Fish assemblage structure response to seagrass bed degradation due to overgrazing by the green sea turtle Chelonia mydas
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Fish assemblage structure response to seagrass bed degradation due to overgrazing by the green sea turtle Chelonia mydas at Iriomote Island, southern Japan Hiroyuki Inoue1 · Akira Mizutani1 · Kusuto Nanjo2 · Kouki Tsutsumi3 · Hiroyoshi Kohno1 Received: 3 April 2020 / Revised: 13 August 2020 / Accepted: 15 August 2020 © The Ichthyological Society of Japan 2020
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 ( 50 cm in leaf length) concentrated near the center of the bay covered a total area of ca. 10 ha in 2013, a 25% reduction more or less of the area covered in 1974 (Takeyama et al. 2014). One of the reasons for the reduction in the area of the beds is considered to be overgrazing by green turtles, since most leaves on the outer edges of the beds were extremely short ( Winter
DB dense seagrass beds, BO boundary areas, GR grazed beds
(10 individuals, 3.7%) and Valenciennea longipinnis (8 individuals, 2.6%) were most abundant in the sandy area. The mean numbers of both species and individuals differed significantly among microhabitats and seasons, an interaction among microhabitats and seasons also being evident (Table 3; Fig. 5). In most (all) seasons, the mean number of species (and individuals) was significantly higher in the dense seagrass beds than in the grazed beds and sandy areas, as well as highest in summer in each microhabitat. The observed fishes included 3 pelagic (2 families), 26 semi-benthic (12 families) and 23 benthic (3 families) individuals (Table 2). The numbers of species and individuals of semi-benthic and benthic fishes, but not pelagic fishes,
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