Cn the Stock of Thiiiigus i3liej?t£iig (Temminck & Schlegei) 

 Synopsis in English 



A study on the stock of Th'.:^nnu5 orientalj a (Termiinck 4 Schlegei), in which 

 the catch records for each boc'y-waight class were utilized as bases, showed that 

 survival rate is ,30 for young fish, Tiie.ii. but .75 for the adults, r/hile fishing 

 rate is ,55 for the youngs bat «10 foi* the adults, {end of English synopsisj 



According to Kimura^-^' on the fishing grounds of Shigedera in the north- 

 f^astern corner of Suruga Bay juvenile black tuna are first taken in the large 

 set nsts aroimd July and August. At the end of that yoar and the beginning of 

 the next year they are 2 - 4 kg in weight, and dui-ing the peak season in April, 

 May, and June they weigh about 5 kg. In the spring of their third year they a t- 

 tain a weight of about 10 kg, and around March and April they leave the fishing 

 grtmcdfl. Consequently fish in their third year and older may be regarded as en- 

 gaged in the great migratiors of the species, 



Aikawa end f'ato have determined the ages of the black tuna from the ver- 

 tebrae and have established thn ages for each length-weight class. As they have 

 pointed out, their results disagrfje to soTie extent with those obtained by 

 Kimurad)* One point which is difficult to explain is their fl'^ding that sixth- 

 year fish were not taken on the Shigedera gro'inds during the five years from 192^1, 

 to 1928. Nevertheless, in the present paper the discussion of th© stock of the 

 black tuna will be based tentatively on the findings of Aikawa and Kato. 



The number of fish of each year class tai'en on the Shigedera grounds during 

 the nine years from 192^ to 1932 has been calculated according to the findings 

 of Kimura and Ishii(3). (Table 1), If we seek the survival rate from these 

 figures, we get ,7A,*IP, on the basis of KawRna'sC-^) data (Table 2), we calculate 

 the survival rate for the periods 1922-1925 and 1926-1932, we get .70 and .66 

 respectively.** 



According to Aika^ta and Kato'^\ In the landings at Numazu large fish pre- 

 dominate in the spring i*ile fish of the year and second-year fish predominate 

 in the autumn. Fish of the year, which are taken daring roughly half of the year, 

 comprise A9% of the catch, and second-year fish, which ere taken the year round, 

 make up 4.736, The third-year fish, which appear for only a short time in the 

 spring, form only 4.% of the catch. If we calctilate the survival rates for young 

 fish from these data/ we get ,23 for fish of the year and ,3A for second-year 

 fish. A similar calculation made l>y substituting age groups for length-weight 

 groups in Aikawa and Kato's data on the landings of black tuna at AburatBu in the 

 spring of 1937 (Table 3) gives a survival rate of .57, 



The Fisheries Experiment Station^ 5) reports by weight classes the number of 

 black tuna taken by yellowtcil nets and tuna nets along the coast and by pole 

 fishing, long lines, trolling, drift nets, and harpooning in the offshore waters, 

 A study of this data for the three years 1937, 1938, and 1939 (Table U) gives 

 the survival rates^^"of ,90 for small and medium tuna and , '8 for medium and 

 large tuna. If we assijme further that the natural mortality rate is the same 

 for all fish of the second year and older, we deduce from the ratio of adult to 

 young fish that the fishing rate for the young is .57 and that for the adults is 

 J)60, This leaves us Tdth a nat^iral mortality rate of .20. Of 50 small black 

 tuna released on the Japan Sea side of Hokkaid? in August and September, 1932, 



