Responses of fish assemblage structures to sandy beach types in Kyushu Island, southern Japan

To determine differences in fish assemblage structures between beach morphodynamic types on an exposed sandy shore, three beach types (reflective, intermediate, and dissipative) were sampled at Fukiagehama Beach, Kyushu Island, southern Japan, in May, August, and November 2006 and 2007. Distinct differences in physical conditions and the amount of major prey invertebrates, such as copepods, mysids, and amphipods, were recognized between the three beach types. The reflective beach was characterized mainly by turbulent swashes, coarse sediment, and a low abundance of major prey invertebrates. In contrast, the dissipative beach was characterized mostly by benign swashes, fine sediment, and a high abundance of prey invertebrates. The intermediate beach tended toward an intermediate condition between the reflective and dissipative beaches. The fish assemblage structures also differed clearly between the three beach types, with species and individual numbers being greatest on the dissipative beach and lowest on the reflective beach. A similarity index indicated differences in species composition of the assemblages between the beach types. Such differences arose primarily from the differential distributions of two dominant trophic groups, zooplankton (mostly copepods) and epibenthic crustacean (mainly mysids and amphipods) feeders, between the beach types, both groups tending to increase in species and individual numbers from the reflective beach to the dissipative beach. In addition, the mean standard length of all fish collected in each month tended to decrease toward the dissipative beach. These results suggested that beach types strongly influence sandy beach fish assemblage structures.