マグロ延繩の漁獲機構に関する研究-II

投繩方向と魚群の游泳方向

抄録

In the operation of tuna longline, the fishing boats have usually set the lines in the direction as fishermen think fit without any available data. If the moving direction of tuna school is assumed in advance, very successful catches will be to obtained at the case when the longline is set so as to intersect the moving direction of the school at right angle.
The present paper treates the relation between the moving course of fish and the direct-ion of setting gears, based on the assumptions as follows:
(1) The catch (F) is proportional to number of fish (S) encountering the set of longline.
(2) The school is moving in a certain definite direction with a constant speed (v). From the above assumptions, S may be assessed as follows;
S=Nv| sinθ| ?? L (1) where N is the density of fish in the fishing ground, ?? the angle of the moving course of fish against the direction of setting gear, _??_; the mean soaking time of the gear and L the total length of the lnngline.
Furthermore, F is obtained from the following equation:
F=αNv/sinθ| ?? L (2) when α means individual catching rate of a gear.
From the equation (2) and the data of current direction, estimated moving directions of the schools are shown in Table 1, and consequently, it was found that the moving direction of tuna is related to the current, most of them move against a current, and thus the most effective setting of the gear is achieved at the direction normal to the current.