AIR BLADDER AND SPECIFIC GRAVITY OF FISHES. 



123 



Calculating the specific gravities of a fish on the two figures assumed above (0.925 

 for fat, 1.076 for fat-free substance), we have the following specific gravities of fish 

 with increasing percentage of fat : 



Specific 



Per cent gravity 



of fat. of fish. 



O I . 0760 



2 1 . 0726 



4 1 . 0692 



6 1.0656 



8 1 . 062 1 



10 1.0587 



12 1-0553 



Per cent 

 of fat. 



14- 

 16. 

 18. 

 20. 



22. 



2934 



46.55- 



Specific 

 gravity 

 of fish. 

 0519 

 0486 



°453 

 0419 



0385 

 0260 

 0000 



Thus, a fish which increases in fat content diminishes in specific gravity. At 29.34 

 per cent fat the fish without air bladder would be in equilibrium with sea water and 

 at 46.55 per cent fat (if it were possible) would float in fresh water without air bladder. 



How does the air bladder react to those changing specific gravities? As the fish 

 grows fatter the air bladder must occupy less and less space, as shown in the following 

 table. This table begins with a 10 kg. fish without fat and shows how the air bladder 

 must change as the fish adds fat, so that each weight has the percentage of fat indicated, 

 the other constituents of the fish remaining constant. 



Fat in fish (per cent). 



Total 



weight 

 of fish. 



Specific 



gravity 



Of fish 



substance. 





Volume of 

 solid fish 



substance. 



<:) 



Displace- 

 ment nec- 

 essary to 

 float fish in 

 sea water, 

 specific 

 gravity= 

 1.026. 



V.I.026/ 



Air 



bladder 



volume 



necessary. 



c—d 



4... 



6.. 



8... 

 10... 

 12.. . 

 14... 

 16... 

 18... 

 20.. . 

 22.. . 

 2934 



G. 



10,000 

 10, 204 

 10,416 

 10, 638 

 10,869 

 11,111 

 11.363 

 11,628 

 11.904 

 12,19s 

 12,500 

 12,820 

 14.152 



1.0760 

 1.0726 

 1. 0692 

 1.0656 

 1. 0621 

 1. 0587 

 1.0553 

 1-0519 

 1.0486 

 1.0453 

 1. 0419 

 1. 0385 

 i. 0260 



Cc. 

 9.293 

 9.513 

 9.741 

 9-983 

 10.233 

 10,495 

 10, 767 

 11.054 

 n.351 



11,660 

 11,997 

 12.335 

 13.793 



Cc. 

 9.746 



9.945 

 10,152 

 10, 368 

 «0, 593 

 10,829 

 11.075 

 11.323 

 11,602 

 11,886 

 12,183 

 12,495 

 13.793 



Cc. 



453 

 432 

 411 

 38S 

 360 

 334 

 308 

 379 

 251 



220 

 l86 



160 



These figures demonstrate clearly that as the fish becomes fatter the • specific 

 gravity of the fish substance diminishes and the necessary air-bladder volume becomes 

 smaller and smaller. Thus, in a fish of 22 per cent fat (which is not uncommon in 

 herring and salmon) the fish would be in equilibrium with sea water (specific gravity = 

 1.026), with scarcely more than a third (160 cc.) as much air-bladder volume as would 

 be required for a fish free from fat. At 29.34 P er cent fat the fish would be in equilib- 

 rium with sea water without an air bladder. Thus, fat can take the place of the air 

 bladder and make the latter unnecessary. 



We saw above that a fish must find some means of increasing its displacement if 

 it is to migrate from salt water to fresh water and not sink. With the several incre- 



