Discussion 



Our study suggests that congenital defects led to 

 the absence of a left pectoral fin, the formation of 

 a small right pectoral and left pelvic fins, and to the 

 impaction of two vertebrae A smaller caudal span 

 may also be a result of such defects. On the basis 

 of age studies (Uchiyama and Struhsaker 1981) we 

 estimate that this fish (36.5 L) was about 9 mo old 

 when captured. (But, because of the vertebral 

 damage, the fish is shorter than it should be and 9 

 mo is a conservative age estimate) Thus in spite of 

 significant locomotory handicaps, this fish had been 

 swimming and feeding effectively at the time it was 

 taken by hook and line. 



Morphological comparisons with SIO specimens 

 and with equation-derived values for similarly sized 

 yellowfin tuna did not indicate any major structural 

 differences in the one-finned fish that can be inter- 

 preted as having facilitated its swimming. However, 

 since the absence of one pectoral fin doubtlessly af- 

 fects the minimum speed required for hydrostatic 

 equilibrium, the horizontal stability, and the maneu- 

 verability of a tuna, it is instructive to consider how 

 the loss might have been compensated. Magnuson 

 (1973, 1978) has amply demonstrated the role of the 

 paired fins in providing lift and reducing minimum 

 equilibrium speed. Total lift (L t ) is calculated as 



L t (dynes) = M[l - - (g)\ 



(1) 



where M is fish wet weight, P e is seawater density, 

 Pf is fish density, and g is the acceleration of gravi- 

 ty (980 cm -sec -2 ). The amount of lift needed by the 

 one-finned fish (M = 861 g, P f = 1.08, P e = 1.02 at 

 25°C) is 47,203 dynes. 



The minimum speed for hydrostatic equilibrium 

 U 100 is determined by 



U 



100 



PJ2 (C L A p + C L A k _ 



% 



(2) 



where C L is the coefficient of lift for the pectoral 

 fins (p) and caudal keel (A;) and A p and A k are their 

 respective areas (Magnuson 1973). Pectoral fin lift 

 area includes both fins and the flat section of body 

 between them (Magnuson 1978, fig. 4). This can be 

 calculated from an allometric relationship (Mag- 

 nuson 1973, table 4). 



A v = 0.0609 L 187 , 



(3) 



and, for a 36.5 cm L yellowfin, A p = 50.8 cm 2 . With 

 this value, a measured keel area (Table 1) of 6.2 cm 2 , 

 and assuming a lift coefficient of 1.0 for both sur- 

 faces (Magnuson 1973, table 4) the calculated (Equa- 

 tion (2)) minimum speed for a 36.5 cm yellowfin tuna 

 is 40.3 cm-s -1 . The same calculation for the one- 

 finned fish (A p = 25.4 cm 2 ) yields a minimum speed 

 of 54.1 cm-s -1 , a 34.3% increase The one-finned 

 fish would need to swim faster, and thus expend 

 more energy. Its higher speed would also probably 

 have required it to make continuous velocity and 

 position changes in order to keep pace with a school 

 of, on average, similarly sized and thus slower swim- 

 ming yellowfin tuna. 



Alternatively the fish might have assumed a 

 pitched (i.e, head up) swimming mode in an attitude 

 such that its body surface would have contributed 

 to hydrodynamic lift by having a positive angle of 

 attack relative to the direction of motion, and the 

 C L of the caudal keel would be increased (Magnu- 

 son 1978). Of course this would result in increased 

 pressure drag and require more swimming power, 

 but it might have enabled the fish to swim more 

 slowly. 



Under any conditions, it seems likely that this fish 

 was not highly maneuverable and would have diffi- 

 culty remaining upright (i.e, not rolling to the left). 

 It, of course, could not use its left pectoral for 

 braking and left turns, and its left pelvic fin, which 

 would also contribute to these actions, was less ef- 

 fective than normal because of its small size Tunas 

 normally accelerate with their first dorsal, pectoral, 

 and pelvic fins appressed (Magnuson 1978), but as 

 this fish slowed and needed lift it would have likely 

 began to roll to its left as soon as its right pectoral 

 fin was extended. This could be countered somewhat 

 by its dorsal fin, but the necessity for unilateral use 

 of the right pectoral fin should have always resulted 

 in some amount of leftward roll and a tendency to 

 turn to the right. Both the sharpness of the turn and 

 the net upward or downward spiral movement of the 

 fish would depend upon the degree of fin extension 

 and swimming velocity. 



Finally, to compensate for the tendency to roll it 

 is possible that the fish habitually swam with its body 

 tilted as much as 80° to the right. In this position 

 it would retain the largest possible pectoral lift area 

 and might gain sufficient additional lift from the dor- 

 sal, second dorsal, anal fins and the body surface to 

 more than compensate for loss of keel lift. It is note- 

 worthy that the second dorsal and anal fin areas of 

 this fish are larger than predicted (see above). The 

 fish would be able to roll from its side to an upright 

 position merely by extending its pectoral fin a bit 



468 



