Prevention of hypermelanosis by rearing Japanese flounder Paralichthys olivaceus in net-lined tanks

In artificially reared flatfish, especially the Japanese flounder Paralichthys olivaceus, pigmented skin (hypermelanosis) frequently appears on the fish’s blind side after normal metamorphosis. As no practical prevention method has yet been proposed, we examined the effectiveness of a loose net placed inside the rearing tank that covers the bottom and walls like a pouch. When juveniles (standard length [SL] 6 cm) were transferred to the net-lined tank (mesh size 4 mm) before the first appearance of hypermelanosis, the pigmented area after 2 months covered about 0.5% of the blind side; this is about 1/40th of the area covered by pigment in fish reared in an ordinary tank (20%). Although the initial appearance of pigmentation in the axilla area (the area covered by the pectoral fin) was not suppressed, utilization of a larger mesh size (12 mm) decreased the expansion of pigmentation in this area. Juveniles reared in the net-lined tank were about 5–15% smaller (SL) than those reared in the ordinary tank, but their body depth:SL ratio was closer to that of wild-caught juveniles. From the results of this study, we propose that net-lined rearing tanks with larger-sized mesh are a practical method of preventing hypermelanosis in Japanese flounder aquaculture systems.