SELECTIVE MORTALITY DURING THE LARVAL–JUVENILE TRANSITION IN TWO CORAL REEF FISHES

Abstract

For organisms with complex life histories, processes occurring during transitions between stages can strongly affect population dynamics. The major life history transition for many marine species is settlement from pelagic larvae to benthic or demersal juveniles. We examined differential mortality at settlement as a function of early life history traits (size-at-age, growth rates) in three cohorts of two common Caribbean coral reef fishes, Thalassoma bifasciatum and Halichoeres bivittatus (Labridae). We deployed light traps to collect late-stage larvae of each cohort. We also collected juveniles of each cohort at regular intervals (every second day) for two weeks following their first appearance on the nearshore reefs of Barbados, West Indies, during the spring (April–May) and fall (August–October) of 1997. Comparisons of otolith-derived traits exhibited by younger recruits (initial group) to those exhibited by older juveniles (survivor group) revealed that there was a difference in otolith growth during metamorphosis for both species: survivors had wider metamorphic bands. In addition, surviving H. bivittatus exhibited higher larval growth rates than settlers, while surviving T. bifasciatum had higher juvenile growth during the first day following emergence onto the reef. Differences in otolith growth rates in the absence of similar changes in somatic size indicate that mortality may have acted upon a growth-based characteristic of condition. We hypothesize that larvae settling in better physiological condition differentially survived the early juvenile period. Finally, selective mortality appeared to be greatest during a relatively brief window of time surrounding metamorphosis, indicating that this is a critical period. These results point to the demographic importance of transitional periods between stages for animals with complex life cycles. Furthermore, traits exhibited by earlier stages can significantly influence survival during these transitional periods.