MINIMUM SWIMMING SPEED OF ALBACORE, THUNNUS ALALUNGA 



Ronald C. Dotson' 



ABSTRACT 



Measurements of density and pectoral lifting area of albacore, Thunnus alalunga, were made and 

 compared with those previously described for yellowfin tuna, Thunnus albacares; bigeye tuna, 

 Thunnus obesus; and skipjack tuna, Katsuwonus pelamis. Albacore have densities within the range of 

 yellowfin tuna of similar size. The pectoral lifting area of albacore was always greater than skipjack 

 tuna but similar to yellowfin tuna and bigeye tuna for fish less than 70 cm long. Larger albacore had 

 increasingly larger fins than did the other species. 



Minimum speed necessary for hydrostatic equilibrium of albacore was calculated and compared at 50 

 and 80 cm fork lengths to values calculated for the species above. Albacore minimum speeds were slower 

 than those for skipjack tuna, similar to those of yellowfin tuna, and greater than those of bigeye tuna. 

 Density variations of albacore, attributed to fat content and gas bladder volume, significantly affected 

 estimates of minimum speed. Calculated speeds were slower than those estimated for albacore tracked 

 at sea or estimated from tag returns. 



Albacore tuna, Thunnus alalunga (Bonnaterre), 

 being negatively buoyant in seawater, must swim 

 continuously to maintain their position in the 

 water column. The albacore's long pectoral fins 

 help to compensate for their negative buoyancy by 

 providing lift, thus lowering the swimming speed 

 necessary to maintain hydrostatic equilibrium. 



A model developed by Magnuson (1970) proposes 

 that the minimum swimming speed of a scombrid 

 fish is set by the necessity to maintain hydrostatic 

 equilibrium rather than to provide adequate gill 

 ventilation. When the lift provided by the pectoral 

 fins necessary to compensate for the weight of the 

 fish in water is estimated, the corresponding 

 swimming speed can be considered the minimum 

 necessary for the maintenance of hydrostatic 

 equilibrium. This model was used by Magnuson 

 (1973) to compare minimum speeds of several 

 species of scombrid fishes that diff'ered in pectoral 

 lifting area, body shape, body density, and the 

 presence or absence of a gas bladder. 



The purpose of this paper is to 1) estimate the 

 minimum swimming speed of albacore; 2) compare 

 the minimum swimming speed of albacore with 

 those for other scombrids; and 3) compare cal- 

 culated minimum swimming speeds of albacore 

 with swimming speeds estimated from sonic 

 tracking of albacore at sea and from long distance 

 tag returns. 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, La Jolla, CA 92038. 



MATERIALS AND METHODS 



To compute the minimum swimming speed with 

 Magnuson's (1970) model, it is necessary to deter- 

 mine the mass of the fish, the lifting area, the 

 density of the seawater, and the density of the 

 fish. As the peduncle keels probably provide neg- 

 ligible lift (Magnuson 1973), they are excluded in 

 the computation of minimum speeds. 



To determine the mass of albacore, 477 

 specimens caught between long. 130° and 140°W 

 and lat. 30° and 40°N during June 1974 were 

 weighed to the nearest gram on a magnetically 

 dampened pan balance and their fork lengths 

 recorded to the nearest millimeter. Specimens 

 were weighed and measured within 15 min after 

 capture. 



A regression In M = In a + 6(ln L), where M is 

 mass in grams and L is fork length in millimeters, 

 was fitted to the length-mass data. The resultant 

 equation was 



M = 4.514 X lO-^L^ ^^-^^ 



(1) 



Manuscript accepted May 1976. 



FISHERY BULLETIN: VOL. 74, NO. 4, 1976. 



with 95% confidence limits on the exponent from 

 2.8245 to 2.9246. 



The total pectoral lifting area (A) is equal to the 

 projected surface area of the pectoral fins plus the 

 projected body area between them, due to their 

 analogy to wings in which the pressure distribu- 

 tion set up by the wings extends across the fu- 

 selage (Magnuson 1970). The pectoral lifting area 

 was determined by tracing the outline of the 



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