Spatial patterns and intra-annual variations in subtropical reef fish communities in Okinawa Island, Japan

Abstract

Monitoring coral reef fish communities remains an imperative endeavor as the reefs continue to face a multitude of threats and stressors that may compromise their health, functions, and the ecosystem services they provide. As reef fish communities are shaped by various large-scale and local processes, elucidating their patterns and dynamics across spatiotemporal scales is fundamental in understanding their current status and future outlook. Okinawa Island in southern Japan has high reef fish diversity despite its high latitudinal position, making it an important habitat for northernmost coral reef fish populations. However, this diversity is also under continuous threat by island development and other natural stressors. Furthermore, despite voluminous research on the coral reef organisms on this island, basic information on the patterns and trends of fish communities in various sites in Okinawa Island remains limited. In this study, we described the spatial patterns of reef fish communities among and within sites in Okinawa Island. We also observed how these patterns changed across four timepoints within a year, including after an opportunistic typhoon disturbance. Through underwater visual census at eight fringing reef sites, we examined the species richness, abundance, biomass, Shannon diversity, composition, and overall community of reef fishes at shallow (1–6 m) and deeper (8–15 m) zones in each site. We found high spatial variation among sites and contrasts between depth zones within some sites. Temporal variation in the overall community and selected metrics was detected with overall peaks in species richness and abundance before winter and declines immediately after winter. However, given the limitations of our study, we caveat that these trends represent intra-annual variations rather than pronounced seasonal effects. Further investigations through continuous monitoring will clarify these patterns and provide a more comprehensive understanding of Okinawa’s reef fish biodiversity to inform strategies for its protection.