

122 BULLETIN OF THE BUREAU OF FISHERIES. 



This table shows that if fish live in and are adjusted to fresh water and if they 

 travel seaward it will be necessary for the air bladder to become smaller. In those, 

 fishes in which the air bladder is closed (as in all the Acanthopteri, or spiny-rayed species, 

 which are typically marine) the volume may presumably be reduced by resorption of 

 some gas in the blood and the discharge of it into the sea, or the gas volume may be 

 compressed, in which case the pressures developed in the air bladder corresponding to 

 the various salinities are shown in the table in terms of millimeters of mercury, the 

 assumption being made that when the fish is adjusted in fresh water its air-bladder, 

 pressure is i atmosphere (760 mm.). In nearly all the teleost fishes, except the spiny- 

 rayed fishes, the air bladder is provided with a pneumatic duct connecting either with' 

 the alimentary canal or with the exterior. Presumably, excess of gas may be expelled 

 through this duct. Some of our most important species, such as salmon, shad, and 

 herring, have this duct. 



If the fish lives in and is adjusted to sea water and travels in the direction of a t 

 diminishing salinity gradient, the conditions are entirely different, for in this case a 

 migration toward fresh water will demand an enlarging air bladder. If, however, the 

 air bladder is at 1 atmosphere when the migration begins, then the pressure must become 

 less than 1 atmosphere, or a partial vacuum must be established in the air bladder, 

 which seems quite improbable. As an alternative to this we may suppose the gas to 

 be absorbed into the blood from the surrounding sea water and discharged into the air 

 bladder. Apart from the physicochemical and physiological difficulties involved in this 

 gas transference against pressure, it is obvious that the mere pumping of gas into the air 

 bladder will be without influence on the specific gravity of the fish if it merely develops 

 pressure and will be effective only in so far as it actually expands or stretches the fish to 

 a larger size. This method of reducing specific gravity appears quite as improbable as 

 the method that would involve a partial vacuum. Unless some other means is found 

 the fish will have to maintain itself afloat by constant muscular effort if it goes to water 

 of a lower salinity. 



The specific gravity of a fish varies with the amount of fat present in the tissue. 

 In fact, Bull (1896, 1897) investigated the possibility of determining the fatness of fish 

 quickly and simply by determining the specific gravity of the fish, and his results, while 

 not altogether satisfactory, are promising. The foundation of this work is, of course, the 

 fact that the specific gravity of fish fat or oil is less than 1 (usually about 0.925), while 

 that of fat-free substance is greater than 1 (about 1.076), that is, fats float on water, 

 while fat-free fish substance sinks. When amounts by weight (W t and W 2 ) of two sub- 

 stances of different specific gravities (5, and S 2 ) are combined, the resultant specific 

 gravity of the whole (Si+ 2 ) is given by the formula' : 



 5 '+ 2 W l+ ]v, ir,.s',+H',,s 1 ' ( 



On a percentage basis (where \V i + \V 2 = 100) : 



100 S^g 

 :il+3 ~W 1 S i +W 3 S 1 ' 



1 The formula for this relation given by Bull (1897, p. 641), ~ . is in error. In this formula F and/, Tand/, V-'andl 



are, respectively, the weight and specific gravities of fat, dry susbtance, and water. 



