A COMPREHENSIVE THEORY ON THE ETIOLOGY OF BURNT TUNA 



Cheryl Watson,' Robert E. Bourke,^ and Richard W Brill^ 



ABSTRACT 



Over the past 14 years, the Hawaii handline fishery has experienced phenomenal growth in the catch 

 for large yellowfin tuna, Thunnus albacares, and bigeye tuna, Thunnus obesus. These fish are 

 primarily caught for the sashimi (raw consumption) market but have been continually plagued with 

 a product quality problem known as "burnt tuna" or, in Japanese, yake niku. Not only does this 

 problem significantly reduce the value of the catch, it also limits export markets and expansion of this 

 low-capital, high-return fishery to other areas of the Pacific. Previous research and suggestions for 

 mitigating burnt tuna have centered on the hypothesis that it is caused by high muscle temperature 

 and low pH, which is the result of a violent struggle during capture. 



A new, more comprehensive hypothesis is presented: Burnt tuna is actually caused by the p)ost- 

 mortem activation of enzymes known as calcium-activated proteases and by the enhancement of the 

 effect of these enzymes by high blood catecholamine levels. Previously unexplainable observations, 

 such as the propensity of female fish to become burnt more often during the summer months, the 

 efficacy of brain destruction in preventing burnt tuna, and the lack of effect of cooling on the incidence 

 of burnt tuna, are explainable in light of this new hypothesis. 



One of the largest fisheries in Hawaii is the hand- 

 line fishery for large (>50 kg) yellowfin tuna, 

 Thunnus albacares, and bigeye tuna, T. obesus, 

 caught primarily for raw consumption as sashimi. 

 Yearly landings increased from 89 short tons (ex- 

 vessel value, $131,000) in 1973 to 615 short tons 

 (ex-vessel value, $2.1 million) in 1984 (Yuen 

 1979; Hudgins and Pooley 1987). The total eco- 

 nomic value of the fishery has been estimated as 

 high as $5 million yearly (Ikehara'*). In Hawaii, 

 the night handline fishery is known as ika shibi 

 from the Japanese words for squid and tuna, and 

 the daytime fishery is known as palu ahi from the 

 Hawaiian words for chum and yellowfin tuna. 

 There is also growing international interest in 

 this tjTDe of fishing because of its low initial capi- 

 tal investment, low operating and fixed expenses, 

 strong export markets, and high profitability 

 (Strong 1979; Gibson 1981; Jerrett 1984). Boats 

 can be as small as 6 m and require only one- or 

 two-man crew. Catch rates in Hawaii have 

 ranged from two fish per hook per night (Yuen 



'Pacific Gamefish Research Foundation, 74-425 Kealakehe 

 Parkway, #15, Kailua-Kona, HI 96740; present address: De- 

 partment of Physiology, John A. Bums School of Medicine, Uni- 

 versity of Hawaii, Honolulu, HI 96822. 



2465B Kawailoa Road, Kailua, HI 96734. 



3Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, 

 HI 96822-2396. 



^Ikehara, W. N. 1981. A survey of the ika-shibi fishery in 

 the State of Hawaii, 1980. Southwest Fish. Cent. Adm. Rep. 

 H-82-4C, 12 p. Southwest Fisheries Center Honolulu Labora- 

 tory, National Marine Fisheries Service, NOAA, Honolulu, HI 

 96822-2396. 



1979) to one-half fish per hook per night 

 (Bourke^), illustrating the profitability for the in- 

 dividual fisherman. 



Unfortunately, handline (as well as primarily 

 recreational trolling) fishermen are plagued by a 

 product quality problem known as "burnt tuna" 

 or, in Japanese, as yake niku (literally translated 

 as "cooked meat"). When fish are intended for raw 

 consumption, product quality is of utmost impor- 

 tance. Prime quality tuna flesh should be red, 

 translucent, and firm and have a delicate flavor. 

 Burnt tuna is pale, exudes a clear fluid, and has 

 a soft texture and a slightly sour taste. Although 

 perfectly palatable when cooked or canned, burnt 

 tuna is considered unsuitable for raw consump- 

 tion and commands only a fraction of the price of 

 prime quality fish. Fish are usually exported 

 whole to preserve freshness, so burnt tuna often is 

 not detected until shipping costs have been in- 

 curred. This discourages exports of tuna caught 

 from areas or via fishing techniques with high 

 incidences of the problem. 



Burnt tuna aflects from 5 to 100% of the tissue 

 from an individual fish and ranges from mild to 

 severe. Approximately 25% of the fish caught by 

 the Hawaii handline fishery are burnt, as are 

 50% of the large yellowfin tuna caught by com- 

 mercial and recreational trollers (Bourke fn. 5). 



^Bourke, R. E. 1985. Hilo ikashibi fishery 1984 survey: 

 Problems of a maturing fishery and a potential solution to the 

 burnt tuna problem. Poster presentation at 1985 Tuna Confer- 



Manuscript accepted January 1988. 

 fishery BULLETIN: VOL. 86, NO. 2, 1988. 



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