•cattnred over the wide area extending from the Mandated lelande weat through the Philippine* 

 and the I>uteh East Indies into the Indian Oceaii. The fleh are found e^eryvhere throughoat 

 this enormoas area, although they tend to congregate around the isleuid groups where more 

 food Is presumably available. The density of the populations Is about one percent that of 

 the schools found In Japanese waters. Howeyeri because weather conditions 'are uniform and 

 mild all year round, operation* can be carried on over long periods of time, and vessels 

 can raake over-all catches conqjarable with those In northern waters. 



Many of the eklpjack schools found in the southern regions are composed of very 

 small. Immature fish, Indicating that the spawning grounds may be located In this area. A 

 young specimen measuring six Inches In length, the smallest recorded by the Japanese, was 

 taken In the vicinity of the Bismarck Islands. Other reports indicate that the Molucca and 

 Celebes seas may also be areas of spawning activity. Ripe Individuals have been taken from 

 schools entering Oorontalo Bay (an arm of the Molucca Sea). When leaving the bay the fish 

 had spent ovaries. 



4. Oeeanographlc Data 



Studies have been undertaken to determine the water temperatures and current con- 

 ditions most suitable for large concentrations of skipjacks. Information of this nature 

 could be valuable for predicting the future location of important fishing grounds, but the 

 work on the species is in Its initial phases and does not cover the full possibilities of 

 this type of research. 



Surface temperatures In relation to fishing success were analyzed by Takayama, 

 Ikeda, and Ando (1934, pp 55-56) from Information obtained in 1930 by commercial and re- 

 search vessels operating along the Pacific coast of Japan. The results indicated that the 

 range of teiperatare in which aklpjacks were taken was between 170 and 31°C, with the most 

 favorable temperatures between 20.5° and 26.5''C. As the migration proceeded northward the 

 temperature for the fishing grounds yielding the peak catch changed from the higher (26.5**C) 

 to the lower (20.5'»C). 



The fluctuations of the catch for three prefectures, Miyagl (northern Honshu), 

 Shlzuoka (central Honshu), and Eagoshlma (Kyushu), we.'o correlated with surface water temp- 

 erature over a nine-year period, 1929-37 (Uda, 1938, pp 77-78). Forecast* based on water 

 ten5)erature during the winter indicated the probable productivity of the major fishing 

 grounds during the following spring and summer. In the years when the water temperature dur-' 

 ing the winter was higher than normal, the catch was relatively small In the south and abun- 

 dant in the north. Reverse conditions prevailed when the water teiq>erature In winter was 

 colder than normal. Tor example, cool waters in the vicinity of Kyushu during the winter 

 months were followed by a good catch in that region during the spring, whereas the catch off 

 the northeastern coast of Honshu during the summer was poor. The author believed that when 

 the temperature was comparatively low in the southern waters the schools tended to stay long- 

 er and move about in densely concentrated shoals. Thus fewer fish broke away and migrated 

 into the Tohoku region. 



Fishing conditions for the skipjack in relation to oeeanographlc features such as 

 ocean currents and the vertical distribution of temperature gradients were analyzed by Uda 

 (1940, pp 145-147). In the Tohoku region the best catches were made when the difference in 

 temperature between the surface and 100 meters was at Its maxlmom (10° to 16°C). Whether 

 this difference was instrumental in forcing skipjacks to the surface or the concentrations 

 of sardines and. copepods (natural foods of the skipjack) at the surface was ths determining 

 factor Is not indicated. The peak catches were taken in the Karoshlo Current and were most 

 often found in the tongues of warm water extending into the cold cvirrent. In this connec- 

 tion several research workers have pointed out that during the past 20 years the appearance 

 of the skipjack has been delayed annually, although no effect on the general migration route 

 Is yet apparent. This yearly trend corresponds to the recant tendency of the warm Kuroshio 

 Cxirrent to be deflected southward, as shown in the oeeanographlc charts prepared by the 

 Tokyo Central Fisheries Sxperlmental Station. 



17 



P-6307 



