Population structure of Ctenochaetus striatus (Acanthuridae) in Tutuila, American Samoa: The use of size-at-age data in multi-scale population size surveys

We combined size-at-age and underwater survey (UVS) data to investigate the spatio-temporal distribution of life-history stages of the bristletooth surgeonfish, Ctenochaetus striatus, around Tutuila Island, American Samoa. Three age-based size categories (recruits, juveniles and adults) were determined by examining the relationship between size, age and gonad maturity. The results from combining size-at-age with UVS density data showed varying densities at the scales examined (years, exposure, sector, and habitat) on the fore-reef slopes of Tutuila Is. While recruit densities were uniform, juvenile densities varied by year, exposure and sector, and adult densities were significantly higher at topographic sites that were comparatively more exposed to water motion than inside bays. Densities of recruits were positively correlated with habitat (rubble) illustrating the importance of this substratum for the survivorship of early life history stages. Densities of adults were positively correlated with coralline algae, which may suggest co-occurrence responding to similar physical environment of adult habitat that is possibly indirectly correlated to feeding. However, densities of juveniles were negatively correlated with food source although the variance accounted for was low. We hypothesize that some other substratum may possibly be important for juveniles. There was a negative correlation between recruit and adult densities, indicating the presence of ontogenetic shifts in habitat with age and size. It is proposed that ecosystem-based fishery management of C. striatus in Tutuila Is should consider the high abundance of adults in exposed topographic habitats in the establishment of marine protected areas. We have shown that size-at-age data when combined with underwater fish surveys significantly expands its utility in the study of the population structure of a coral reef fish thus allowing the identification of areas characterized by high abundance of particular life stages for management.