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.

中文翻译：

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日本南部西表岛绿海龟 Chelonia mydas 过度放牧导致海草床退化的鱼类群落结构响应

在日本南部的西表岛，通过对海床内和周围的几个微生境中的鱼类物种进行目视普查，研究了鱼类群落结构对由于绿海龟 (Chelonia mydas) 过度放牧而导致海草床快速退化的反应（即，密海草床、放牧床、它们的边界和相邻的沙区）。海草微生境的鱼类组合结构不同，与过度放牧的床和未植被的沙地相比，有海草的微生境的物种和个体数量更高，以及不同的物种组成。在密集海草床和边界区（密集海草床和放牧区的边界区域），与海草相关的鱼类，如兔鱼 Siganus fuscescens 和红衣主教鱼 Ostorhinchus ishigakiensis，数量丰富，可与该地区其他海草床中的鱼类相媲美。此外，这些微生境中的鱼类组合随季节变化，夏季由于补充水平较高，鱼类丰度更大。在放牧床中，底栖虾虎鱼，如 Ctenogobiops crocineus 和 Cryptocentrus caeruleomaculatus 占主导地位（在没有植被的沙质地区也是如此），与密集海草微生境中的相比，此类组合的季节性变化相对较低。放牧地区海草相关鱼类的丰度和多样性下降的部分原因是缺乏全年用于庇护和觅食的植被（叶长 < 20 厘米）。