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マダイの視力とその照度による変化を学習法により求めた。最小視認闘によるマダイの視力は500lxで0.24であった。照度を段階的に減少させたところ,視力はこれにともない減少し,1lxで0.10となった。さらに照度を0.1lxに減少させたが,視力は1lx時と同じ0.10であった。網膜の順応状態を調べたところ色素指数では,1lxと0.1lxの間で明順応から暗順応に変化していた。これらから,マダイの網膜で機能的な視細胞は1lxと0.1lxの間で錐体から桿体に移行することが示唆された。

SUMMARY: 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 attributed to visually disoriented behavior of the fish. To examine this possibility, the authors carried out retinomotor response experiments with juvenile 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 at the culture site at dawn. The light intensity at which the transition from 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 increased from scotopic light intensity level and attained photopic light intensity level in 10 min. This incompatibility of the retinal adaptation with the change in the ambient light intensity could cause the visual disorientation of the fish. It is therefore possible that the visually disoriented juveniles cannot control their high power swimming and thus bump into the walls at dawn.

The retinal adaptations were examined for the walleye pollock Theragra chalcogramma taken at different times of day and night from different depths of trawling in the north Pacific. Even in the daytime, some retina sampled from depths of 150 to 200m showed a tendency towards dark adaptation. This suggests the inactive response of the walleye pollock against the gear due to reduced visual acuity and colour vision. To confirm the results from the on-board sampling, the retinomotor response was also examined by laboratory experiment. The light intensity for darkadaptation of walleye pollock was revealed to be much lower than 10-2 lx. These results are dissussed in relation to the ambient light intensity in the sea, and the visual function and swimming ability of fish during the capture process by trawl net.

In order to investigate the differences in retinomotor responses between pre- and post-midnight phases, the responses in chub mackerel angled under a fishing lamp and those in sardine caught by nighttime purse seining were examined. The adaptation ratio determined in the cone and pigment layers of chub mackerel indicated no direct relationship to illuminance at angling point, but showed a large variation. Chub mackerel were mostly angled at a light intensity ranging from 10-1 to 10-3 lx, and were angled more frequently in darker areas post-midnight than pre-midnight. The correlation between cone migration and pigment migtation was high previous to midnight, but was low subsequent to midnight. This tendency was observed in both species. If these results are due to circadian rhythms corresponding to retinomotor responses, operating time in lamp fishing operations should be considered in commercial endeavors.