メバチの研究-II

竿釣りメバチの魚体組成についての一考察

抄録

The present paper deals with analysis of length frequency of the young big-eyed tuna caught by pole and line. Especially a relationship between the fishing localities and the discrepancy in the mean length which was evident in the same modal group has been discussed.
1. Data used for the study are the measurement of body length obtained from numbers of samples and the pertaining information furnished by fishing boats at two ports during the period from June to July 1953.
2. In the length frequency two dominant modes, A and B, were recognized not only throughout the samples (Fig. 2) but also in the mean value of each school from 1 to 17 examined nearly in the same period. These findings have led the writers to an assumption that modal group A may from an independent unit different from group B in regard with length frequency.
3. As no uniformity in the length frequency has been found among the samples belonging to the same group (Table 1), correlation between the mean length of the groups and fishing locality has been examined, substituting the size composition sorted by fishing cruise for that by school of fish, because few data on the latter were available for this purpose.
As a result it was found that the mean length of the fish in both groups A and B was small in the western area of the fishing grounds and tends to become greater as one goes east (Table 2, Fig. 3).
4. The fact that the survey period fell on the months of June to July when the fish would be migrating northward (Fig. 3) seems to suggest that the easterly increase in the mean length may be attributable to different tendencies of the migrating behavior between the fish in the eastern area and those in the west. In other words, the fish of both groups A and B in the eastern area are supposed generally larger in size than those of the corresponding groups in the west, while all migrating nothward.