| Co-Investigator(Kenkyū-buntansha) |
SATOH Hiroo Tokyo Univ. of Fish. Assis. Prof., 水産学部, 助手 (60114914)
MORINAGA Tsutomu Tokyo Univ. of Fish. Associ. Prof., 水産学部, 助教授 (90088196)
中村 善彦 (中村 喜彦) 東京水産大学, 水産学部, 助教授 (10017072)
TAKAHASHI Tadashi Tokyo Univ. of Fish. Associ. Prof., 水産学部, 助教授 (50017042)
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| Budget Amount *help |
¥2,200,000 (Direct Cost: ¥2,200,000)
Fiscal Year 1991: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 1990: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
In tuna longline fishing, the branch lines made of wire used in the early days are replaced with those made of nylon gut in these days. Such an improvement makes better in catch efficiency. But, there is no justification for its reason. And it is important to realize, in advance, a distance in visibility of tuna inhabiting in the different oceanic regions.
1. The author et al. investigated and made public how much catch efficiency could be improved by applying suitable materials to the gear. (1990)
(1)The sea regions surveyed in the Bay of Bengal, Indian Ocean, have very clear water of which turbidities are 0.11 to 0.22m^<-+1> with small quantities of suspended particles.
(2)The inherent contrasts of the materials under the same lighting conditions as those of the sea regions surveyed are 1.1 for the #150 nylon gut(diameter 2. Omm)and 6.3 for the #28 wire(diameter : 1.7mm).
(3)If visual acuity of tuna is estimated to be 0.90 and the data stated in the above(1)and(2)are taken into account,
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the underwater visible limits in distance of the branch lines should be 3.6m for the wire leader and 1. lm for the nylon gut. And the visual distance for a piece of bait mackerel(fork length : 25cm)is 27.5m under the same conditions stated above. This indicates that the visible distance of tuna varies with different kinds of fishing-gear materials and is much less for nylon than wire. Such a point of view, therefore, can duly explain the main reason for the improvement of catch efficiency.
2. The author et al. calculated the visual distances of tuna in the different sea regions based on the data obtained through opt-oceanographic surveys as well as others' references. (1991)
(1)The practical operation conducted extends over the Indian Ocean, the Pacific Ocean as well as the Atlantic Ocean, and the Mediterranean Sea. Each optical nature of sea water in those oceans corresponds to "Oceanic type I, II and III" of the Jerlov's water-mass optical classification(1976), respectively.
(2)When the materials specified in the above 1-(1)are used in those different waters, the daytime visible limits in distance in the layer of 50m deep are 1.3m, 0.9m and 0.8m for nylon and 3.8m, 3.6m and 2.1m for wire, respectively. Less
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