FISHERY BULLETIN: VOL. 74, NO. 4 



Table 1. -Density changes in albacore following gas bladder deflation and resultant eflFect on estimation of minimum speed. 



Albacore no. 



Characteristic 



Fork length (mm) 



Density with gas bladder (g/ml) 



Density without gas bladder (g/ml) 



Change in density due to gas bladder (g/ml) 



Minimum speed with gas bladder (cm/s) 



Minimum speed without gas bladder (cm/s) 



Change in minimum speed due to gas bladder (cm/s) 



Figure 2.-Computed densities for 

 three groups of albacore with group 2 

 densities (crosses) recomputed after 

 correction for fat loss. Recomputation 

 of group 2 densities is explained in the 

 text. 



I 



>- 

 H 

 if) 

 Z 

 UJ 



a 



I.IIO- 



i.ioo - 



1.090 



1080 



1.070 



1.0601- 



1.050 



45 



50 



this equation assume 100'^ extension of the pecto- 

 ral fins. 



The mass of the fish [M) and the lifting area (,4) 

 can be calculated using Equations (1) and (2), 

 respectively. The density of the environment {D^) 

 is 1.025 g/'ml. If we use M = 2,540 g, D, = 1.082 

 g/ml, A = 77.4 cm- for a 50-cm fish, the calculated 

 minimum speed T' is 54 cm/s. 



Density variations due to fat content and gas 

 bladder volume can affect the minimum swimming 

 speed necessary to maintain hydrostatic equilib- 

 rium. For a 65-cm albacore, a loss of 10*^ of its body 

 weight in fat would result in a 10*^ increase in 

 minimum speed. Loss or emptying of the gas 

 bladder results in an 8% increase in minimum 

 speed. Minimum speeds calculated from data on 

 fish with full gas bladders and in good condition 

 are therefore considered to be the minimum ob- 

 tainable while retaining hydrostatic equilibrium. 



COMPARISON OF MINIMUM 

 SPEEDS OF FOUR SCOMBRIDS 



Minimum speeds were calculated for albacore; 

 yellowfin tuna, Thunnus albacares;higeye iunsL, T. 

 obesus; and skipjack tuna, Katsunvnus pelamis, at 

 fork lengths of 50 and 80 cm. The speeds are given 

 in Table 2 with the density, mass, and pectoral 

 lifting area used in the computations. 



The minimum swimming speed of albacore 



55 60 65 70 75 



FORK LENGTH (cm) 



80 



85 



T.\BLE 2.-Estimated minimum speeds of four species of scom- 

 brids at fork lengths of 50 and 80 cm. The mass of the fish (M), 

 pectoral lifting area (A), and density of the fish (D) used in the 

 computations are also given. 



Data from present paper. 

 ^M, A, and D, from Magnuson (1973). 

 ^Extrapolated value. 

 *M calculated from Chatwin (1959), 

 (1973). 



A and 0, from Magnuson 



decreases from 57 cm/s when they are 50 cm FL to 

 45 cm/s at 80 cm FL. The decrease is a direct result 

 of the allometric growth of the pectoral fins 

 (Yoshida 1968) and the gas bladder (Gibbs and 

 Collette 1966). The gas bladder of albacore does 

 not have significant development when the fish is 

 less than 55 cm long, but has considerable volume 

 at a fish length of 65 cm, and apparent complete 

 development when the fish has reached 80 cm FL 

 (data, this paper). Combined with the increasing 

 length of the pectoral fins, the result is a relatively 



958 



