Growth Curves of Blue-Fin Tuna and Yellow-Fin Tuna Based on The Catohes 



Synopsis [in English] 



No investigation has been made on the scale and otolith of the tuna from 

 the Seas of Japan with reference to the growth rate of the fish. In the pres- 

 ent paper are given frequency histograms (Figs, 2 and 5) showing body-weight 

 distribution of bluefin tuna ( Thunnus orientalis Tenuninok fc Schlegel) and yel- 

 lowfin tuna ( neo thunnus macropterus Temminok & Schlegel) from Shigedera fishing 

 ground which is situated at north-eastern corner of the Suruga Ray (Fig. 1). 

 The data were obtnined from 1924 to 1931 inclusive, and can be divided into 

 several age-groups as plotted in Figs. 4 and 6, in which solid circle indicates 

 individual fluctuation of the body-weight and encircled dot shows the average 

 body-weight of a large number of specimens which were caught there at the same 

 time and nearly of the same weipht. The curves are free-hand smoothing. 

 [ end of English synopsis ] 



It is difficult to find out the growth of fishes which migrate widely 

 in the ooeans, and the study of annuli and otoliths is still not very far ad- 

 vanced. The author has used data on the weights of individual fish from the 

 records of catches of black tuna and yellowfin tuna on the Shigedera fishing 

 grounds in the northeastern corner of Suruga Bay (Fig. 1), and has deduced the 

 growth rates of the fish taken on those grounds on the basis of graphs of the 

 weight distributions. 



Yoimg black tuna are first taken in the large set-nets of these fishing 

 grounds around July and August, and thereafter some are taken every day. From 

 the end of the year through the early part of the following year these fish 

 aire from 2 to 4 kg in weight; in the peak season of April, May, and July they 

 weight about 5 kg; and in the spring of their third year they a.ttain a weight 

 of about 10 kg. During this period they are called small meji, medium meji, 

 and large meji depending on their weight. In the spring of the third year 

 from about March and April these second-year fish (large meji ) disappear com- 

 pletely from the catch, and the only fish taken after this time are either 

 first-year fish or those much larger than the second-year fish. If we except 

 these young fish and separate the large black tuna which are taken by weight 

 groups, we get a graph of weight distribution like that shown in Fig. 2. This 

 graph can be divided according to the differences in the weights of the fish 

 into three groups of small, nediian, and large tuna. Although from 1924 to 

 1928 the average weights of the fish in the small and medium groups tended 

 gradually to increase, it can be seen in Fig. 3, which shows the fishing season 

 for each group, that the season for small and medium fish was coming later 

 each year and it may be thought that the increase in the average weights was 

 related to this lag. 



Since it was already possible to ascertain clearly the rate of growth for 

 fish under the second year of age, we were able, by considering the above- 

 mentioned three groups of small, medium, and large fish to be respectively 

 third-, fourth", and fifth-year fish, to draw a growth curve for the blaok 

 tuna «LS shown in Fig. 4, However, the rate of increase in the body weight in 

 the summer season appears to be greater than that shown for the summer season 

 in the corresponding portion of the smoothed growth-rate curve covering a six- 

 year period, and marked differences are notloeable in the case of the fourth- 

 and fifth-year fish. This leads one to believe that there is a marked differ- 

 ence between summer and winter growth. Only one or a few fish of the sixth 

 year and older are taken each year and it is therefore not possible to find 

 their rates of growth, 



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