g. Relation of Fish Catch to Lunar Age 



Many of the Kushiro drift-net fishermen, believing that the fishing 

 is poor before and after the full moon, rest from fishing and mend their 

 nets during this period o Iherefore the total catch is markedly small 

 around the full moono In order to determine whether or not the fishing 

 was, after all, poor in the light of the moon and good on dark nights, 

 the average number of fish taken per boat per trip for each day from 1929 

 to 1931 was calculated and the values were arranged in the order of lunar 

 age as in figure 6„ This shows that the catches are largest from the 

 third to the eleventh day of the mqono 



h. Conditions for Good Fishing Over a Long Period 



As conditions for good fishing over a long period we have recorded 

 the small vertical differential in water temperature j, the great breadth 

 of the coastal stream of the Chishima cold current in the waters off 

 Kushiro, and the failure of the third branch of the warm current to 

 break through it„ 



On the basis of the facts given above the writer wishes to propose 

 the following hypothesis with regard to the tuna fishery? 



In the case of a warm-water migratory fish like the tuna^ the local 

 fishing situation and the amount of the catch are controlled less by the 

 total nunibers of the fish present in the ocean than by the breadth or 

 narrowness of the area of migration as influenced by the position of the 

 ocean currents and their relative strengtho Since this area is the limit 

 of the distribution of the tuna, this hypothesis is most appropriate o 



To judge from past facts, in years like 1926 and 1929, because the 

 warm currents were not strong, the tuna schools did not go north into the 

 Chishima area and their area of migration was restricted, giving rise to 

 the phenomena of poor fishing at Etorofu and good fishing at Kushiro, In 

 a year like 1932, when the cold water zone expanded greatly off Erimosaki, 

 the third branch of the warm current was lacking, and the migration of 

 small tuna into the Kushiro area was blocked, resulting in good fishing 

 at Urakawa and poor fishing at KushirOo In 1927, because of the great 

 strength of the warm current, the tuna schools reached far into the 

 Chishima area, and the area of migration was extended with the result 

 that the tuna schools were scattered, and Kushiro had poor fishing, 

 while the fishing was good at BtorofUo 



The hypothesis presented above is confirmed by the inverse cor- 

 relation between the tuna fisheries of southern Japanese waters and of 

 the adjacent waters of Hokkaido, and by the fact of an inverse corre- 

 lation between the black-tuna catch and the broadbill-swordfish catch 

 in HokkaidOo 



22 



