(no gas bladder) than for Ac. solandri, which 

 has a large gas bladder. These differences would 

 also significantly influence the speed required 

 to maintain hydrostatic equilibrium, and on 

 this basis species without gas bladders are 

 expected typically to swim faster than those 

 with gas bladders. 



Variations in the Lifting Area 

 of Pectoral Fins 



The lift produced by a hydrofoil is directly 

 proportional to its lifting area, thus those 

 species with a larger lifting surface should 

 be capable of swimming at a slower speed, all 

 other things being equal. The primary lifting 

 surfaces of E. affinis are the pectoral fins 

 (Magnuson, 1970), although other surfaces 

 such as the caudal keel may counter some 

 of the weight. The lifting area of the pectoral 

 fins is the area of the extended pectoral fins 

 as viewed from above plus the area of the body 

 between the fins. It was determined for the 

 various species from photographs of dead fish 

 as described by Magnuson (1970). 



Pectoral fins of scombroids are analogous 

 to variable sweepback wings; they can be fully 

 extended, swept back, or laid against the body 



FISHERY BULLETIN: VOL. 71, NO. 2 



(Magnuson, 1970). Species such as E. affinis 

 and K. pelamis swam with the fins swept 

 back about 40°, but the two species with the 

 largest pectoral fins, T. albacares and T. obesus, 

 swam with the fins swept back about 60° (Table 

 2). Sweepback angles measured from dead fish 

 used for morphometric measurements were 

 not as great as those from fish swimming 

 in tanks. Measurements from dead fish were 

 7° and 14° for E. affinis and K. pelamis and 

 29° and 36° for T. albacares and T. obesus. Thus, 

 some bias occurs in the estimation of the lifting 

 area for swimming fish from the dead fish. 

 This bias is probably slight because, although the 

 actual area of the pectoral fin decreases rapidly 

 as the fins are swept back, the total lifting 

 area does not (Magnuson, 1970). Measurements 

 of sweepback from other dead fishes used for 

 morphometric measurements averaged 10° for 

 Sc. japonicus, 24° for Ac. sola)idn, 16° for 

 Sa. chiliensis, and 10° for Au. rochei. 



Differences in pectoral fin lifting areas 

 among scombroids are illustrated in diagram- 

 matic dorsal views of seven species at the 

 same length (Figure 5). Lifting areas differ 

 markedly among species and among sizes for 

 the same species (Figure 6). T. obesus has, 

 among the species here, the largest lifting 



Table 2. — Average sweepback of pectoral fins for swimming captive fishes. 

 n equaled the number offish. A was averaged for left and right fins. 



Fed and not fed 

 combined 



Day fed 



Species 



A (degrees) 



Day not fed' 

 A (degrees) 



A (degrees) 



' Two days after fed for E. affinis and T. albacares; 1 day after fed for K. pelamis 

 and T. obesus. 



r/Hjtmus obeaus 



Thunnus albacares 



Eyfhyofius affinis 



Katsuwonus pelamis 



Sarda chiliensis 



Figure 5. — Diagrammatic dorsal views 

 of five scombroid species to dramatize 

 the variation in the lifting areas of 

 the pectoral fins for various species. 

 Drawn from photographs of dead fish 

 ca. 50 cm in fork, length. 



342 



