It is, of course, difficult to make a hard and fast distinction between 

 these three systems and they appear to fluctuate with natural changes in 

 the oceanographic eondition,3c The schools in system (a) are shoal=fish or 

 island~fish with a limited range of movement^ and they have a tendency to 

 stay in one place„ Systems (b) and (c) compidse the so-called migratory 

 stockc Their range of movement is broad, and in the course of their great 

 migrations they respond to variations in objective factors such as currents, 

 water temperatures, character of the sea water, food, and so fortho They 

 may adopt -various patterns of migration such as taking on tendencies 

 similar to those of system (a) and remaining throughout the year on reefs 

 and shoals or around islands in the Satsunan and Zunan areas, or turning 

 south midway in the migratory path of system (c) and failing to reach the 

 Northeastern areao The schools of system (b) are the main element in the 

 skipjack of Japanese waters and the presence or absence of these fish 

 determines the year to year success of the skipjack fishery, 



1, The make=-ap of the skipjack population in Japanese waters 



Investigations in all areas have revealed that the fish which are 

 taken in Japanese waters are mainly 4- to 5 years old and that the peak 

 seasons come ©very four yearSo 



Skipjack are classified by weight j those above 1 kan / 8„27 lbSo_7 

 being called large skipjack, those from 500 to 1,000 momme /"/!tol3 to 8„27 

 lbSo_/ medium skipjack, and those below 500 momme small skipjack,. Investi- 

 gations in various areas shcv? some variation from year to year, but the 

 overall picture is generally as follows. 



(1) Satsunan area 



Schools of small skipjack appear aro-and March and are most numerous 

 from April to August o They make up about 50^ of the skipjack schools which 

 appear in this area„ and also s.:::count for the greatest part of the catch„ 

 The schools of medir.."! skipjack are mingled with the schools of small fish 

 in March and reac' uheir greatest abundance in April and May after which 

 time they decline in numbers„ They reappear again in August and September, 

 but in this area they form the smallest element in the skipjack population 

 and in the catch, being only 20^ of the total. Schools of large fish 

 appear and are taken in the greatest numbers from April to Augusts They 

 form 30^ of the total catch in this area„ Thus in this area schools of 

 ycung skipjack are the most important, followed in order by old and 

 middle-i -;ed fish, which are thought to be shual-dwelling schools, 



(2) Xirian area 



This area is in the path of migration of the schools and the length 

 of time they remain in the area is extremely snort „ From the latter part 

 of March to the middle of May the number of schools which appear and the 

 catch ratios are extraordinarily small in comparison with other areas. 

 Small fish (60%) make up the greater part of the population, followed by 

 medium fish (40%), and almost no large fish appear in the area. 



20 



