An effective and practical method of net settings in rearing tank to suppress hypermelanosis in Japanese flounder

Abstract

In Japanese flounder aquaculture, hypermelanosis occurs widely on the blind side. Rearing flounders in a net-lined tank was recently reported to prevent hypermelanosis. To effectively apply this method to larger tanks for aquaculture farming, the net setting method was examined. Juvenile flounders without darkened areas on the blind side [total length (TL) 13 cm] were selected, and reared for 6 months (TL 32 cm). In the control tank without a net, the median value of darkened area ratio (darkened area to blind side area) was 46%. By only covering the tank bottom with slack net, the darkened area ratio was suppressed to 8%, less than 1/5 of that in the control tank. At the end of experiment, bottom coverage ratios of 0%, 10%, 30%, 50%, and 100% revealed a negative correlation between bottom net coverage and darkened area ratio. In this experiment, the darkening area in the tank with 50% bottom net coverage decreased to 1/3 of the control. Although the occurrence of hypermelanosis differs depending on the production lot, these results are expected to serve as a reference for selecting the suitable net size to meet the level of clearness of the blind side.