groups of fish that do not breed with fish from other 

 groups, will display distinct proportions of blood types or 

 other characteristics; these proportions change only very, 

 very slowly with the generations. Thus, in theory, if the 

 fish from the eastern Pacific, say, never breed with fish 

 from off Japan, then in the course of time distinguishable 

 subpopulations would be established. 



In practical terms, this would also mean that if the popu- 

 lation from off Japan, as an example, were brought to so 

 low a level it could not reproduce itself, the area would 

 not be repopulated by fish from the eastern Pacific, for 

 they might never reach Japan. If, on the other hand, a 

 single great freely intermingling population exists, the 

 depletion of fish in any single area would probably be 

 followed in time by a replenishment of the supply. 



To throw light on the subpopulation structure of the 

 tunas, the Laboratory in Honolulu has established a Tuna 

 Blood Group Center. To it come samples from all over 

 the world. In recent months, shipments of skipjack tuna 

 blood, for e.xample, have been received from the Gulf of 

 Guinea (off west Africa), and from the Trust Territory 

 of the Pacific Islands in the western Pacific; and Fujino 

 journeyed to Tahiti and more recently to Ecuador to obtain 

 samples. 



In all, 14,000 samples of tuna blood have been analyzed 

 or arc waiting for analysis at the Tuna Blood Group Center 

 fig. 14). Because of the importance of the species to 

 the future of tuna fisheries in the central Pacific, more 

 blood samples (12,000) have been taken from skipjack 

 tuna than from any other species, but albacore are repre- 

 sented by 300 samples, bigeye tuna by 500, yellowfin tuna 

 by 700, .southern bluefin tuna by 300; and there are still 

 others. 



From this storehouse of data, Fujino has drawn the 

 materials for a .series of scientific papers for journals in 

 this country and abroad. In a paper published in 1967 he 



outlined what is known of the subpopulation structure of 

 the skipjack tuna in the Pacific Ocean. Writing in the 

 "Proceedings of the Forty-Seventh Annual Conference of 

 the Western Association of State Game and Fish Com- 

 missioners," he reported that the skipjack tuna of the 

 tropical western Pacific, those taken in the waters of the 

 Trust Territory, belong to a subpopulation that does not 

 appear in the Hawaiian fishery. Also different from the 

 Hawaiian fish are those taken in Japanese coastal waters. 



The Dividing Line 



Seven Japanese fishing vessels operating in the wide 

 and empty reaches of the North Pacific Ocean west of 

 the international date line in the winter of 1967-68 made 

 a catch unexpectedly valuable to science. 



The craft were seeking large albacore, a tuna taken by 

 longline fishing. The longlines also caught specimens of 

 skipjack tuna. Smaller than albacore, the skipjack tuna 

 is taken only infrequently on longlines, most of them being 

 caught at the surface. 



Through arrangements made by Japanese scientists sam- 

 ples of blood were drawn from 94 of the .skipjack tuna at 

 the Japanese port at which they were unloaded. These were 

 then shipped by air to the Tuna Blood Group Center. There 

 the serum was extracted and subjected to electrophoresis. 



One of the many constituents of serum is an enzyme 

 called esterase, whose biochemical function is to accelerate 

 the synthesis or breakdown of an ester, a compound of an 

 acid and an alcohol. Like many other proteins, the e.sterase 

 in serum varies in the amount of electric charge the indi- 

 vidual molecule carries. This means that otherwise indistin- 

 guishable esterases will move at different rates when they 

 are subjected to an electric field of direct current under 

 certain chemical conditions. In electrophoresis, a few 

 milligrams of a sample of the sub.stance being tested are 

 placed on a plate of starch gel; when current is applied, 



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