Variation in the population of protogynous coral reef fishes over tens of kilometres

Abstract

This study describes changes in the timing of maturation and sex reversal and shifts in the sexual composition of unfished local populations of protogynous reef fishes across the continental shelf of the Great Barrier Reef. On outer shelf reef crests, both Chlorurus sordidus and Scarus frenatus matured as females and underwent protogynous sex reversal at reduced sizes and ages relative to fish from equivalent midshelf habitats 20 km away. Earlier maturation at smaller body sizes on the outer shelf is likely to be an adaptive response to the higher mortality rates in these habitats compared with the middle shelf. Furthermore, reduced size and age at sex reversal on the outer shelf is consistent with the reduced growth and increased mortality regimes in these locations. The sexual composition of local populations changed markedly across the shelf for C. sordidus but not for S. frenatus. High densities of C. sordidus on the outer shelf were characterized by threefold higher proportions of both terminal-phase and primary males than on midshelf reefs. Differences between local populations separated by tens of kilometres highlight the danger of assuming homogeneous sexual compositions and life histories when managing exploited reef fishes over larger spatial scales.