STATIONARY ABILITY OF BASTARD HALIBUT PAR ALICHTHYS-OLIVACEUS AND LONGSNOUT FLOUNDER LIMANDA-PUNCTATISSIMA TO WATER CURRENT AT THE BOTTOM OF A NET CAGE

To estimate and control the water current velocity at the bottom of a net cage for aquaculture of benthic fishes, it is necessary to know th e stationary-holding ability of the fish in the net. In this experimen t, bastard halibut Paralichthys olivaceus and dead longsnout flounder Limanda punctatissima were used. A net was laid horizontally inside a small flume tank. Then the fish was carefully placed inside in such a way that its body axis was parallel to the stream line and its head wa s at the upper stream side. For bastard halibut, each fish was allowed to stay approximately 5 minutes in the tank. A slow water current was applied and increased gradually. The current velocity at the time whe n the fish slipped and could not hold the same position inside, was re corded and termed as the slipping speed. The coefficient of static fri ction between the fish body and the net plane was calculated by the an gle of the slope. The relationship between total length l and slipping speed u(s) was expressed by the function u(s) = k3square-root(l) in b oth two species. The value of proportional constant k3 was 3.2 (cm/s2) 1/2 in bastard halibut, 3.3 (cm/s2)1/2 longsnout flounder. For bastard halibut of 1.0 kgf weight and 45 cm total length, the average slippin g speed was 21 cm/s.