DOTSON: SWIMMING SPEED OF ALBACORE 



the gut cavity. All developed gas bladders ap- 

 peared full with two exceptions, and these may 

 have been damaged during capture or dissection. 



Seven albacore caught in September with fork 

 lengths of 63 to 68 cm were examined to determine 

 the effect of the gas bladder on density. Gas was 

 removed from the bladder by a cannula (inside 

 diameter = 1 mm) which was inserted through the 

 ventral surface of the body while the fish was 

 submerged in seawater, and the fish was then 

 weighed while still submerged. The mean density 

 increase with gas extraction was 0.007 g/ml (Table 

 1). Although this is probably a conservative es- 

 timate, the difference in density calculated before 

 and after gas removal is used as the effect of the 

 gas bladder on fish density. In an albacore less 

 than 56 cm FL, the small gas bladder is not 

 expected to affect density whereas the large and 

 fully developed gas bladder of albacore greater 

 than 80 cm FL should reduce density to a greater 

 extent than was measured on the smaller fish 

 above. 



Densities of group 2 fish were considerably 

 higher than those of similar size fish in groups 1 

 and 3 (Figure 1). Seasonal variations in density 

 due to changes in fat content have been described 

 for other pelagic species by Aleev (1963). Mass 

 estimates were calculated from the length for each 

 fish in all three groups using Equation (1), and 

 compared with the observed values. The mean of 

 the observed values for group 2 fell 403 g below the 

 estimate from the regression line, ranging from 

 172 g greater to 999 g less. Because fish in group 2 

 had apparently just migrated into the area of 

 capture, presumably from the central or western 

 Pacific, the loss in mass was assumed to have been 

 caused by the utilization of fat during migration. 

 Group 1 would not yet have utilized this fat, and 

 group 3 is assumed to have added fat by feeding in 

 the rich coastal waters. 



The densities in group 2 were recomputed on the 

 assumption that the mass difference between the 

 individuals and the regression curve is attributed 

 to fat loss. An equation was developed by Magnu- 

 son (1970) relating the density {Df) of a scombrid 

 without a gas bladder to the percentage (P) of the 

 total body weight that is fat. The equation 



Df = 1.100- 0.0017 P 



(3) 



was used to recompute densities for the fish in 

 group 2. The effect of the gas bladder on density 

 was assumed to be 0.007 g/ml because fish in group 



2 were in the same size range as the above fish for 

 which gas bladder measurements were taken. This 

 value was added to the observed density and the 

 percentage body weight in fat calculated. The 

 difference in mass (assumed to be fat loss) of each 

 individual was then added and new densities 

 determined with the increased percentage of body 

 fat. The density effect of the bladder was sub- 

 tracted from this value to yield a density adjusted 

 for fat loss. When determining fat content in the 

 fish, the density effect of the bladder was taken 

 into account, except for those fish with measured 

 densities greater than 1.100 g/ml, which is the 

 level Magnuson (1970) chose as the density for a 

 scombrid without a gas bladder. Fish with densi- 

 ties greater than 1.100 were assumed to have 

 empty or damaged gas bladders, and the density 

 difference due to the gas bladder was subtracted 

 from the recomputed density. 



Recomputed densities of group 2 are plotted in 

 Figure 2 with the measured densities of groups 1 

 and 3. The close fit of the recomputed densities 

 appears to support the assumption that fat con- 

 tent and gas bladder volume can account for the 

 disparity in densities observed for group 2 in the 

 original data. Density values are, therefore, ex- 

 pected to vary considerably depending on the 

 development and condition of the gas bladder and 

 the fat content of the fish when it is caught. 



DETERMINATION OF 

 MINIMUM SPEED 



To estimate the minimum speed for hydrostatic 

 equilibrium, it is necessary to calculate the amount 

 of lift a fish must produce. The lift (Zy) required by 

 a scombrid to attain hydrostatic equilibrium, 

 expressed in dynes, is determined from the rela- 

 tion (Magnuson 1970) 



Lf = M 



^l-^)980cm/s2 . 



(4) 



When the lift is assumed to be provided solely by 

 the pectoral fins, and the coefl^cient of lift for the 

 pectorals is assumed to be 1.0, then the equation 

 for minimum swimming speed becomes 

 (Magnuson 1970) 



V = 



t 



i^i 



/2{A)j 



(5) 



Calculations of minimum swimming speed from 



957 



