. hardwicke populations might respond to habitat degradation and/or changing coral communities. Two habitat attributes—density of branching coral Pocillopora spp. and presence/absence of territorial damselfish Stegastes nigricans—explained >77% of the variation in site quality. The strength of densitydependent mortality experienced by cohorts of T. hardwicke was consistently weaker at sites where S. nigricans was present, and declined with increasing coverage of Pocillopora. However, further sampling suggests that intrinsic quality of juvenile T. hardwicke covaries with these same habitat attributes: cohorts collected from sites with relatively high cover of Pocillopora had proportionally more lipids and faster otolith growth rates as larvae relative to cohorts collected from reefs with noor low coverage of Pocillopora. We expect that habitat attributes and individual traits both contribute to “site quality”, and suggest that a better understanding of the sources of variation in site quality (i.e., density dependence mediated by habitator individual traits) may be necessary to predict the fates of coral reef fish populations with any reasonable accuracy.