14 



Subpopulations 



Identification of reproductively isolated groups, or sub- 

 populations, within tuna species is the principal goal of the 

 Subpopulations Program. While identifying these subpopula- 

 tions, laboratory biologists also hope to gain new insight into 

 the relationships between tuna subpopulations and their oceanic 

 environment. Knowledge both of the identity of specific sub- 

 populations and of their environmental boundaries is necessary 

 to the calculation of abundance of populations as a whole and 

 number of fish that can be taken annually without decreasing 

 maximum long-term yields. 



Serological techniques are used to identify tuna subpopu- 

 lations. Serology , unlike other available methods, offers rea- 

 sonable assurance that the characteristics studied are under 

 genetic control and are not the result of environmental differ- 

 ences. Through these studies the Biological Laboratory has 

 made two important contributions to knowledge of tuna sub- 

 populations: (1) disco\?ery of a blood-group system in bigeye 

 tuna similar to the A-B-O system long known in man; (2) def- 

 inition of several skipjack subpopulations in the Pacific. 



studies may uncover reagents capable of demonstrating similar 

 or related systems in other tuna species. 



Concurrent studies of skipjack tuna blood groups revealed 

 the existence of subpopulations in widely divergent areas of 

 the Pacific. Samples used in these studies were obtained from 

 the waters around the Hawaiian Islands, Christmas Island, 

 Marquesas Islands, Tuamotu Archipelago, Society Islands, 

 western Carolines, and near Baja California. Further studies 

 are expected to reveal the existence of still other skipjack 

 subpopulations. 



Table 1. --Agglutination reaction patterns of 

 A-B-0 blood groups in bigeye tuna and in 

 man 



Discovery of the A-B-O type blood-group system in the 

 bigeye tuna was a significant contribution to the identification 

 of tuna subpopulations. The study demonstrated that two re- 

 agents, when combined with individual blood samples from a 

 large number of bigeye tuna individuals, showed four kinds of 

 reaction patterns analogous to the A-B-O blood-type system 

 of man (table 1). Hardy-Weinberg analyses of the relative 

 proportions of these four blood types agree with a hypothesis 

 that the blood-group system is determined by the inheritance 

 pattern of three related allelic genes. 



These serological findings in bigeye tuna are significant 

 because it can be shown by an analysis of the expected and 

 observed proportions of these four blood types in a represent- 

 ative population sample whether the sample has been drawn 

 from a single subpopulation or from more than one subpopula- 

 tion. Discovery of an A-B-O type of blood-group system in 

 the bigeye tuna suggests the further possibility that future 



Two skipjack subpopulations recently identified in the Ha- 

 waiian fishery have been the subject of intensive research at 

 the Biological Laboratory to develop greater precision in our 

 distinguishing of closely related subpopulations. Another as- 

 pect of the study concerns the association of these subpopula- 

 tions with specific physical features of the major ocean cur- 

 rents. We have noted that certain changes in the dynamic 

 features of the ocean near Hawaii appear to influence the move- 

 ment of skipjack in Hawaiian waters. 



Earlier oceanographic studies of the physical properties 

 of the waters near the Hawaiian Islands showed that the bound- 

 ary between the North Pacific Central Water and the California 

 Current Extension Water moved northward in the spring and 

 southward in the fall. The emigration of large skipjack into 

 Hawaiian waters in the spring and out in the fall appears to be 

 associated with the north-south movement of the California 

 Current Extension Water. In 1963, laboratory biologists col- 



