Abstract
The fish assemblage structure response to rapid degradation of Enhalus acoroides seagrass beds due to overgrazing by green sea turtles (Chelonia mydas) was investigated at Iriomote Island, southern Japan, by visual censusing of fish species in several microhabitats in and around the beds (i.e., dense seagrass bed, grazed bed, their boundaries, and adjacent sandy area). Fish assemblage structure differed among the seagrass microhabitats, both species and individual numbers being higher in microhabitats with seagrass compared to overgrazed beds and unvegetated sandy areas, together with different species composition. In the dense seagrass beds and boundary areas (the border area between dense seagrass beds and grazed areas), seagrass-associated fishes such as the rabbit fish Siganus fuscescens and cardinal fish Ostorhinchus ishigakiensis were abundant and comparable with those in other seagrass beds in the area. In addition, the fish assemblages in those microhabitats varied seasonally, fish abundance being greater in summer due to higher levels of recruitment. In the grazed bed, benthic gobies, such as Ctenogobiops crocineus and Cryptocentrus caeruleomaculatus, were dominant (similarly so in unvegetated sandy areas), and seasonal variations in such assemblages were relatively low compared to those in dense seagrass microhabitats. The decrease in the abundance and diversity of seagrass-associated fish in the grazed areas could be explained partly by the shortage of vegetation (< 20 cm in leaf length) for shelter and feeding throughout the year. The results suggested that the degradation of seagrass beds due to overgrazing by green turtles has a deleterious effect on fish assemblages, and that future conservation and management strategies for seagrass beds should consider both their associated fish communities and green turtles.
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Acknowledgments
We are grateful to K. Sakihara for assistance with fieldwork. Constructive comments on the manuscript from Graham Hardy and two anonymous reviewers were much appreciated. This study was supported by a grant for Study and Education from the Okinawa Regional Research Center, Tokai University.
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Inoue, H., Mizutani, A., Nanjo, K. et al. Fish assemblage structure response to seagrass bed degradation due to overgrazing by the green sea turtle Chelonia mydas at Iriomote Island, southern Japan. Ichthyol Res 68, 111–125 (2021). https://doi.org/10.1007/s10228-020-00775-1
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DOI: https://doi.org/10.1007/s10228-020-00775-1