The influence of allochthonous macroalgae on the fish communities of tropical sandy beaches

This study evaluates the hypothesis that detached macroalgae (drift algae) influence the structure of fish communities in the surf-zones of two tropical sandy beaches from the central coast of Brazil. Surf-zone seine hauls were conducted weekly during three consecutive 6-week periods according to the occurrence of drift algae - in pre-drift, drift and post-drift periods. Drift algae comprised 33 macroalgae species and likely came from offshore rhodolith beds. Fish biomass, density and species richness increased from pre-drift to drift periods, and species composition differed significantly between periods, particularly in the more sheltered of the sampled beaches. The density of young-of-the-year (YOY) fishes also increased significantly from the pre-drift to drift periods, suggesting the importance of drift algae to early fish stages. The gut contents of the two most abundant species during the drift algae period, Trachinotus falcatus and Trachinotus goodei, demonstrated the importance of macrophyte-associated amphipods in their diets, suggesting that the drift algae may be used as feeding habitats for these species. Our results showed that the influx of allochthonous drift algae may structure fish communities of tropical sandy beaches and that it may also represent an important alternative resource for YOY fish by providing shelter and food during autumn and winter.