FISHERY BULLETIN: VOL. 86, NO. 2 



clearly show the effectiveness of this technique in 

 preventing burnt tuna. A drop from 30 to < 10% of 

 the tuna becoming burnt was noted when the 

 fisherman changed his killing technique from 

 shooting to the use of a brain spike. 



A MORE COMPREHENSIVE 



HYPOTHESIS ON THE ETIOLOGY 



OF BURNT TUNA 



The hypothesis presented by Hochachka and 

 Brill (1987) does not explain all the observed fac- 

 tors leading to tunas' different propensity to be- 

 come burnt. It cannot explain why tunas strug- 

 gling for <7 minutes rarely become burnt 

 (Nakamura fn. 7), nor can it account for the obser- 

 vations of Davie and Sparksman (1986) and 

 Bourke (unpubl. data), who found that tuna 

 struggling for extended (>1 hour) periods of time 

 have a lower probability of becoming burnt. It 

 also cannot explain why female fish become burnt 

 more often than males (Nakamura et al. 1977), or 

 why fish caught in summer become burnt more 

 often (Fig. 3). 



Our revised hypothesis is presented in Figure 

 4. We have incorporated into the Hochachka and 

 Brill (1987) hypothesis, the action of the neuro- 

 transmitters-hormones norepinephrine (NE) and 

 epinephrine (E) (collectively referred to as cate- 

 cholamines). Periods of intense physical activity 

 or capture stress increase the levels of circulating 

 catecholamines by about 10-200 times in fish 

 other than tunas (Nakano and Tomlinson 1967; 

 Ling and Wells 1985). Most likely a similar situa- 

 tion occurs in tuna during hooking, fighting, and 

 capture. The importance of elevated circulating 

 catecholamines in this particular schema comes 



INTENSE MUSCLE ACTIVITY 



i 



LACK OF 02 + ATP 



i 



METABOUC COLLAPSE OF MEMBRANE 



i 



RISE IN INTRACELLULAR CALCIUM 



i 



ACTIVATION OF CANP 



STRESS OF CAPTURE 



NE & E RELEASE 



■INCREASE OF CANP ACTION  



from their potentiating effect on the action of 

 CANP. Catecholamines cause the phosphoryla- 

 tion of troponin, resulting in a more rapid and 

 prolonged proteolysis of this muscle structure 

 (Toyo-oka 1982); in other words, high circulating 

 levels of catecholamines greatly increase the ef- 

 fectiveness of the enzymes responsible for a tuna 

 becoming burnt. ^ 



The gills are the organ primarily responsible 

 for the degradation of catecholamines, and circu- 

 lating catecholamines are rapidly cleared from 

 the blood (Nekvasil and Olson 1986). We estimate 

 circulating catecholamines in tuna have a half- 

 life of 30 minutes or less. Therefore, we hypothe- 

 size that, in a longline-caught fish remaining in 

 the water for several hours after hooking (Davie 

 and Sparksman 1986), plasma catecholamines 

 are reduced to low levels before the fish is landed 

 and killed, thus resulting in a low percentage of 

 burnt fish. Similarly, fish caught by rod and reel 

 or by handline and fought for several hours would 

 have the low circulating catecholamine levels and 

 low propensity to become burnt. Indeed, Gibson 

 (1981) recommends that handline-caught fish be 

 attached to a buoy and left for an hour prior to 

 being brought on board and killed, as a measure 

 to prevent burnt tuna. That this technique pre- 

 sumably lowers blood catecholamine levels prior 

 to death could explain its efficacy. On the other 

 hand, we hypothesize that a fish landed during 

 the peak of its blood catecholamine concentra- 

 tions would have the greatest propensity for be- 

 coming burnt. This indeed appears to be the case. 



The mechanism that ties catecholamines to the 

 observed seasonality and increased number of fe- 

 male fish becoming burnt lies in the biochemical 

 structural similarity of catecholamines and re- 

 productive steroids, particularly estrogen. These 

 steroids reach a peak during spawning season, 

 which is May through October for yellowfin tuna 

 in Hawaii (June 1960). A corresponding increase 

 occurs in the percentage of the tuna catch that 

 becomes burnt during this season (Fig. 3). In the 

 presence of tyrosine hydroxylase, estrogen is con- 

 verted to catecholestrogen and consequently com- 

 petes for the same degradative enzymes in the 

 gills, therefore slowing the clearance of cate- 

 cholamines from the blood (Nekvasil and Olson 

 1986). Female fish, with high circulating estro- 

 gen levels, would then be expected either to reach 



UNDESIRABLE MUSCLE TEXTURE 



Figure 4. — Comprehensive theory of the etiology of burnt tuna 

 proposed in this paper. 



^Although Cortisol is also known to be released during stress, 

 its mechanism of action is much slower than NE or E, making 

 it unlikely that it could exert catabolic effects within 15 min- 

 utes. 



370 



