Retinomotor responses of juvenile bluefin tuna Thunnus thynnus

In bluefin tuna culture, a high mortality of juveniles is caused by bumping into the tank and net-pen walls at dawn. This bumping can possibly be attr ibuted to visually disoriented behavior of the fish. To examine this possib ility, the authors carried out retinomotor response experiments with juveni le bluefin tuna (50.7-96.8 mm total length, at which they were transferred from a indoor tank to a sea net-pen) and measured ambient light intensity a t the culture site at dawn. The light intensity at which the transition fro m scotopic to photopic vision takes place was 7.52 lx and the time taken by the transition was 15 min. At dawn, the ambient light intensity rapidly in creased from scotopic light intensity level and attained photopic light int ensity level in 10 min. This incompatibility of the retinal adaptation with the change in the ambient light intensity could cause the visual disorient ation of the fish. It is therefore possible that the visually disoriented j uveniles cannot control their high power swimming and thus bump into the wa lls at dawn.