FISH MASS 100 kg 



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FISHERY BULLETIN: VOL. 71. NO. 2 

 FISH MASS I kg 



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Figure 10. — Surfaces of minimum speeds required for hydrostatic equilibrium computed with different values of 

 Af and {D^ - D^) for hypothetical scombroids with a mass of 1, 10, or 100 kg. Speeds were computed from equa- 

 tion 1 with C/7^set at 1.0 and D ox p set at 1.025 g/ml. 



should be able to maintain speeds near 100 

 cm/sec according to data from nonscombroids 

 summarized in Figure 18 of Magnuson (1970). 

 If there were some disadvantage to having a 

 gas bladder, the loss of the bladder would not 

 impose impossible levels of activity. Thus, the 

 absence of a gas bladder and the occurrence 

 of small pectorals among some small scom- 

 broids are consistent with the expectations of 

 Figure 10 for 1-kg fish. 



If hypothetical 10-kg and then 100-kg scom- 

 broids are considered (Figure 10), the pres- 

 ence of a gas bladder can be seen to be 

 increasingly important to retain low levels 

 of swimming activity. Required speed decreases 

 rapidly with increasing pectoral size up to 

 about 100 cm' for 10-kg fish and up to 200 cm^ 

 for 100-kg fish compared to 50 cm- for 1-kg 

 fish. For a 100-kg fish and {Df - D^) = 0.025, 

 as lifting areas increase from 200 to 700 cm-, 

 required speed declines to about 100 cm/sec. 

 This constitutes a significant reduction in 

 speed and helps explain why 100-kg Tluiniius 

 have pectoral lifting areas as great as 700 cm-. 

 Large scombroids could reduce energy ex- 

 penditures with larger than expected pectoral 

 fins. 



Computed minimum speeds were slower 

 than 40 cm/sec for a 100-kg Tliionius (Figure 

 8). If its pectoral fins were as small as those 

 of K. pelamis, it would have to swim at least 

 90 cm/sec; and if it also had no gas bladder, 

 its minimum speed would have to be 160 cm/sec. 



Adaptations that serve to make low activity 

 possible for large scombroids and xiphoids may 



reduce required speed well below the endurance 

 speed or even below the most physiologically 

 efficient speed for a species to migrate. No 

 data on maximum endurance speeds are avail- 

 able for fish this large, but endurance speeds 

 of sockeye salmon, Oucorhynchus )ierka, in 

 centimeters per second, increases as length 

 increases (Brett, 1965). The relationship, speed 

 = 19 P-^, was determined from O. )ierka, 8 to 

 54 cm FL, over periods only as long as 1 hr. 

 Another data set (Hunter, 1971) for the jack 

 mackerel, TrachuruH symmetticus, is similar 

 but based on shorter fish, 9-18 cm long, for 

 longer periods, 6 hr. The relation was speed 

 = 22 P-^. Extrapolation of these relationships 

 from a salmonid, 50 cm long, or a carangid, 

 18 cm long, to scombroids, about 80 and 180 

 cm long, seems a bit unrealistic, but if done 

 indicates that the required speeds, even for a 

 large hypothetical fish with short pectoral and 

 no gas bladder, are considerably below the 

 extrapolated endurance speeds. And a 100-cm 

 T. albacares with its gas bladder and large 

 pectorals swims at speeds less than one-sixth 

 the extrapolated endurance speeds of a salmonid 

 and less than one-fourth the speeds predicted 

 by Shuleikin (1966) as most efficient for migra- 

 tion. Shuleikin's theoretical model was based 

 on the speed at which the fish expended the 

 least energy to overcome internal friction of 

 muscles and external friction from movement 

 through water. 



Perhaps for the large fish the capability for 

 low speed, makes possible speeds which are 

 energetically prudent in an ecological sense. 



352 



