MAGNUSON: ADAPTATIONS OF SCOMBROIDS AND XIPHOIDS 



area at a given length, and Ac. sola)idri has 

 the smallest. For example, at a fork length 

 of 125 cm, T. ohesus has a lifting area greater 

 than 500 cm^, whereas Ac. solandri has a 

 lifting area near 150 cm- (Figure 6). These 

 differences should have a marked effect on 

 the minimum speed required to maintain 

 hydrostatic equilibrium. Those with larger pec- 

 torals such as T. albacares and T. obesus should 

 have a slower minimum speed than those 

 with smaller pectorals. 



Predicted Swimming Speed 



The model used by Magnuson (1970) to pre- 

 dict the minimum speed required for hydro- 

 static equilibrium was 



L 



V- 



100 



E 



't 



p/2 (C^Af^ 



+ C, 



,M 



V2 



(0 



All terms used here and elsewhere in this 

 paper are listed below: 



Lt = 

 Lf = 



^ft = 



^100 = 



A = 



De = 



Df = 



Mf - 



fork length (cm) 



total weight of fish in water (dynes) 



all lift produced by the total lifting area 

 of the pectoral fins (dynes) 



lift produced by keel (dynes) 



area of pectoral fins 



total lifting area of pectorals (cm^) 



lifting area of keel (cm-) 



coefficient of lift for the pectoral fins 

 based on all lift produced by pec- 

 torals and on total lifting area of 

 pectorals 



coefficient of lift for the keel 



speed of fish with pectorals con- 

 tinuously extended (cm/sec) 



sweepback angle of pectoral fins 

 (degrees) 



p = density of water == 1.022 g/ml 

 in Kewalo tanks, 1.025 g/ml at sea 



density offish (g/ml) 



fish mass. 



To test predictive value of this model for a 

 number of species, computed speeds were com- 

 pared with observed speeds. Typical swim- 

 ming speeds were computed twice: first, as- 

 suming the keel provided significant lift, and 



800 



7D0 



600 



CO 



2 500 

 u. 



-I 

 < 



o 



t- 



y 400 



0. 



li. 

 o 



< 



5 300 



Thunnis obesus 

 Thunnus a/bocares 

 Euthymus of finis 

 Kotsuwonos pelamis 

 Scomber /oponicus 

 Auxis rochei 

 Sarda cMiensis 



8 Acanthocybium solandri 



200 



100 



50 75 100 



FORK LENGTH (cm) 



Figure 6. — Comparison of lifting area of pectoral fins 

 against length relationships among eight scombroid 

 species. Information on regression equation in Table 4. 



second, assuming all the lift was provided by 

 pectoral fins. Results from these computations 

 and the data used to make them are presented 

 in Table 3. Regressions in Table 4 were used 

 to determine the mass of the fish and the 

 area of the lifting surfaces for calculations 

 of minimum hydrostatic speeds. The mass of 

 the fish was converted to required lift by 

 using the conversion factors from Table 1. 

 Lift coefficients were from Table 7 of Magnuson 

 (1970) — those determined for E. affinis on the 

 basis of total lifting area of the pectorals. 



Several comments should be made about 

 the use of the lift coefficients from E. affinis 

 for all species considered in the present text. 

 The lift coefficient calculated on the basis 

 of the total lifting area of pectorals repre- 

 sents lift on the pectorals, interference lift 

 on the body owing to the pectorals, and the 

 interference lift on the pectorals owing to the 

 presence of the body. At best the coefficient 

 is only a convenient but arbitrary standard 

 frequently used in aerodynamic literature. 



343 



