魚群探知機の記録から見たイワシ群の生態-III

外敵の攻撃より逃避するイワシ群の魚探記録

抄録

A series of field tests has been carried out at a fishing ground (Fig. 1) to obtain schooling behavior of sardines gathering under a fishing lamp and their reaction when they were attacked on by dolphins, which are often the cause of fishermen's failure in catching sufficient amounts of fish.
Plotted on Figs. 2 to 10 are images of sardines and dolphins recorded by a fish-detector. Each small spot near the surface represents an individual of dolphin, while a cloudy shadow is the reflection from a sardine school. Judging from these results we may presume that the sardine would escape very rapidly from an approaching shadow of the dolphin, and that the dolphin do not likely to swim deeper than 40 meters at any time as far as the present findings are concerned.