Fish assemblage structures in planted seagrass habitats, compared with natural seagrass habitats: a field experiment in an intertidal bay in Trang, southern Thailand

Abstract Fish assemblage structures in 1 × 1, 3 × 3 and 5 × 5 meter planted seagrass habitats, natural seagrass habitats and bare sand/mudflats in the intertidal bay at Trang Province, Thailand, were examined in detail by visual census, in order to clarify if the fish assemblage structures in the planted habitats had become similar to those in the natural seagrass habitats. Although the establishment of the smallest planted habitats required a relatively long time, their tidal inundation eventually resulted in harbored fish assemblages of similar structures to those in the intertidal natural seagrass habitats. Of 63 species recorded in the natural seagrass habitats, 47 occurred also in the planted habitats, with common dominant species (e.g., Siganus fuscescens , Halichoeres bicolor and Pelates quadrilineatus ), and density patterns of species assemblages in both habitat types becoming relatively similar over time. Total species numbers tended to be greater in both planted and natural seagrass habitats covering greater areas, probably a result of higher fish encounter rates and greater probability of discovery of the larger habitats. On the contrary, total fish and dominant species densities tended to be higher in seagrass habitats with smaller areas. Relatively greater densities of some prey items in smaller habitats, in addition to overcompensation of lower encounter rates and probability of habitat discovery by large groups of dominant-species’ juveniles within smaller areas may at least partly explain such phenomena. The present study suggested that intertidal planted seagrass habitats eventually play important roles for fish assemblages, much as natural seagrass habitats, and that seagrass-planting in intertidal zones should contribute to rehabilitation of damaged coastal ecosystems. However, intertidal bare sand/mudflats, when inundated, also support unique fish assemblages, including juveniles of fishery target species. Accordingly, seagrass-planting in such habitats without careful consideration could result in negative effects on overall coastal ecosystem biodiversity and the sustainability of local fisheries. Further study, considering a variety of factors which affect assemblage structures, is needed to develop appropriate seagrass-habitat restoration designs.