Takaya Kusaka

STOPPING EFFECTS OF ELECTRIC FISH SCREEN

The author has done to see stopping effects of electric fish screen in place of leading net. Fish materials used were 10 Carassius auratus Goldfish of total length 5.0cm., and 10 Oryzias latipes Japanese Killifish of T. L. 2.5cm. as both live in fresh water, and 12 Sphaeroides niphobles Japanese common puffer T. L. 5-7.0cm. in sea-water. Two electrodes are set in the upper and lower sides at the center of tank, and were closed alternate electric current of sine wave of 50 cycles regulating its voltage and amperage by transformers. (Fig. 1). Frequencies of fish individuals pass through the center of tank were compared on cases closed and off the electric current. The result is in Tables 1-4. which are taken efficiencies of passage and those significancy of stopping effects were judged. (Table 5). In general, stopping effects of electric current in fresh water is about 50% on better condition, as if sufficiently available. The otherhand, the case of sea-water is about 20% and also, less availability, because it is required several hundreds times of amperage than the case of fresh water.

FISH GATHERING EFFECTS AND SUBMARINE ILLUMINATION OF THE INCANDESCENT AND FIUORESCENT MERCURY-VAPOUR LAMPS

On the Tachibana Bay of Nagasaki Prefecture purse sein anchovy fisheries, using lights, is carried on (Fig. 1). The author investigated the fish gathering effects of lamps. One Incandescent lamp of 2 KW, (1 KW, 2 globes, D. C. 100-120 V) was generally used another Fluorescent mercury-vapour lamp of 2 KW, (1 KW, 2 globes 100 V, A. C. 60 cycles) was specially prepared for this experiment with a 6 KW A. C. dynamo on a boat (Plates). The author compared the submarine illumination under each lamp and also the fish gathering effects of each lamp. The fish gathering effect of the mercury vapour lamp was better than the incandescent lamp. This was especially true during the early period after lighting, when fishes of comparatively medium length gathered (scomber and so on). In this case the fishery was mainly for anchovy. The fish school that gathered under the mercury-vapour lamp were swimming actively. However, during the later stages on the illumination of the mercury-vapour lamp it was found necessary either to weaken the illumination to change the lamp to the incandescent variety or find some means by which the illumination of the mercury-vapour lamp could be maintained. The author supposes that the effective illumination for the gathering of anchovy is below 0.1 Lux. The illumination of the mercury lamp is about 30% more than that of the incandescent lamp and this is proportional to one dimension of length in water. (Figs. 2 & 3).

Fishing Effects of a Model Lift Nets, regard on Relation between Mesh Size and Fish Length

As a fundamental experiment concerning effect of lift net, the authors studied the relation between mesh sizes and fish lengths of catches in model lift nets. The set of the observation was shown in Fig. 1. as a material of fish was used Carp Cyprinus carpio. Population of the carps in the tank is explained on the upper column of Table 1. as the total number 197 individuals. They were taken into consideration on the population to be approached in the natural condition. Net materials were used of mesh sizes 6, 3 and 1.5cm. and the lifting speeds of the net were selected 3 grades at 5, 10 and 20 seconds per 88cm. The results obtained were shown in Table 1. and Fig. 2. for the number of the fish lifted and the catching rate. Fig. 3. were shown a effect what is a indicator of catching efficiency against a certain mesh size and fish length.

Difference on the Frequency of Transfer of Fishes with Different Degrees of Inclinations of Under-water Fencing Net

The experiment on inclinations of a fish net, was done in a water tank to observe the frequency of transfer of fishes over several models of fencing net. The tank was of wood, 72cm. long, 52cm. wide, with water 20cm. in depth, the tank being painted white inside. The fish used were 10 individuals of Acheilognathus lanceolate (Yari-tanago) with an average total length of 4.5cm. and were liberated in the tank. As model of under-water fencing nets, 4 tin-plates with white enamel paint were bent at 90, 60, 45 and 30 degrees of facial angle against the horizontal surface and they were kept at a height of 14cm., keeping a space of 6cm. between the water surface and the top of plate (Fig. 1). The number of transfer of fishes over the tin-plate were counted during 10 minutes for each inclination. This was repeated 10 times during a period of several days (Table 1). Averages of the frequency of transfer on each angle is shown in Fig. 2. These averages, when compared with each other, show significant differences. In almost all combinations the significance level was 1 or 5% excepting for the combination of 60° and 45° (Table 2). The 90° plate had more stopping effect than any of the others. Decreasing the inclination of under water fencing net has, less stopping effect against the transfer of fishes; instead it accelerates the transfer. Therefore, the author suggests that attention should be paid to the inclination of the fish net.