Spatial variation and habitat preference of reef fish and gastropod assemblages from two coastal habitats in subtropical reef, Port Shelter, Hong Kong

The complexity of coastal habitats could influence the biodiversity in such areas due to the variability of food availability and refuge protection. Coral reefs and macroalgal beds are two important coastal habitats for sustaining a greater abundance of fishes and invertebrates in marine ecosystems because of high productivity and structural refuge. In Hong Kong, there were highly diverse coral communities and seasonal Sargassum beds. A survey of reef fish and gastropod assemblages was conducted in five coral communities and five Sargassum beds in Sai Kung for three dates by SCUBA diving from March 2023 to May 2023. We found that the mean abundance of reef fish (278.58 ± 40.39 individuals 60 m⁻²) in Sargassum habitats was higher (p < 0.01) than that (134.44 ± 35.13 individuals 60 m⁻²) in coral habitats. The mean gastropod abundance (207.56 ± 25.58 individuals 60 m⁻²) in Sargassum habitats was higher (p < 0.001) than that (64.89 ± 6.65 individuals 60 m⁻²) in coral habitats. The abundance of reef fishes was found to be positively correlated to coral cover (r_S = 0.464, p < 0.01). Moreover, species richness and abundance of reef fishes and gastropods showed no relationship with Sargassum density (Species richness: reef fishes: r_S = 0.013, p > 0.05; gastropods: r_S = 0.100, p > 0.05; Abundance: reef fishes: r_S = 0.231, p > 0.05; gastropods: r_S = 0.217, p > 0.05). Furthermore, the species composition of reef fishes and gastropods from coral habitats was also significantly different (reef fishes: p < 0.001; gastropods: p < 0.001) from Sargassum habitats. Overall, some fish species exhibited habitat specificity by possibly migrating from coral communities to Sargassum beds during the Sargassum growth season or as juveniles to inhabit Sargassum beds for food resources and to avoid predation. In addition, most gastropod species were not habitat-specific, but there might be favorable characteristics in Sargassum beds resulting in higher gastropod abundance. To better understand the coral-reef fish relationship, the abundance of reef fishes was found to be negatively correlated to coral genus diversity (rs = −0.413; p < 0.01). Reef fishes and coral composition also significantly differed (reef fishes: p < 0.001; corals: p < 0.001) among sites, and the reef fish composition can be structured by the coral composition. Acropora, Platygyra, and Pavona were significant variables shaping the reef fish composition, suggesting their presence and coverage as important factors in structuring the reef fish assemblage in a habitat.