Effects of habitat change from a bare sand/mud area to a short seagrass Halophila ovalis bed on fish assemblage structure: a case study in an intertidal bay in Trang, southern Thailand

Fish assemblage structures in an intertidal sand/mud area invaded by short seagrass Halophila ovalis and a nearby non-invaded sand/mud area in Trang Province, Thailand, were examined in detail by visual census to elucidate the effects of such habitat change on assemblage structure. The assemblage structure in the newly established seagrass bed showed a significant shift from that in the sand/mud area, despite the total fish species numbers remaining similar to each other (i.e., 30 and 29 species, respectively). Total fish density was significantly lower in the seagrass bed. In addition, differences in density patterns of component species between seagrass and sand/mud fish assemblages were evident, some fish species being restricted to or more abundant in the alternative habitat. Such differences may have arisen, at least in part, from differences in practical food availability/accessibility between the two habitat types, and/or specific microhabitat preferences of the resident fishes. Although seagrass habitats are often regarded as supporting a richer fish assemblage compared with bare sand/mud areas, the present study clearly indicated that the latter also supported a unique fish assemblage, including juveniles of fishery target species. Accordingly, both seagrass and bare sand/mud habitats should be taken into consideration for both the conservation of greater overall biodiversity in the coastal environment and the continued existence of local fisheries.