At the time when they are moving northward through Hokkaido waters the 

 tuna schools are seen in the waters of the warm-current system. However, 

 they gradually become accustomed to cold water and many of them pursue 

 schools of sardines even into zones of low-ten5)erature, turbid water (water 

 color below 6, transparency less than 5 meters), so it is difficult to 

 infer the area of migration from water temperature c Furthermore, because 

 the water temperatures at which the fish occur vary from year to year 

 and from area to area, it is hard to detennine accurately what is the 

 most favorable temperature. In the foregoing only the surface tempera- 

 ture has been considered. It is, nevertheless, an error to think that 

 the vertical movements of the tuna take place only between the surface 

 and a depth of 20 meters, for in the waters east of Erimosaki they go far 

 deeper, down to 100 meters, as is presumed from the fact that they are 

 hooked on octopus long lines. Even in the waters east of Erimosaki, where 

 the temperatures at the deeper levels are low, the fish must descend to 

 depths greater than iiO meters judging by the length of the long-line 

 branches, Ihere must be cases in which the water temperatures are 5oO°iG 

 or lower, and from this it can be seen that the adaptability of the tuna 

 to ten^^erature is surprisingly great, Figure 2 shows the average surface 

 temperature and the catch by 10-day periods along a line 10 to 50 miles 

 south from Kushiro, 



Co Inhabiting Zones of Mixed Water 



Tuna are abundant in areas where different water masses are mixedo 

 Examples s Near Shiriyasaki, Tsugaru warm current / coastal stream of 



cold current 

 Near Erimosaki, third branch of the warm current / Tsugaru 



warm current / coastal stream of cold current 

 Coastal grounds off Kushiro, third branch of warm current / 



coastal stream of cold current 

 Kushiro off-shore grounds, second branch of warm current / 



coastal stream of cold current 

 Near Shikotan Island, second branch of warm current / Okhotsk 



current / coastal stream of cold current 



lliis may be because the fish are attracted by the abundant production of 

 such natural foods of tuna as sardines and squid. 



HI, Observations Concerning Oceanographic Conditions 



ao Pattern of Cold Currents -^n the Pacific Coast of Hokkaido" (Summer) 



The following is an account of the Chishima cold current as observed 

 by this Station within a radius of 200 miles from shore. The current 

 runs south along the east coast of the Chishima chain to about 30 miles 

 south of Noshappu where it is cut by water of the warm-current system 

 into two brancheso One branch runs from about 10 miles south of Kushiro 



15 



