COMPARATIVE STUDY OF ADAPTATIONS FOR 

 CONTINUOUS SWIMMING AND HYDROSTATIC EQUILIB 



SCOMBROID AND XIPHOID FISHES 



John J. Magnuson'- - 



ABSTRACT 



Scombroid fishes swim continuously with pectoral fins extended as lifting hydrofoils 

 to counter their weight in water. Their gas bladder is often reduced or absent. Typical 

 speeds were observed from seven species and compared with the speeds expected from 

 them to maintain hydrostatic equilibrium. The expected speeds were computed from 

 a model I developed using data on the lifting area of extended pectoral fins and the 

 weight of the fish in water. 



Lifting areas of the pectoral fins were determined from photographs of dead fishes. 

 Mass-length relations were determined and converted to weight in water from data on 

 water density and fish density. 



Typical observed speeds varied among species from 0.33 to 2.19 lengths/sec. Scom- 

 broids of the same fork length differed greatly in mass, density, and pectoral fin areas. 

 Swimming speed required for maintaining hydrostatic equilibrium should be greater 

 for fishes with greater mass, higher density, and smaller pectorals. Expected speeds 

 were computed. They were highly correlated with observed speeds, ;• = +0.89. Thus, the 

 model accurately predicted typical swimming speeds of scombroids, which suggests 

 that variations in mass, density, and the lifting area of the pectoral fins, through the 

 mechanism of maintaining hydrostatic equilibrium, can account for about SO'yf of the 

 variation in typical swimming speeds. Expected speeds (centimeters per second) gen- 

 erated from the model either increased with fork length or reached a maximum and 

 declined for larger fish. The latter pattern occurred if gas bladders were relatively 

 larger in older fish or even absent in younger fish. 



Scombroids with faster typical speeds have larger dark muscles and higher con- 

 centrations of blood hemoglobin than those with slower speeds. Differences in body 

 shape were not closely related to differences in typical speeds. 



Larger fishes should have more difficulty maintaining a speed which is sufficient 

 for hydrostatic equilibrium without a gas bladder or large pectoral fins. The occurrence 

 of gas bladders and long pectoral fins were compared for small, medium, and large 

 scombroid and xiphoid fishes. Both were more common among larger species. Each 

 species with a maximum mass ^ 70 kg had a gas bladder while only 26"?^ of those 

 species ^ 6 kg had a gas bladder. Further analyses suggested that a gas bladder and 

 large pectorals may be increasingly more important for large species if they are to 

 swim at energetically prudent speeds. 



Scombroid fishes are negatively buoyant, pela- 

 gic, oceanic fishes. The scombroids include 

 the mackerels, tunas, and bonitos. They swim 

 continuously with pectoral fins extended. This 

 behavior produces lift that balances their 

 weight in water. Some attain large size. Xi- 



' Laboratory of Limnology, Department of Zoology, 

 University of Wisconsin, Madison, WI 53706. 



- Visiting Scientist, Southwest Fisheries Center, Na- 

 tional Marine Fisheries Service. NOAA, Honolulu, HI 

 96812. 



phoids are the largest teleosts and powerful 

 swimmers that share the pelagic environment 

 with the scombroids. 



General considerations of buoyancy mecha- 

 nisms of negatively buoyant fishes are dis- 

 cussed by Aleev (1963) and Alexander (1967. 

 1968). 



In regard to scombroids. Magnuson and 

 Prescott (1966) observed that continuous swim- 

 ming was apparently not entirely determined 

 by a need for gill ventilation, as Pacific bonito. 



Manuscript accepted November 1972. 

 FISHERY BULLETIN; VOL. 71. NO. 2. 1973. 



337 



