Analysis of the Fish Finder Records―II Some Acoustic and Ecological Informations on Tuna and Others

摘要

Recentry the fish-finder for tuna fishing has been developing and there are many reports on the echogram of fish-finder ,recording the swiming depth of the fish and their density, by HASHIMOTO, NISHIMURA and MANIWA. In the tuna fishery, however, the fish-finder by ultrasonic wave has not been used so effective as in trawl and purse sein. For the study on the method of its field operation, the auther had some experiments with the fish-finder at Philippine Sea,in June and November 1962. The fish-finder wich where installed on M. S. NAGASAKIMARU, the fishery training vessel, Nagasaki Univ., 130 GT, has charactors shown inTabie 1. The one NEC 1620 type, was used for detecting tuna shoal, another 1600 type, for tracing the tuna longline under waterco ntribution. From these experiments,it was recognized that the fish-finder could sharply catch the reflaction from the fish and was very covenient for the study of their ecology. And from echograms that the ability of sounding capacity was about 8000m by 13.5KC and 800m by 200KC under cruising by 9 knots on the slight or moderate sea,and detecting a sigle tuna (20 db, bcdy weight 35kg) it was 250m by 13.5KC (Fig.2&3). The results obtained are summarized as follows. 1) Their typical echograms are shown in PLATE Ⅲ-2.3 and detecting area of tuna echosunder the above conditions is shown in Fig. 5. Furthermore,it was caluculated from Fig. 5 that the fish-finder could cover the conical volume, 108m3 per hour under cruising by 9 knots. 2) It was measured that the running ultrasonic noise was 40 db under cruising by 9 knots against the wind being 12m/sec, and the reflection loss of yellow-fin tuna (B.W. 30kg) was 22db (Fig. 7). The single tuna, however, was recorded distinguishably as "Λ" on the paper which was filled with ultrasonic noise (PLATE Ⅲ-2). As the result from analysis of those echograms, it was estimated that the tuna were in 40-200m depth layer and most of them were in "resting state", and some-times a few of them moved with 1-5 knots reactively against the aproaching plancton net or fishing gear. (PLATE Ⅳ-5,6) 3) Especially, the fact that there were no correlation between the density of their population and the daily catch per hook; will have a important point on the estimation of their population (Fig. 10, Table 3). 4) From the PLATE Ⅳ-7,Ⅴ-8,9 shown the daily migration of D.S.L. and small fishes,it was observed that the small fish (less than about 10 kg in body weight) would be moving vertically with the migration of D.S.L.. From the nacturnal echogram, it was shown that the D.S.L. contained small single fish from surface to 120m depth, but a few large fisfes (more 10kg body weight) were scattered from 100 to 200m depth, and it was observed that D.S.L. began to dive suddenly with the speed of 200-400m per hour at begining of morning twilight and it proceeded to dive until arriving at the 230-300m or 550-700m depth at half an hour after sunrise, but during the daytime it has no migration. And it was observed that in the evening, D.S.L. began to refroat toward 0-120 depth under the same speed as in morning diving, following with the small fish and a few large fishes.