3. Mimura, K. - Studies on Indo - maguro 



4. Clemens, H. B. - The distribution of California bluefin tuna in the easternNorth 



Pacific 



5. Clemens, H. B. - The distribution of albacore in the North Pacific 



The initial discussion centered on long- 

 line fishing in the Pacific and Indian Oceans. It 

 was brought out that the Pacific Ocean in general 

 is marked by a series of current systems extend- 

 ing in an east -west direction. These current sys- 

 tems are characterized as distinct environments 

 with distinctive fisheries. For example, the 

 North Pacific Current north of 28° N. is noted 

 for heavy catches of albacore, while the area 

 from 5° N. to 10° S. (parts of the Equatorial 

 Countercurrent and the South Equatorial Ourrent) 

 is noted for heavy catches of yellowfin. Migra- 

 tions of tunas in these current systems were 

 hypothesized as occurring (a) within current sys- 

 tems, where the movements are slow and easy 

 to follow, and (b) between current systems, 

 where they are rapid and difficult to follow. Evi- 

 dence for (b) is the abrupt seasonal change in the 

 size composition of the catch. 



The next phase of the discussion pointed 

 out there was considerable variation in the abun- 

 dance of tunas according to years. For example, 

 the 1953 catch of albacore was good, relative to 

 the catch in an average year, and catches of big- 

 eye tuna were also good then. The 1953 catch was 

 attributed in part to the dominance of 5-year-old 

 albacore. It was also noted that 1953 was an 

 atypical year in many other ways. El Nino off 

 western South America was well developed, tuna 

 catches in the eastern Pacific were affected, the 

 Equatorial Countercurrent was weak, Chinook 

 and sockeye salmon catches were poor, and the 

 eastern Pacific was abnormally warm. 



North -south hemispheric differences 

 were shown to exist in longline catches. In gen- 

 eral, catches of tunas and marlins were heaviest 

 in the Southern Hemisphere . The significance of 

 this is unknown. 



The discussion then shifted to the length 

 distribution of yellowfin in various regions of the 

 Pacific longline grounds. Catch data were clas - 

 sified by length, area, and season. Yellowfin 

 smaller than 120 cm. were largely restricted to 

 waters of east longitude, while fish larger than 

 140 cm. were mostly distributed in waters of 

 west longitude. Intermediate -sized fish, 121-140 

 cm., were widely distributed, showing a rela- 

 tive shift inabundance to the east when compared 

 to the small fish. This shift in size was present 

 throughout the year, with no marked seasonal 

 differences. Three possible artificial causes 

 for it were evaluated: effect of fishing, gear 



selectivity, and the location of the fishing grounds 

 with respect to land masses. The difference in 

 size groups was hypothesized as a migration of 

 the smaller yellowfin from west to east. It was 

 suggested that the shoaling thermocline to the 

 east in equatorial waters may be responsible for 

 bringing the larger yellowfin into the depth range 

 of the longline. It was further pointed out that 

 returns from tagging in the western Pacific have 

 been mostly short term (i.e., 1 year or less) . 



Indian Ocean bluefin tuna, fished by the 

 Japanese in waters north and west of Australia, 

 have recently become the object of an important 

 longline fishery. There are two grounds for 

 these bluefin, clearly separated from each other. 

 The Old Fishing Ground is located between the 

 Lesser Sunda Islands and Australia and, since 

 1958, the New Fishing Ground off the Australian 

 west coast from 20° S. - 30° S. No bluefinhave 

 beenfound peripheral to these grounds, and only 

 scattered fish are found on the grounds outside 

 the months of September through April. The 

 catch rate in the Old Fishing Ground has two 

 peaks, one in September - October and the other 

 in February. On the New Fishing Ground the 

 single peak occurs in January - February and is 

 twice as great as those of the Old Fishing Ground. 

 The size of bluefin taken on the Old Fishing Ground 

 is uniformly large throughout the season, while 

 on the New Fishing Ground their size decreases 

 at the height of the season. Bluefin on the Old 

 Fishing Ground have relatively large gonads 

 (1,000 -4,000 g.) throughout the season, while 

 fish on the New Fishing Ground have gonads 

 mostly smaller than 1, 500 g. , with a change in 

 relative size at various times during the season. 

 It was pointed out that gonads from large western 

 Atlantic bluefin are difficult to evaluate by the 

 method of gonad weight because they contain a 

 large amount of fat. Western Atlantic bluefin 

 make a seasonal migration, a situation also hy- 

 pothesized for these two groups of Australian 

 bluefin. 



The bluefin tuna of the eastern North 

 Pacific is taken primarily in coastal waters, in 

 contrast to the North Pacific albacore, which is 

 primarily a pelagic species . Both species range 

 from Baja California to Alaska, and both are the 

 object of seasonal fisheries off the west coast of 

 the United States and Baja California. The size 

 of bluefin caught in the last 50 years has changed 

 considerably. In the early 1900' s, many large 

 bluefin (200 - 300 lb.) were caught by sports 



