7 o The sources of the skipjack of the Northeastern Sea Area. 

 Looking at the distribution of the positions of catches of migratory 

 and sedentary schools (figo 7), it can be seen that the sedentary 

 schools are fished in the greatest numbers along the Ogasawara and 

 Zunan archipelagoes 5 and they are a^." taken in lesser numbers off 

 Sanrikuc In contrast to thiSj the migratory schools are fished 

 not only in the Zunaa and Ogasawara islands^ but also in the South- 

 western sea- area to the westj andj like the migratory schools of 

 the Ryukyu Sea Areag the r ange of their distribution is nob select- 

 ive. The sedentary schools of the Northeastern Sea Area swim 

 from the Ogasawara chain to the Zunan islands and then migrate on 

 farther to the waters off Sanriku, and therefore they should be 

 called the Ogasawara stocko They are not as bound to islets and 

 shoals as the' sedentary schools of the Ryukyu Sea Area„ In contrast 

 to theses ^^^ migratory schools move into the Northeastern Sea Area 

 from the Rjnikyu Sea Area by way of the Southwestern Sea AieSj and 

 therefore can be called the Ryukyu stocko It is deduced from the 

 fact that a school of fourth=year fish of the migratory type was 

 fished by the Fusa Maru in the vicinity of Douglas Shoal on 

 May Ij 1930s that migration is not restrirted to the routes follow- 

 ing the coast of Honshu ^ but that the fish also migrate through 

 the offshore waters to the southo The migratory- fourth=year fish 

 of the Ryukyu Sea Area and the migratory fourth-year fish of the 

 Northeastern Sea Area resemble each oth^r very closely in the 

 trend of change of the average and mode of their body lengths 

 {table 9)0 TlJhat are thought to be facts that enable us to 

 deduce that the schools of fouxth-year migratory fish of the 

 Ryukyu Sea Area do migrate away are? (1) The proportion of 

 migratory fourth=year fish in the catch becomes lower as one goes 

 toward the more northern fishing grounds (table 1)| (2) In years 

 when migratory schools are plentiful the amount of the catch in 

 the Ryukyu Sea Area diminishes (1935) « These phenomena are 

 thought to be ascribable to the fact that fourth-year fish do not 

 remain permanently on the fishing grounds o The skipjack schools 

 that are fished in the Southwestern Sea Area are migratory fourth- 

 year fishji and tiieir age composition and condition factor resemble 

 those of the migratory fourth-year fish of the Ryukyu and North- 

 eastern Sea Areas (table 10) » No connection can be found between 

 the sedentary schools of the Northeastern Sea Areas that is, the 

 Ogasawara stock, and the sedentaiy schools of the Ryukya Sea Area, 

 Schools of fifth-year fish are scarce in the Ogasawara stock in 

 the Northeastern Sea Area^ but it is thought that in the C^asawara 

 archipelago and farther south fifth-year fish are much more 

 numerous o The skipjack taken by the Fuji Maru in the middle of 

 Mays 193^s i" "th® waters adjacent to Marcus lo (153 59 'E, 

 2k° 11 «N) were sedentary fifth-year fish with an average body 

 weight of Uo 5 kg s body length of 60 3 cm ^iCc Probably 63 cm J, 

 and condition factor of 17-i8o 



Thus there are two distinct stocks that come into the 

 Northeastern Sea Area, Fluctuations in the catch are probably 

 closely related to the volume of migration of each group,, Future 

 investigations should pay considerable attention to this poxnt„ 



