ブリの成長と年令に関する研究-I

Abstract

The present paper is the first installment of the author's work on the age estimation of the yellow tail, Seriola quinqueradiata T. & S., dealing with a fundamental character of the vertebra in estimating the age of the fish. The author has used for his experiments two different groups of 20 fishes measuring 20cm and 80cm in fork length, respectively, which were caught in Wakasa Bay during the period between July, 1957 and January, 1958. The vertebral column of the fish consists of 11 abdominal, and 13 caudal, vertebrae, generally 24 in all. Each centrum of the column is measured in three parts, as shown in Fig. 1. The results obtained are as follows: 1) In Table 1 are shown the mean and coefficient of variation (C. V.) of the length of those three parts (T, R and D) of each centrum in the 20 fishes measuring 80cm in fork length. As is seen in Table 1, there is no marked difference in coefficient of variation (C. V.) in the sequence of centra, excepting the first and the last several ones which vary considerably. The relative size of each centrum is represented by the ratio of its longitudinal length (T) to that of the first centrum, the ratio being given in Fig. 2. As shown in Fig. 2, it is obvious to us that the centra from the 7th to the 20th vertebra are larger in T and R than those of the rest, particularly so in those of the four caudal vertebrae from the 15th to the 18th, and that there is no marked difference in D among the centra, excepting those of the few smaller vertebrae at the caudal end. 2) Significant difference is seen in the growth-rate of the centrum in the sequence of the vertebra, as shown in Fig. 3. The growth-rate of the centra of the first and the last two or three vertebrae is lower than the average value of all the 24 vertebrae, but the value of the centra in the middle part of the column is higher than the average value, the value of the seven centra from the 13th to the 19th vertebra being especially higher. 3) In order to clarify the degree of similarity in the mode of ring formation among the centra of the fish, the author has examined by means of F-distribution method whether the relation between R and γi is linear or not, and found that the linearity between the two is efficient enough to be a means for the age estimation of the fish. An example of the relation is shown in Fig. 4, in which the last three vertebrae are excluded on account of their being small in size and indistinct in ring mark. From those interesting facts mentioned above, we may conclusively say that, in estimating the age of a fish in general through its vertebrae, we have to examine as many centra as possible. If it is found very difficult to do so in case of the yellow-tail, those vertebrae from the 15th to the 18th had better be used for the purpose, because they have the following characters, not only advantageous but also necessary for the work of this kind: 1) clear ring mark, 2) large centrum, 3) small coefficient of variation in size of centrum, 4) high growth. rate of centrum, and 5) great similarity of ring mark among the centra.