Age and Growth of Tunas 



From data obtained by the Laboratory in Honolulu, 

 Eugene L. Nakamura and James H. Uchiyama calculated 

 curves of average weight against length. Their data show 

 that skipjack tuna have a maximum length of about 27 

 inches and weight of 37 pounds. 



As has long been known, yellowfin tuna grow much larger 

 than the skipjack tuna. The Honolulu data show that they 

 attain a length of about 5 feet and a weight of more than 

 250 pounds. Bigeye tuna are larger yet, reaching a length 

 of almost 6 feet and a weight of about 330 pounds. Albacore 

 are smaller. The largest are about 4 feet long and weigh 

 under 100 pounds. 



Richard S. Shomura prepared a study of age and growth 

 of the Pacific tunas for the Governor's Conference. He found 

 the greatest discrepancies of reported age and growth in the 

 several studies that have been made on the growth of 

 albacore. Workers are uncertain whether the young albacore 

 enter the fisheries at 1 or 2 years of age. Usually these fish 

 are about 18 inches long, although some are as short as 

 12 inches. 



Few studies have been made of the age and growth of 

 skipjack tuna, Shomura found. The most recent, made by 

 Brian J. Rothschild, indicates that the .skipjack tuna in 

 Hawaiian waters are about a year old and 12 inches long 

 when they enter the fishery. Fish 2 years old and about 20 

 inches long form a considerable part of the Hawaiian skip- 

 jack tuna catch. 



Most of the evidence on the yellowfin tuna suggests that 

 it grows rapidly during its early life, about 14 inches to 

 almost 20 from its first to its second year. Bigeye tuna 

 display much the same growth pattern. 



Est-imates of Tuna Abundance 



The studies just cited summarize what is known of the 

 distribution of the commercially caught tunas in the central 



Pacific Ocean. The chief business of the Governor's Confer- 

 ence was to evaluate the evidence to see whether the central 

 Pacific catches might be increased. The conferees found that 

 for the yellowfin tuna and the skipjack tuna, substantial 

 increases should be possible. 



After reviewing the material on the yellowfin tuna, the 

 working group estimated an increased yield of about 30,000 

 to 50.000 tons per year from the central and western Pacific 

 if yellowfin tuna smaller than those now taken by the 

 Japanese longline fleet could be caught. 



The estimate for bigeye tuna was made by Ralph P. Silli- 

 man. He used data from the Japanese longline fishery to 

 arrive at an estimate of a maximum sustainable yield of 

 about 80,000 tons a year. The fishery is already far more 

 than this ; Silliman believes, therefore, no significant increase 

 in the catch of the large bigeye tuna is possible. 



Silliman also estimated the potential catch of skipjack 

 tuna. Here his basic data came from the eastern Pacific 

 fishery. He estimated a maximum yield of 225,000 tons, or 

 about three times the amount now being taken. 



Also using data from the eastern Pacific fishery, Brian 

 J. Roth.schild treated his material somewhat differently from 

 Silliman and arrived at a different estimate. He set the 

 potential yield at between 2 and 17 times the current 

 70,000 tons a year. (His higher figure would make the 

 central Pacific skipjack tuna stock provide almost as much 

 fish as the present catch of tuna for the entire world.) 



Rothschild points out that his estimate of yield ignores 

 the skipjack tuna stocks of the South Pacific, which may be 

 vast. 



Underlying both Silliman's and Rothschild's calculations 

 is the assumption that the hypotheses concerning spawning 

 and migration of the skipjack tuna advanced by Rothschild 



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