Effect of water current on the body size and occurrence of deformities in reared Japanese eel leptocephali and glass eels

Abstract

Research efforts to achieve the production of artificial seedlings of the Japanese eel Anguilla japonica have progressed in recent decades. However, morphological deformities have been frequently observed in reared leptocephali and glass eels. We examined the effect of water current velocities (5.7–8.3 cm/s) on the body size and morphology of reared leptocephali and metamorphosed glass eels. As the current velocity increased, the size of leptocephali became smaller and the occurrence rate of notochord curvature increased. However, even in low velocities, water current had a long-term negative impact on their morphology. Sixty-five percent of metamorphosed glass eels had one of the eight types of vertebral deformities: compression, luxation, fusion, brachyspina, modification, lordosis, kyphosis, or scoliosis. Although their occurrence rate was unrelated to current velocity, there was a tendency for some deformities to be localised in a certain area of the vertebral column. In particular, compression frequently occurred in caudal vertebrae in faster currents. Most vertebral deformities began before the completion of metamorphosis. Therefore, appropriate management of the water current during the leptocephalus stage is important for establishing mass production of morphologically normal glass eels.