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

Koei Mizutani,Keita W Suzuki,Reiji Masuda,Masatomo Tagawa,Kuniaki Nakata,Toshiyuki Yamada

doi:10.1007/s12562-019-01369-1

Abstract

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.

Highlights

Most teleosts undergo striking bodily changes during their development from larva to juvenile
We first showed that net-lined rearing tanks clearly suppress hypermelanosis in Japanese flounders, both it its expansion in juveniles with small darkened areas and its significant initial appearance in juveniles without darkened areas
Nakata et al (2017) demonstrated that an undulating bottom surface, such as that provided by a net lining, suppressed the “expansion” of hypermelanosis in juveniles that had already expressed a small degree of darkening

Summary

Introduction

Most teleosts undergo striking bodily changes during their development from larva to juvenile (e.g., metamorphosis). First localized pigmented/darkened areas appear on the skin, followed by gradual expansion of the pigmented areas on the blind side of the flounder after the completion of normal metamorphosis. The market value of flounders with hypermelanosis, both those reared to harvest size in aquaculture systems and those released into the sea for recapture by fishermen, is about 20–70% lower than that of fish with normal coloration (Kaji and Fukunaga 1999). Dietary studies suggest that vitamins A (Tarui et al 2006) and D (Haga et al 2004) have a stimulatory effect on hypermelanosis. Despite these extensive and intensive efforts, hypermelanosis has proven difficult to prevent on the industrial scale

Methods

Results

Conclusion