MAGNUSON: ADAPTATIONS OF SCOMBROIDS AND XIPHOIDS 



(1966) as most efficient for migration by aquatic 

 organisms. 



Among the species with gas bladders, T. 

 albacares and T. obesus have the largest pec- 

 torals, but Ac. solandri with small pectorals 

 swims the slowest. The difference appears to 

 be primarily from differences in gas bladder 

 volume and resulting density. The large gas 

 bladder and low density of Ac. sola)idri more 

 than compensate for small pectoral fins. 



At larger sizes all species swim slower in 

 body lengths per second (Figure 8b), but owing 

 to the allometric growth of the gas bladder, 

 both large T. albacare^ and T. obesus can 

 swim at slower speeds even in absolute units 

 (centimeters per second). Their weights in 

 water increase more slowly than their lifting 

 areas after the fish attain approximately 50 

 cm FL. 



A word of warning about Figure 8 is that 

 the predicted speeds are extrapolated outside 



160 



a 120- 



a. 

 in 



Z 



3- 



O 

 UJ 

 UJ 



a. 

 in 



z 

 » 



in 



25 50 75 100 



FORK LENGTH (cm) 



125 



150 



Figure 8. — Estimated minimum swimming speeds of 

 seven scombroid fishes for maintaining hydrostatic equi- 

 librium in (a) cm/sec and (b) body lengths/sec. Estimates 

 from equation 1 and data in Table 4. See text for 

 explanation. 



of the range for which morphometric and body 

 density data were available. In addition, the 

 density data for Ac. solandri was based only 

 on animals 125 cm long. Likewise, the density 

 data on T. obesus was limited. 



OTHER FEATURES 

 ASSOCIATED WITH BASAL SPEED 



Scombroid fishes have many adaptations for 

 efficient continued movement through the 

 water. They have a high hemoglobin level 

 (Klawe, Barrett, and Klawe, 1963), a large 

 dark muscle (Kishinouye, 1923) for continuous 

 locomotion (Rayner and Keenan, 1967), and 

 a streamlined shape (Walters, 1962; Aleev, 

 1963; Alexander, 1967). The following analyses 

 were to determine whether these adaptations 

 were strongly associated with the basal or 

 typical swimming speed. If differences in 

 these characteristics varied closely with dif- 

 ferences in typical speed, then I would argue 

 that these adaptations were primarily selected 

 for continuous slow swimming rather than 

 maximum, burst speeds. Data on the burst 

 or maximum speeds of these species are limited 

 but do not indicate a strong association with 

 typical speed. The burst speeds of 6-21 //sec 

 for T. albacares and 12-19 //sec for Ac. solandri 

 measured by Walters and Fierstine (1964) 

 with a piscatometer at sea were approximately 

 the same even though typical speed observed 

 for Ac. solandri was less than one-fourth 

 those of T. albacares (Table 3). Maximum 

 speeds of four species measured photographi- 

 cally at sea or in shoreside tanks differ from 

 species to species but not in relation to the 

 typical speeds (Table 6). 



Table 6. — Comparison of typical and maximum speeds 

 observed from four scombroids. 



347 



