MAGNUSON: ADAPTATIONS OF SCOMBROIDS AND XIPHOIDS 



The observations that scombroid fishes swim 

 continuously (Figure 2) would appear to be 

 generalizable but the speeds, expressed either 

 as body lengths per second (//sec) or centimeters 

 per second (cm/sec), differ considerably from 

 one species to another or within species de- 

 pending upon the size. Typical speeds ranged 

 from 0.33 //sec for Ac. soland7i to 2.19 //sec 

 for the bullet mackerel, Auxis rochei. All species 

 other than Ac. solandri, namely the skipjack 

 tuna, Katsuivonus pelamis, Sa. chUiensis, 

 E. affi)iis, T. ohesus, T. albacares, and Au. rochei, 

 had typical speeds greater than 1 //sec. None 

 of the species ever stopped swimming. The 



3.0 



2.5 



2.0- 



:i 1.5 



o 



UJ 



a. 



1.0 



0.5- 



• DAY FED 



o DAY NOT FED 



* 1ST NIGHT AFTER FED 

 t 20 NIGHT AFTER FED 

 - MEAN 



X SLOWEST OBSERVED 



I 



I 



I 



I 



I 



<^ 



I 



I 



I 



3 



a 

 < 



■6 



.1 



I 



SPECIES AND FORK LENGTH (cm) 



Figure 2. — Typical swimming speeds of seven scombroids. 



slowest speeds observed ranged from 0.12 //sec 

 for Ac. solandri to 1.97 //sec for E. affinis. 

 The typical speed of T. ohesus, 36 cm FL (fork 

 length), was 1.31 //sec but the larger 55-cm 

 T. obesus swam only 1.08 //sec. Three length 

 groups of the K. pelamis were also observed 

 and their speeds differed among sizes. 



SPEEDS REQUIRED 

 FOR HYDROSTATIC EQUILIBRIUM 



Variations in the typical swimming speeds 

 among scombroids would result from differences 

 in both the weight of the fish in water and 

 the lifting area of the pectoral fins. Weight 

 in water depends on mass and density of the 

 fish. All specimens of chub mackerel. Scomber 

 japo)iicus, and Saj'da chiliotsis used for mor- 

 phometric and mass measurements were col- 

 lected near Los Angeles, Calif. Specimens of 

 all other species were collected in the central 

 Pacific almost always near the Hawaiian 

 Islands. 



Variations in Weight 



Not all scombroids have the same mass- 

 length relationship (Figure 3). K. pelamis 

 was heaviest and Ac. solandri lightest at a 

 given length. For example, at a length of 

 75 cm, Ac. solandri has a mass less than 2 kg 

 and K. pelamis greater than 9 kg. Other 

 species were intermediate. 



When submerged in water the fish displaces 

 water which also has mass. If the gas bladder 

 is large enough, the volume of water that 

 a fish displaces may have the same mass as 

 the fish. The weight of the fish in water, or 

 conversely the lift required to keep the fish 

 from sinking, is equal to the excess mass of 

 the fish times the acceleration of gravity 

 (Alexander, 1967; Magnuson. 1970). The 

 excess mass of the fish is the difference be- 

 tween the mass of the fish and the mass of 

 the water it displaces. It can be computed by 



Dp 



multiplying the mass of the fish by (1 --p^) 



Df 



where Z)g is the density of the environment or 



seawater and Df is the density of the fish. The 



339 



