LIFE-HISTORY CYCLES IN FLATFISH FROM THE NORTHWESTERN PACIFIC, WITH PARTICULAR REFERENCE TO THEIR EARLY LIFE HISTORIES

Nearly 300 species of Pleuronectiformes are found in the western Pacific, of which 120 species are recorded around Japan. In this area the geographical distribution of flatfish is generally related to family. Thus two major groupings are apparent; those from cold water, principally the major members of the Pleuronectidae, and those from warm-water habitats including members of the Citharidae, Paralichthyidae, Bothidae, Soleidae and Cynoglossidae. Notably, the family Scophthalmidae is not found in the northwestern Pacific. Japan's temperate location between subarctic and subtropical zones results in an overlap of the two groups. Most species inhabit the continental shelf, but some occupy waters deeper than 200 m and a few are found below 1000 m. Spawning occurs throughout the year at low latitudes, but tends to concentrate in summer at high latitudes. Flatfish tend to move to shallow water to spawn. Nursery grounds for juveniles are shallower than those of the spawning grounds. Size at metamorphosis and pelagic-life span vary among species, but there is a general trend of deeper-dwelling species having a longer pelagic life span and a larger size at metamorphosis. In general, flatfish do not exhibit large-scale migration, but most of them move to shallower waters in spring and return to deeper waters in autumn. Most species change their diet from zooplankton to benthic animals as they settle on the bottom. The Paralichthyidae mainly feed on mysids, the Pleuronectidae on polychaetes and the Soleidae on gammarids. There are significant differences in the ecology of flatfish between, and in some cases within, families. These differences make co-existence possible for the great number of flatfish species found on the continental shelf and slope in the northwestern Pacific.