Spawning grounds and maturation status in adult flathead flounder Hippoglossoides dubius off Niigata Prefecture, Sea of Japan

Investigations were conducted on the flathead flouder Hippoglossoides dubius to analyze the environmental conditions around the spawning grounds and the maturational status of spawners in the Sea of Japan off Niigata. In both sexes, larger fish matured earlier than smaller individuals, and participated in breeding from the early part of the spawning season. Males were always the predominant sex found within the spawning ground, likely because males spent longer periods in the spawning grounds than females. This finding further suggests that males matured earlier and maintained sexual activity for longer periods than females within the spawning grounds. Submarine topography and water temperature were concluded to be important factors affecting the formation of the spawning grounds. The spawning ground was located within a restricted area where bathymetric lines curve towards a shallow area. Adult fish usually occur in the Japan Sea Proper Water (<1 °C), but spawners gathered just below the surface mixing layer where water temperature was 5–10°C. Hence, it was thought that the flounder spawn at the upper tolerable water temperature limit (i.e. the shallowest accessible depth) to release their offspring into the surface layer.