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

We examined the effects of variation in intra- and intercohort density on t he magnitude and form of per capita juvenile mortality rates of three speci es of damselfish (Dascyllus spp.) at Moorea, French Polynesia. Patterns of mortality over a 2-wk period were estimated from daily counts of new settle rs and of the next older age class (<1 mo old) following a natural settleme nt pulse to standard amounts of suitable microhabitat. Two spatial scales w ere explored: among 11 lagoon sites dispersed evenly around the 60-km perim eter of the island and among microhabitats within a site. For each species at both spatial scales, per capita mortality rates of new settler cohorts i ncreased monotonically with density, whereas those of the next older cohort were density independent. Intra- and especially intercohort processes indu ced density-dependent mortality in new settler cohorts. Despite experimenta l densities that were only 15-25% of ambient, similar to 50% of the spatial variance in settler abundance was reduced by density-dependent mortality i n 2 wk. The relative contributions of primary recruitment limitation and su bsequent density-dependent loss in setting the average abundance of 2-wk-ol d recruits were estimated to be similar to 70% and similar to 12%, respecti vely. Our findings demonstrate that density-dependent mortality may only oc cur for a brief period immediately after settlement of at least some reef f ishes and that its influence can be relatively large at comparatively low d ensities. 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.