FISHERY BULLETIN: VOL. 86, NO. 2 



Annual losses to the handline fishery are an esti- 

 mated 169^ of the value of the total catch (Cramer 

 et al. 1981). The problem has also been reported 

 in fish intended for raw consumption caught by 

 purse seine (Nakamura et al. 1977). Surprisingly, 

 the problem rarely occurs in fish caught by 

 longlining (Williams 1986). 



PAST RESEARCH ON BURNT TUNA 



The Japanese were the first to investigate the 

 causes of burnt tuna and possible mitigating 

 strategies (Itokawa 1968, 1969). The first con- 

 trolled laboratory investigations were those of 

 Nakamura et al. (1977) and Konagaya and Kona- 

 gaya (1978, 1979). Nakamura et al. (1977) con- 

 cluded that high muscle temperature and low 

 muscle pH caused myofibrillar protein denatura- 

 tion and also noticed that, once denaturation 

 began, it continued even if the tissue was kept at 

 0°C. Because of the relatively high thermostabil- 

 ity of tuna myofibrillar protein and because yake 

 niku occurs in species (e.g., frigate mackerel and 

 sardine) that do not generate high muscle tem- 

 peratures during struggling, Konagaya and Kon- 

 agaya (1978) concluded that acid denaturation of 

 myofibrillar proteins at moderate temperatures 

 was the underlying cause. 



Cramer et al. (1981) studied handline-caught 

 yellowfin tuna in Hawaii and found that the oc- 

 currence of burnt tuna did not correlate with 

 muscle temperature at time of landing and corre- 

 lated only loosely with extracellular muscle pH. 

 Ikehara^ conducted an engineering study to de- 

 velop methods to cool large yellowfin tuna more 

 rapidly, the presumption being that rapid cooling 

 would prevent muscle degradation. Although suc- 

 cessful in developing a technique to increase cool- 

 ing of deep muscle temperature, as shown in Fig- 

 ure 1, there was no apparent correlation between 

 rate of cooling and incidence of burnt flesh. In 

 spite of this lack of directly observed correlation, 

 some publications designed for fishermen still 

 stress that high muscle temperatures and low 

 muscle pH are the prime causes of burnt tuna 

 (Gibson 1981). Others have expressed doubt as to 

 the validity of this hypothesis (Jerrett 1984). 



The high muscle temperature-low pH hypothe- 

 sis appears to fit with what is known about tuna 

 physiology, in that these fishes are capable of pro- 



ducing muscle temperatures significantly above 

 ambient (Carey et al. 1971; Carey 1973) and ex- 

 hibit some of the highest rates of muscle glycoly- 

 sis (production of muscle lactate and concomitant 

 production of acidity) observed in nature 

 (Hochachka et al. 1978; Hochachka and Momm- 

 sen 1983). Yet some observations do not fit this 

 hypothesis. For example, burnt tuna occurs more 

 frequently in summer, more frequently in female 

 fish, and more frequently in fish fought for short 

 periods of time than in fish fought for long periods 

 (>7 minutes or <2 hours) (Davie and Sparksman 

 1986; Nakamura^). Furthermore, burnt tuna oc- 

 curs rarely in longline-caught fish and in fish 

 subjected to brain or spinal column destruction 

 immediately following capture (Nakamura et al. 

 1977; [Suisan Sekai] 1977; Cramer et al. 1981; 

 Davie and Sparksman 1986; Nakamura fn. 7). 

 The hypothesis that burnt tuna is caused by high 

 muscle temperature and low muscle pH does not 

 seem to directly fit with any of these observa- 

 tions. 



A NEW ANALYSIS OF THE BURNT 

 TUNA PROBLEM 



At the biochemical level, the high muscle 

 temperature-low pH h3rpothesis would predict 

 that the observed drop in extracellular pH would 

 be accompanied by a similar drop in intracellular 

 pH and activation of lysosomal proteases. These 

 proteases would then degrade actin and myosin, 

 the dominant muscular proteins, resulting in the 



'^R. Nakamura, Department of Animal Sciences, University of 

 Hawaii, Honolulu, HI 96822, pers. commun. October 1987. 



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ELAPSED TIME (h) 



^Ikehara, W. N. 1981. Development of a small-boat chill- 

 ing system for the reduction of burnt tuna. Final Report for a 

 Pacific Tuna Development Foundation contract, var. pag. 



Figure 1. — Data are from Ikehara (text fn. 6). Initial and final 

 deep muscle temperatures are plotted on semilogarithmic axes 

 to linearize the rate of temperature change. Numbers in paren- 

 theses are the body weight (kg) for each fish. 



368 



