8そう張網操業中に観察される集魚灯による魚群の誘引

抄録

The main aim of this study is to investigate the behaviour of fish in relation to light stimuli, based on the data on logbooks (1960) of a commercial eight-boat lift net, which has been carried out with the aid of artifical lights in the waters west coast of Noto Peninsula, Sea of Japan.
The operation takes place in steps as follows:
Light-boat attracts the fish directly under the lamp spending minimum to 30min. max. 6 hours firstly, next, she pulls toward other light boat waiting in the center of net which had already been laid out. The latter boat, by putting on light, receives the fish school or schools which were led by the former. The school of fish here is defined as that of Nicholsky (1962).
The results obtained are as follows:
1) The depth of sardine schools in ordinary circumstances and their floating-up toward surface by the influence of light as detected by fish finders, differ depending on seasons (Fig. 1). The interpretation of these phenomenon may not be clearly made as to whether they change due to sea-condition. such as sea-temperature or they behave by internal factor like maturity of gonad.
2) Horse-mackerel school and the school of mackerel mixed with horse-mackerel often encountered in this area may be divided into 2 categones; migrating group and stationary group, the latter located usually on banks or reefs. It is not radily made to attract the schools of stationary groups toward surface even by the use of light, especially in the month of June when the temperature of water is lower than the optimum level for these fish. The depth of the migrating group generally become deeper from July to September or early October, and, during this period the fish are well attracted by light (Fig. 2).
3) The light intensity in terms of brightness and time of exposure or, on the side of fish, their phototaxis determines the number of fish led onto the net, i.e. the catch. As shown in Fig. 2, the operation of the present fishing gear finds that sardine schools led onto the net decrease in size 2 hours after the initation of light stimuli, which, the schools of mackerel, horse-mackerel and the mixture of these two species show a similar tendency 3 to 3¹/₂ hours after the first lighting.