HUNTER: SPEED OF JACK MACKEREL 



level in the muscle (Pritchard et al., 1971). 

 These results are inconsistent with the conclu- 

 sion that at sustained cruising speeds, fish use 

 lipid metabolism to drive red muscle (Bone, 

 1966; Gordon, 1968; Blaxter, 1969) and that 

 no oxygen debt is incurred (Brett, 1963). Re- 

 liance on glycogen as the principal fuel pro- 

 bably severely limits the time a speed can be 

 maintained as compared with one where lipid 

 metabolism is used exclusively. Thus the bio- 

 chemical evidence indicates that the 6-hr speed 

 threshold for Trachurus probably could be main- 

 tained only for a period of hours or perhaps 

 days but certainly not weeks as one would ex- 

 pect if fat were used as fuel. The 6-hr thresh- 

 old was also considerably above sustained speed 

 thresholds for other fish where presumably fat 

 may be employed as fuel. Brett (1967), in a 

 study directly comparable with the current one, 

 found the 50% fatigue time for sockeye salmon 

 was 4 L/sec (about 11.3 U"^) whereas for comp- 

 arable size jack mackerel it would be about 7.6 

 L/sec or 22.0 L"^. Other less comparable data 

 give sustained or cruising speeds in the range 

 of 3 to 4 L/sec (Blaxter, 1969). Thus, Trach- 

 urus has special physiological and structural 

 adaptations that permit swimming for periods 

 of hours at elevated speeds and it was the thresh- 

 old for this swimming behavior that was meas- 

 ured. Other fishes, especially the scombroid 

 fishes, may have similar abilities. For example, 

 skipjack tuna can swim at 8 knots, or about 43 

 L"^, for over an hour (Commercial Fisheries 

 Review, 1969) and yellowfin tuna and skipjack 

 tuna have higher levels of white muscle gly- 

 cogen than many other species of fish (Barrett 

 and Connor, 1964). 



It seems possible another speed threshold may 

 exist for Trachurus below the present one where 

 fat is the principal fuel, only red muscle is used 

 for locomotion, and swimming can be main- 

 tained almost indefinitely. It would not be sur- 

 prising if this lower threshold were closer to 

 those determined for other fishes. 



LITERATURE CITED 



Bainbridge, R. 



1958. The speed of swimming of fish as related 

 to size and to the frequency and amplitude of 



the tail beat. J. E.xp. Biol. 35: 109-133. 

 1960. Speed and stamina in three fish. J. Exp. 

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1962. Training, speed and stamina in trout. J. 

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Barrett, I., and A. R. Connor. 



1964. Muscle glycogen and blood lactate in yellow- 

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Blaxter, J. H. S. 



1969. Swimming speeds of fish. FAO (Food Agr. 

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1966. On the function of the two types of myotomal 

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Brett, J. R. 



1963. The energy required for swimming by young 

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1965. The relation of size to rate of oxygen con- 

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1967. Swimming performance of sockeye salmon 

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Commercial Fisheries Review. 



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Finney, D. J. 



1952. Probit analysis. 2d ed. Cambridge Univ. 

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Gordon, M. S. 



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Gray, J. 



1968. Animal locomotion. Weidenfeld, London, 

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1971. Swimming speed, tail beat frequency, tail 

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Magnuson, J. J. 



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1971. The relation between exercise and biochem- 

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