Detecting age-structured effects in growth performance of coral reef fish juveniles

Abstract

The growth performance of coral reef fish juveniles collected in different habitats is often used as a proxy for habitat quality for juveniles. However, back-calculated growth trajectories of juve- niles may be age-structured, for instance, because of potential differences in initial offspring size and/or quality or size-selective mortality. A novel approach is proposed to isolate growth performance of coral reef fish juveniles from potential age-based factors. Juveniles of Chromis viridis (Pomacentri- dae), Lethrinus genivittatus (Lethrinidae) and Siganus fuscescens (Siganidae) were collected from waters around inshore and offshore islets in a coral reef lagoon. Individual growth trajectories were back-calculated from otolith increments and compared with repeated-measures generalised linear models (RM-GLMs). Settlement marks in otoliths were used to differentiate larval and juvenile growth trajectories for each individual. For the 3 species, fish from around the offshore islet presented signifi- cantly larger size-at-age during their larval stage than those from from around the inshore islet. Juve- niles of L. genivittatus and S. fuscescens from around the offshore islet remained larger than inshore juveniles at the same age, while growth curves of C. viridis from the 2 islets crossed at settlement so that inshore fish were larger as juveniles than offshore individuals. RM-GLMs revealed that the growth trajectory was significantly age-structured for C. viridis only. These results suggest that post-settlement age may be used as a covariate in comparative analyses of larval growth in order to isolate growth per- formance from potential age-based factors.