Mortality of juvenile damselfish: Implications for assessing processes that determine abundance

We examined the effects of variation in intra- and intercohort density on the magnitude and form of per capita juvenile mortality rates of three species of damselfish (Dascyllus spp.) at Moorea, French Polynesia. Patterns of mortality over a 2-wk period were estimated from daily counts of new settlers and of the next older age class (<1 mo old) following a natural settlement pulse to standard amounts of suitable microhabitat. Two spatial scales were explored: among 11 lagoon sites dispersed evenly around the 60-km perimeter of the island and among microhabitats within a site. For each species at both spatial scales, per capita mortality rates of new settler cohorts increased monotonically with density, whereas those of the next older cohort were density independent. Intra- and especially intercohort processes induced density-dependent mortality in new settler cohorts. Despite experimental densities that were only 15-25% of ambient, similar to 50% of the spatial variance in settler abundance was reduced by density-dependent mortality in 2 wk. The relative contributions of primary recruitment limitation and subsequent density-dependent loss in setting the average abundance of 2-wk-old recruits were estimated to be similar to 70% and similar to 12%, respectively. Our findings demonstrate that density-dependent mortality may only occur for a brief period immediately after settlement of at least some reef fishes and that its influence can be relatively large at comparatively low densities. These results have major implications for current assessments of the relative importance of the processes that drive abundance and dynamics of species with demographically open populations.