Seasonal growth of macroalgae (Sargassum) shapes juvenile fish assemblages on subtropical reefs

Seasonal habitats formed by the brown algae Sargassum in Hong Kong provide important nursery grounds for juvenile fishes, including several commercially relevant species. This study used purse-seining and cage-trapping to explore the spatial-seasonal variation of juvenile fish assemblages in three coastal habitats, namely Sargassum beds, rocky reefs, and mangrove mudflats in three regions in Hong Kong, China. It also examined the influence of Sargassum canopy complexity on juvenile fish assemblage. Around 110,363 juvenile fish from 145 species were collected in total, including 58 unique to Sargassum. Over 92% of fishes were <3 cm long, mostly found during the peak Sargassum season. Differences in fish assemblages amongst the three types of coastal habitats and among the three regions were driven by seasons. Several commercially important species (e.g. Rhabdosargus sarba and Pagrus major) were at least twice as abundant in Sargassum beds than rocky reefs during the peak season. Furthermore, a large group of juvenile fishes (61 species) had a substantial proportion (> 50%) of their relative abundance within Sargassum beds, in contrast to 13 and 26 species in rocky reefs and mangrove mudflats, respectively. Juvenile fish assemblages were structured by habitat-scale three-dimensional complexity, particularly the presence of canopy-forming Sargassum, whereas within-bed variation in canopy density or biomass had limited effects. The subtropical Sargassum beds host a greater diversity and biomass of juvenile fish when Sargassum flourishes. We conclude that seasonal Sargassum in subtropical coastal waters provides an important habitat refuge for juvenile fishes and contributes to fish recruitment. Such macroalgal habitats should be preserved and protected.