fishsry for black tuna / ~Thunnus oriantalis / in the coastal waters of Hyuga 

 begins in the middle of i)ecember >, At first longlines are used (fishing 

 depth 76-luOin) and the fish are taken where the surface temperature is 

 22°Cj but in the middle of "January a change is made to trolling gear and 

 the surface temperatures are 18°~19 Co Early in karch the fish go deep, 

 the trolling lines no longer catch fish, and longlines come into usn again 

 with the surface temperature at 21o5°Co In the middle of April the fishing 

 comes to an end °'' , I'here has not been as yetj however, any thorough study 

 of the depths at which the fish are taken, and many items have not been 

 clarif iedo 



Favorable temperatures for catching tuna (.a) Northern part of the 

 t^orbhcastern 5ea Area o I shall attempt to treat the data from the Sakigake 

 iii^ru, of Aomori Prefecture, the Daito_Liaru, of i-iyagi Prefecture, the Akita 

 iuarUs of Akita Prefeccure^ and the Chokai i-^ru. of Yamagata Prefecture, 

 all of which vessels took temperatures at the upper and lower levels during 

 the cooperative tuna fishing experiments of 1950, These observations were 

 made from June to December, 1930, a season when the difference between the 

 temperatures at the upper and Icwer levels is great, and covered an area 

 centering off Erimosaki and extending betv/een 141° - 144 °E, 37°- 42°K. 

 First of all, Figol a), b) are graphs of the distribution of catches' and 

 niunbers of fish caught (total of black tuna, albacore, big-eyed tuna, and 

 yellowfin) plotted against surface water temperatures and against the 

 temperature of the level at which the fish v/are taken according to the 

 above-mentioned fishing logs (30m for the Dait"© Laaru, 18-36m for the Akita 

 iJaru, 46-53i;-; for the Chokai iferu^ reco^'ds lacking for- the Sakigalct Maru but 

 assumed from Table 1 to be 50m}. From these graphs it can be seen that in 

 these waters the most fish were taken where the surface temperature was 

 19°-20°C but at the 25-50m levels, where the fish were caught, there are 

 two modes at 13°-15°C and 18°-19°C„ It is still difficult to tell for sure 

 whether the former is an apparently favorable temperature based on schools 

 especially concentrated by an upwelling of cold water from the lower level 

 of the Oyashio Current system or whether it is the temperature at which 

 black tuna of a certain age-class swimo However., the fact that in the 

 coastal waters of Izu and Sagami the large set nets take the most black 

 tuna at temper.:- vures of 14°-16*t presents useful data for the future con- 

 sideration o' this questiono 



b) Favorable temperatures on the winter grounds in Zunan waters » Fig . 

 2 a), bl shows the relationship between the number of tuna taken and the 

 number of catches made and the water temperatures at the surface and at 

 lOOra (according to Table 1 100m is approximately the level at which the 

 fish were caught, except that the Kiyo i^laru fished at 70m), the data being 

 drawn from the records of the Kiyo iuaru, Kaiko' t^aruj Fuji Maru, and K!oh"o 

 Maru's operations during the winter months of December, January, February, 

 and Marchg when the temperatures at the upper and lower levels are con- 

 sidered to be comparatively uniforaij, in the cooperative tuna fishing 



S) Summary of the Activities of the kiyazaki Prefecture Fisheries 

 Experiment Station for 1931 <> 1933 » ppo 10-18« 



