抄録
In the present work, the opercular bones are used for the age estimation of the yellowtail, Seriola quinqueradiata T. & S. One of the objectives of our work is to determine whether or not the opercular bone can be used for estimating the age of this species, and we have found the bone to be extremely suitable to our particular requirement of analysing the growth of the fish, and in some cases the bone is preferable to scales for the purpose intended, although scales still remain most widely used for the age estimation of the fish owing to their having clear structure in their younger stages and our sampling methods from the fish body being simple.
We have examined the opercular bones both right and left of the 435 yellow-tail, which were caught in Wakasa Bay during the period from November, 1956 to January, 1958. The opercular bone has alternating bands of translucent and opaque character, as seen in Fig. 7. These characteristic lines marked between the two bands we call here operculum marks. Our successive collections of the bone all through the year have led us to believe that the operculum mark is formed every winter; in other words, most of the samples collected seem to have had their operculum marks formed in December, January and February (Fig. 2). But the first mark can hardly be seen in the fish larger than about 30cm. in length owing to many keels radiating from the junction of the bone with the hyomandibular bone to the posterior. Therefore, in the case of older fish, it is necessary for us to add one more mark to the observed ones for the accurate estimation of the age of the fish.
Table 2 gives the sizes of the fish as they are estimated from the operculum marks, and Fig. 5 is obtained by plotting them. In Fig. 5 the sizes of the fish with the marks formed are given as follows: the fish is about 19cm. in fork length when its first operculum mark is formed; about 43cm., in the second mark formation; about 59cm., in the third: about 71cm., in the fourth; about 79cm., in the fifth and about 85cm., in the sixth; respectively.
In Fig. 6 is shown the average growth curve obtained from Fig. 5, which shows that the fish respectively attains about 29cm. in fork length in a year; about 49cm. in two years; about 63cm. in three years; about 73cm. in four years; about 81cm. in five years and about 86cm. in six years.
