number of fishreaching 6 years of age, the pre- 

 dominant age group in the South Pacific longline 

 fishery. This condition is reversed in the South 

 Pacific , which may be the reason why the 6 - 

 year-old and older age groups are abundant 

 enough to sustain a commercial fishery. 



Uda, Michitaka 



Cyclical fluctuation of the Pacific tuna 

 fisheries in response to cold and warm 

 water intrusions. /Conference Paper 

 V - 7.7 (In press (1962) in Journal of 

 Tokyo University of Fisheries. ) 



Peak landings of skipjack from Japanese 

 waters have occurred at irregular intervals 

 since 1912. A study of oceanographic conditions 

 showed that good catches were made during years 

 with warm water temperatures, whereas poor 

 catches were made during years with cooler 

 water temperatures. These differences in tem- 

 peratures were primarily relatedto the relative 

 strengths of the warm Kuroshio and the cool 

 Oyashio. 



It was postulated that recruitment of 

 skipjack is favored by the enriched zones asso- 

 ciated with upwelling in the Equatorial Counter - 

 current and near the Equator. From these trop- 

 ical waters there is a migration of skipjack 

 (composed mainly of medium- sized fish) into 

 Japanese waters. When the warm Kuroshio 

 water spreads over a broad area, more skipjack 

 become available to the Japanese fishery. 



Conversely a strong Oyashio current 

 (cold water) hinder s the migration, and catches 

 are low. However, good catches may also be 

 made when both the Oyashio and Kuroshio are 

 strong. Under the latter conditions skipjack 

 are concentrated along the boundary between 

 the warm and cold water (Polar Front). It is 

 postulated that the yield of skipjack from Japa- 

 nese waters varies inversely with that from 

 American waters. 



Albacore catch records also show peak 

 landings at irregular intervals for both the Japa- 

 nese and the U. S. fisheries. It is postulated 

 that the variations between these two areas occur 

 in a reciprocal manner, as was the case with 

 skipjack. It is further postulated that the tern - 

 peratures in the eastern and western Pacific 

 vary in a reciprocal manner, with pulsations 

 traveling west to east in the region of the domi- 

 nant westerlies, returning in the lower latitudes 

 dominated by the northeasterly trades. Intru- 

 sions of cold and warm waters into the coastal 

 areas may be related to fluctuations in the at- 



mospheric pressure systems. During 1955-59, 

 an increase in pressure difference among the 

 Siberian and North Pacific Highs and the Aleutian 

 Low corresponded with a period of cooler tem- 

 peratures in the western Pacific, warmer in the 

 eastern Pacific. 



Geographical variations in the location 

 of the North Pacific High in winter and spring 

 also affect the temperatures. Movement to the 

 northwest results in warm intrusions in the 

 western Pacific and good albacore catches, re- 

 ciprocal in eastern Pacific. Movement of the 

 high to the southeast corresponds to cool water 

 in the west and poor catches, warm water in the 

 east and good catches. 



Bluefin catches off the coasts of Japan 

 have also shown periodic fluctuations, with a 

 decline in catch associated with periods of cold 

 water intrusion, an increase in catch with per- 

 iods of warming. The cause of these fluctuations 

 maybe related to dominant brood strength during 

 periods of rising temperatures. When cool 

 waters intrude southward into the spawning 

 grounds, one or more year classes are seri- 

 ously affected. However, these cooler waters 

 are comparatively rich in nutrients and, with a 

 reversal in temperature toward warming, suc- 

 ceeding year classes find more plentiful food 

 and favorable temperature conditions for their 

 migration northward into Japanese waters. Thus 

 we have a cyclical situation — a reduction of 

 year-class strength and the fishery during per- 

 iods of cool surface waters, followed by an in- 

 crease in brood strength during the subsequent 

 period of warming and a more favorable eco- 

 logical situation resulting from the enrichment 

 of the surface waters during the cool period. 



Uda, Michitaka 



Localized concentration of tunas in t h e 

 eddies along oceanic fronts. /Confer- 

 ence Paper V - 8. J (In press (1962) 

 in Journal of Tokyo University of 

 Fisheries. ) 



Aggregations of tuna aretobe found along 

 fronts, localized in eddies, cool or warm, and 

 in zones of upwelling. Variations in concentra- 

 tion and location of the tuna are associated with 

 growth, decay, and change in position of the 

 eddies. Such eddies are to be found associated 

 with the Polar Front and the subtropical conver- 

 gence and near the Equator. 



Albacore catch records for the period 

 1951-60 and surface temperature data were con- 

 sidered for the areabetween 140° E. and 154° E. 



39 



