Experiments on the Recognizable Part of the Gill Net and the Process of Entanglement of Bottlenose Dolphin Tursiops truncatus

Abstract

Recently, the dolphin’s and porpoise’s entanglement in gill net becomes an issue. The situation requires that the effective counter-measure should be done. We reserched the dolphin’s entanglement condition and their ability of recognizing the gill net. We used bottlenose dolphin Tursiops truncatus. The net enclosure in Taiji port in Wakayama prefecture was the experiment field. We observed the dolphin’s behavior related to barriers such as a gill net, float, rope, and netting only (without float line and ground line). The dolphin got entangled only in the condition that it swan at night and the barrier is netting only. At daytime, bottlenose dolphin might mainly use the sense of sight. At night it might use echolocation. We concluded that at daytime dolphin can recognize the net only, at night it can recognize the existence of the net, but can scarcely recognize the detail of the net’s figure by echolocation. On the other hand, at night the dolphin can recognize sufficiently the float by echolocation. © 1991, The Japanese Society of Fisheries Science. All rights reserved.