Seasonal habitats formed by the brown algae Sargassum in Hong Kong play a vital role in global fisheries by offering key nursery habitats for many commercial species. This study used purse-seining and cage-trapping to explore the spatial-seasonal variation of juvenile fish assemblages in three coastal habitats. It also examined the impact of Sargassum canopy complexity on juvenile fish assemblage. Around 110,300 juvenile fish from 145 species were collected, including 58 unique to Sargassum. Over 92% were less than 3 cm long, mostly found in winter. Differences in fish assemblages amongst the three types of coastal habitats and among the three regions were strongly driven by seasons. Several commercially important species (e.g., Rhabdosargus sarba and Pagrus major) were at least two times more abundant in macroalgal reefs than rocky reefs during winter. Furthermore, a large group of juvenile fishes (61 species) had a substantial proportion (> 50%) of their relative abundance within macroalgal reefs, in contrast to 13 and 26 species in rocky reefs and mangrove mudflats, respectively. Juvenile fish assemblages significantly varied with canopy complexity in winter; biomass and density of Sargassum accounted for 25% of the variation in juvenile fish composition. The subtropical macroalgal reef hosts a greater diversity and biomass of juvenile fish during the winter period when Sargassum flourishes. We conclude that seasonal Sargassum in subtropical coastal waters, like those in Hong Kong, provides an important habitat refuge for juvenile fishes and contributes to fish recruitment. Such macroalgal habitats should be preserved and protected.
 

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