Abstract
The location of ground fish by a fish-finder is one of impotant problems in the Japanese trawl fishing. The authers have accumulated various fundamental experiments on this problem, the results of which have been already published in our Technical Report. On the basis of those experimental results, we have tentatively manufactured the three fish finders of new types. These three apparatuses are designed under the conditions for increasing the discrimination power between the fish school and the sea bed. The main characteristics of these apparatuses are as follows ; [table] These three apparatuses are installed aboard one of the two boat trawler of 100 gross tons ; and the transducers on her same side planking. The followings are the experimental results. (1) The capacity of each apparatus for detecting a single ground fish were measured 180 meters when the boat was stopping. When the boat was cruising, the capacity of 50kc apparatuses drop to 100 meters because the running attenuation of ultrasonic wave and the running noise were measured, while the 200kc apparatus is not disturbed by any influences. (2) After we made a continuous observation of the ground fish behavior with the fish-finders, the whole day from morning to night every day, the fish school was clarified to move with the migration of D.S.L. ; namely, during the night the recording paper was filled only with single fishes or D.S.L. and any fish school did not appear. In the morning when the sky was gradually turning gray, the D.S.L. began to go down slowly at the rate of 20 to 30 meters per hour ; and about sunrise, above the D.S.L. appeared the fish school, which went down slowly and finally reached in the neighborhood of sea bed when the D.S.L. disappeared near sea bed within two or three hours after sunrise. In the day time, we could find many fish schools near the sea bottom. But in the evening when the D.S.L. which began to rise slowly in the rate of 20 to 30 meters per hour appeared on the bottom. the fish school also began to rise toward the sea surface and disperse about sunset and finally the record showed the state of night. (3) We have observed the vertical distribution of ground fishes in the three layers of 0 to 2 meters, 2 to 4 meters and 4 to 10 meters above the sea bed. We sumed up the total length of fish schools appeared on the recording paper every trawling time and layer during the whole experiment period. The results show that in the major tide the fish school is more numerous mainly in the layer of 2 to 10 meters ; but in the neap tide mainly in the lowest layer. In this fishing ground there was a tendency that the catch of ground fishes at the major tide was smaller than that at the neap tide. I believe that this tendency is supported by the abovementioned results.
