166 EIGHTH PACIFIC SCIENCE CONGRESS 



the recent twenty-year records of Japanese Tuna fleet activities. His 

 work was done mainly to see geographical distribution as regards the 

 density of fish in fishing grounds throughout the seasons. To measure 

 the density of fish, he used the number of certain species of fish caught 

 by one hundred hooks in a long line by the records brought by com- 

 mercial boats, which is a sort of average catch, catch per amount of fish- 

 ing effort. Many maps showing the density distribution of each fish 

 species for each month of each year and of the average of the past 

 twenty years have been prepared for printing. Here a part of it is in- 

 troduced as an example. 



Thunnus orientalis. Though Kishinouye (1923) thought the 

 southern limit of the distribution would be 30°N, Nakamura (1936, 

 '38, '43, '49, '51) reported the existence of fishing ground from March 

 to June in the Northeastern part of the South China Sea, North from 

 15 °N in the East of Luzon, and near Formosa and the Ryukyu Islands. 

 Nakamura also thought that the fish in the East of Luzon would be in- 

 dependent from the fish that appear near Tanegashima (130°E, 30 °N) 

 from March to December, judging from their size composition and 

 sexual maturity. Beside these, Shimada (1951) reported the occurrence 

 of this species in the equatorial area from June to September, and Jor- 

 dan & Evermann (1926) reported its occurrence in Hawaii and in the 

 U.S. Pacific coast. The northern limit reported by Isahaya (1935) is 

 near 50°N in Karafuto. The amount of catch of this species by the 

 Japanese fleet was near to nothing in 1940, but has been much better 

 since 1950. 



Gerino germo. Uda and Tokunaga (1937) thought the fish caught 

 by the Japanese fleet can be divided into three groups: a small-sized 

 group in coastal waters off Honshu, a medium-sized group in the grounds 

 further off shore, and a large-sized group found further East from the 

 former. Each of the three groups migrates clockwise, and intermingle 

 at the point near 155^E, 40^N, having its southern limit in subtropical 

 convergence. From the data Nakamura compiled, he thought that al- 

 though it is evident that the bigger size of fish is found in the more 

 eastern grounds, it is hard to separate the group as the change is so 

 continuous, and it might be caused by the economical size selection of 

 fishing by the commercial boats. The fishing ground usually goes South 

 during the fishing season at a speed of 2-6 miles per day, and in March 

 it arrives at the Southern extremity, that is, subtropical convergence, 

 and it goes back to somewhat North in April. The size of the fish in the 

 front line of this southward migration is usually bigger than that at 

 the tail, which is found somewhere North from the former. South from 

 the subtropical convergence to the equator, a large-sized group is found, 



