FISHERY BULLETIN: VOL. 71, NO. 2 



Sarda chiliensis, swam with their mouth closed 

 as much as 40% of the time. Presumably the 

 fish could have conserved energy by swimming 

 more slowly with the mouth open more often. 

 Minimum speed was also not regulated for 

 food search (Magnuson, 1969); kawakawa, 

 Euthynnus affinis, swam at the same speed 

 day and night even though they were exclu- 

 sively diurnal feeders. When deprived of food 

 for several days, they swam slower and slower 

 in absence of food stimuli. If typical speed 

 had been regulated by feeding motivation, the 

 fish would have been expected to swim faster — 

 become more active — as motivation to feed 

 increased. 



Typical swimming speeds of E. affinis were 

 well explained by the minimum speed required 

 to produce sufficient lift on the pectoral fins 

 to keep from sinking. Magnuson (1970) pre- 

 sented a model to predict minimum speed 

 required of E. affinis to maintain hydrostatic 

 equilibrium. Minimum speeds were estimated 

 from the animal's weight in water and the 

 lifting area, primarily of the pectoral fins. 



Differences between the morphology and 

 typical speeds of yellowfin tuna, Thminus alba- 

 cares, and E. affinis (Magnuson, 1966a) sug- 

 gested that the variations in area of pectoral 

 fins and volume of gas bladder might also 

 be explained as various solutions for countering 

 negative buoyancy. Many other specializations 

 of scombroids related to swimming speed and 

 activity may be adaptations to the speeds 

 required for hydrostatic equilibrium rather 

 than maximum, burst speeds (Magnuson, 1970). 



In regard to xiphoids, a recent review on 

 their functional morphology by Ovchinnikov 

 (1970) did not consider the gas bladder or 

 buoyancy mechanisms in discussions on the 

 function of pectoral fins. 



Purposes of the present paper are to (1) 

 test whether the model mentioned above gen- 

 erally predicts typical swimming speeds of 

 scombroid fishes, (2) consider the adaptive 

 radiation in the morphology especially of the 

 gas bladder and pectoral fins which together 

 with swimming speed contribute to the mecha- 

 nism by which scombroids maintain hydro- 

 static equilibrium, and (3) consider problems 

 associated with large body size and mainte- 



nance of hydrostatic equilibrium among scom- 

 broid and xiphoid fishes. 



SWIMMING SPEEDS 



Typical swimming speeds were observed for 

 five scombroid fishes in 7.2-m diameter swim- 

 ming pools at Kewalo Basin, Honolulu, Hawaii, 

 from underwater photographs of wahoo, Acan- 

 thocybium solandri, swimming at sea, and 

 from previously published records on Sa. 

 chiliensis (Magnuson and Prescott, 1966). Ob- 

 served swimming speeds were determined or 

 available for seven species of six different 

 genera. 



Methods for obtaining swimming speeds 

 were described by Magnuson (1969). Fishes 

 were observed for short periods at 1- to 4-hr 

 intervals (0000-2400) during their first month 

 in captivity. Observations were made for two 

 consecutive 24-hr periods during which the 

 fish were fed and not fed (Figure 1). On fed 

 days they were fed to satiation with thawed 

 smelt or shrimp once or twice usually at 0900 

 and 1600 hr. Data collected during the "day 

 not fed" and the "night after not fed" were 

 used for the estimates of the minimum typical 

 swimming speed to compare with body mor- 

 phology. These estimates were based on 19-212 

 min (median 68 min) of recorded speeds for 

 each species. With the exception of 36-cm long 

 bigeye tuna, Thioums obesus, where only 1 fish 

 was observed, measurements from 6 to 40 fish 

 (median 12 fish) made up each estimate. Water 

 temperatures were 23°-26°C and salinity 33o/oo. 



Typical speeds of Ac. solandri were from 

 17 cinema sequences averaging 2.59 sec each. 

 The films were taken from underwater viewing 

 ports in the RV Charles H. Gilbert and an 

 observation raft described in Nakamura (1972). 



I 



M 



I 



{night 



iAFTEF^ 

 NOT 



FED 



DAY FED 



NIGHT AFTER 

 FED 



t t 



FED FED 



DAY NOT FED 



NIGHT 



AFTER 



NOT 



24 



_L_ 



48 



_1_ 



72 



TIME IN HOURS 



Figure 1. — Observation schedule for measurements of 

 typical swimming speeds presented in Figure 2. 



338 



