Pacific albacore; however, the conclusion was 

 that there was possibly some mixing between the 

 Indian Ocean and southwest Pacific albacore. 



In respect to yellowfin, Hiyama concluded 

 that morphometrically there are at least three 

 independent populations in the equatorial Pacific 

 betweenl30° W.andl30° E. Yellowfin in waters 

 adjacent to the Lesser Sunda Islands were found 

 to be distinct from fish of the equatorial Indian 

 Ocean. 



No conclusions were made on population 

 differences in bigeye because of insufficient 

 data. 



Discussion brought out that the albacore 

 has the widest distribution of any of the tunas in 

 the Pacific and also the smallest differences 

 between populations. The yellowfin apparently 

 has small independent populations scattered over 

 the Pacific; the bigeye distribution is larger than 

 the yellowfin but smaller than the albacore 

 distribution. 



Royce compared yellowfin samples from 

 the Pacific, Atlantic, and Indian Oceans. In the 

 yellowfin along the Pacific Equator from the 

 eastern Pacific to the Caroline Islands, the ex- 

 istence of a cline or character gradient was 

 noted. A sample from the Atlantic closely re- 

 sembled a sample taken between Costa Rica and 

 the Line Islands. The Somaliland sample was 

 the most diverse, the specimens showing particu- 

 larly short fins, a deep body, and a long distance 

 from the snout to the insertion of the ventrals. 



The conclusion was that east-west migra- 

 tion is limited, because the overlap of samples 

 from along the Pacific Equator is inversely re- 

 lated to distance between samples; the average 

 overlap between samples 1, 500 miles apart was 

 less than 50 percent, 3,000 miles less than25 

 percent, and 6, 000 miles less than 6 percent. 



Royce concluded that the name for 

 the yellowfin should be Thunnus albacares 

 (Bonnaterre) 1788, because the yellowfin has a 

 continuous distribution and also because the full 

 range of characters which have been used to 

 distinguish species occurs in the series of sam- 

 ples from the Pacific Equator. 



Discussion on yellowfin subpopulations 

 brought out that among the yellowfin in the east- 

 ern Pacific, ranging from Baja California to 

 Chile, there is evidence of isolation among the 

 stock, with intermixing on the order of about 20 

 to 25 percent. North of latitude 15° N. , the 

 stock responds differently from stock to the south 



of 15° N. An example noted was that the northern 

 stock spawns in summer while the equatorial 

 stock spawns in winter. 



Discussion brought out that plastic pheno- 

 typic characters c a n be modified by diet and 

 temperature . Conditions that modify growth may 

 change not only from place to place, but also 

 from year to year. As a result, we may be 

 measuring the changes in the fish or changes in 

 the environment. It was remarked, unfortu- 

 nately, there is no way to separate morphological 

 changes due to genetic factors from those due to 

 the environment. However, even if the charac- 

 ters are phenotypic, there is still an indication 

 that intermingling between fish from one area 

 and those of another area is highly unlikely. 



It was stated that knowledge of the sub- 

 populations of the tunas is essential, since each 

 population unit will respond, perhaps uniquely, 

 to fishing effort. Such knowledge of the popu- 

 lation pattern can help materially in the manage - 

 ment of the fishery resources. 



Discussion on genetic exchange brought 

 out that there may be intermingling between sur- 

 face and subsurface aggregations and also during 

 the early years of life. Yellowfin were noted to 

 be responsive to oceanographic changes and, 

 under conditions of abnormal warming or cool- 

 ing, they may move to other areas. Thus ex- 

 change of genetic material may occur through 

 the movements of a few fish. Further evidence 

 of possible genetic isolation was cited. Along 

 the Equator from 120° E. to 110° W. there does 

 not appear to be much variation in spawning. 

 Size at maturity appears to be at about 110 cm. , 

 although a few individuals between 80 to 110 cm. 

 may be mature. Off the American coast, how- 

 ever, a very large percentage of fish over 80 cm. 

 were found to be mature. Therefore, one might 

 possibly conclude that the fish in t h e eastern 

 part of the Pacific are of a different population 

 than those of the western part. 



Diversity of opinion was noted among 

 some contributors as to the extent of inter- 

 mingling of yellowfin population units in the 

 equatorial Pacific. Because of the possible 

 need for management in some yellowfin fisheries 

 in the near future, it was thought that the inves- 

 tigators concerned should make special effort 

 to resolve this problem. 



Discussion brought out that the term 

 subpopulation should refer to reproductive units, 

 and that the use of this term for aggregates of 

 individuals apart from this framework would 

 lead to confusion. 



