Nursery Habitats for Semi-Endemic Fish Species in the Chikugo River Estuary, Ariake Sea, Japan

To understand the characteristics of fish nursery habitats for semi-endemic species (i.e. species found only in the Ariake Sea within Japan), the juvenile fish community was investigated in the Chikugo River estuary on a monthly basis in 2005 and 2006. While in total 77 species/types (including seven semi-endemic species) were identified, the number of species/types found in the estuary increased with temperature (7–31ºC). The number of individuals of the two semi-endemic species Coilia nasus (Engraulidae) and Neosalanx reganius (Salangidae) ranked first and second in the total catch, respectively. In the upper estuary (salinity 0.1–10.1), C. nasus and N. reganius occurred almost every month, accounting for 86% in number. In the lower estuary (salinity 0.7–27.3), the seven semi-endemic species (29%) were less abundant than the other species/types (71%). In summer 2006, when floods occurred repeatedly, catch per unit effort was extremely small in the upper estuary, whereas in the lower estuary it increased with freshwater discharge levels. The present study indicated that the upper estuary is a specific nursery habitat for C. nasus and N. reganius and that the lower estuary is a general nursery habitat for many coastal fishes including semi-endemic species especially during flood periods.