FISHERY BULLETIN: VOL. 71, NO. 2 



X 



in 



< 



40 



35 



30 



25 



20 



10 



Kotsuwonus pelamis 

 Thunnus obesus 

 Euthymus off mis 

 Thunnus albacares 

 Scomber japonicus 

 Sardo chiliensis 

 Aconthocybium solandri 



25 



50 75 100 



FORK LENGTH (cm) 



Figure 3. — Comparison of mass against length rela- 

 tionships among seven scombroid species. Information 

 on regression equations in Table 4. 



acceleration of gravity, g, is equal to approxi- 

 mately 980 cm/sec'^. 



The influence of a gas bladder can be clearly 

 seen by contrasting data for K. pelaniis with 

 T. albacares. Gas bladder volumes were deter- 

 mined with an x-ray method of Chang and 

 Magnuson (1968) on fish immediately after 

 they were killed. K. pelamis has no gas bladder 

 but T. albacares has a gas bladder which grows 

 allometrically, becoming progressively larger 

 as the fish increases in mass (Figure 4a). 

 T. albacares, 2 kg or less, has no gas in the 

 bladder, but the gas bladder then grows rapidly 

 until the fish is at least 8 kg. These dif- 

 ferences in gas bladders of K. pelamis and 

 T. albacares influence the density of the two 

 species (Figure 4b). The density of K. pelamis 

 remains approximately the same over the 

 entire size range. However, once the gas bladder 

 begins to grow in T. albacares the density of 

 the fish becomes less, decreasing from approxi- 

 mately 1.09 g/ml for 2-kg fish to 1.05 g/ml 

 for 10-kg fish. These differences in density 

 have a corresponding influence on the lift 



required to keep the fish from sinking (Figure 

 4c). The lift required for K. pelamis increases 

 in almost direct proportion to the increase 

 of mass. But the lift required to keep T. 

 albacares from sinking increases more slowly 

 as the density of the fish becomes less. In fact, 

 the lift required to keep T. albacares from sinking 

 appears to approach asymptotically a maxi- 

 mum near 200 x 10^ dynes. 



Variations in the densities of various species 

 and sizes can be examined in Table 1, where 

 the mean densities of various species are 

 given and the multipliers are computed to 

 convert the mass of the fish to the lift re- 

 quired to keep them from sinking. All species 

 with markedly lower densities, such as Sc. 

 japoyiicus, T. albacares, T. obesus, and Ac. 

 solandri, have gas bladders at some point in 

 their life history. The more dense species, 

 Sa. chilioisis, Au. rochei, E. affinis, and K. 

 pelamis, have no gas bladder. 



Lift to prevent the fish from sinking is 

 10 to 20 times greater for larger K. pelamis 



Kotsuwonus pelamis 



It 



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X 



4 

 3 

 2 



I 





 I 12 



I 10 



loe 



1.06 

 1.04 

 1.02 



'«t^ 



_l \ I 1 L. 



2 4 6 8 10 12 2 4 6 8 10 12 14 



MASS OF FISH (kg) 



Figure 4. — Comparisons between Katsuwonus pelamis 

 and Thunnus albacares of the relations between fish 

 mass and (a) gas bladder volume, (b) fish density, and 

 (c) lift required for hydrostatic equilibrium. 



340 



