Specific 

 Percent gravity 



of fat. of water. 



14 1 . 0097 



16 1 . 0084 



18 1 .0062 



20 I. 0040 



22 1 .0025 



25.3 I. 00OO 



124 BULLETIN OF THE BUREAU OF FISHERIES. 



ments of fat the fish would be in equilibrium with water of the specific gravities shown 

 below without any change of air bladder volume of 453 cc. per 10 kg. body weight. 



Specific 



Per cent gravity 



of fat. of water. 



o 1.0260 



2 1 . 0239 



4 1 . 02 1 7 



6 1-0193 



8. . : 1.0171 



10 1. 01 58 



12 1. 01 27 



It appears from these figures that a fish living at sea and accumulating fat would 

 find itself more at home physically in fresher water without the necessity of enlarging 

 the air bladder. May this not be the influence which directs salmon and shad from 

 salt water to the mouths of rivers? Indeed, it seems unavoidable to conclude that such 

 difficulties in navigation as are introduced by 20 per cent and more of fat must have 

 a profound influence on the movements of the fish. The changes that take place in 

 composition of salmon have been extensively studied. The fluctuations of body com- 

 position of the salmon at different stages of the life cycle were studied by Miescher- 

 Ruesch (1880), Paton (1898), Greene (1914, 1919), and those of the herring by Hjort 

 (1914). 



Briefly, the career of the Atlantic salmon at sea is as follows: Two or two and 

 one-half years getting its growth, developing muscle and bony tissue (a period obvi- 

 ously of high body specific gravity), then in the third year the accumulation of much 

 fat, whereupon the fish moves to fresh water in the fourth year, when the spawning 

 migration is performed. The large accumulation of fat is consumed in the develop- 

 ment of the reproductive organs and in supporting the fish during the journey in fresh 

 water when no food is taken. We do not know so much of the shad as we do of the 

 salmon, but what information we have agrees in a general way with the above. 



Apparently, therefore, the fish can not well go into fresh water before a sufficient 

 quantity of fat has been accumulated, because of difficulties in keeping afloat. After 

 the fish has accumulated the fat there would seem to be a strong influence directing it 

 to fresh water. 



There is another possible means of overcoming the excessive buoyancy of fat in sea 

 water. That is, the fish may descend until the pressure of the water, by reducing the 

 volume of the air bladder, reduces the displacement of the fish to the necessary extent. 

 This reduction of displacement, with increase in specific gravity, must always occur, in 

 any event, when a fish containing an unprotected air bladder descends. The deeper the 

 fish goes into the water the more easily it descends, the excess weight of the fish becoming 

 greater and greater. If the fish begins to rise, the excess weight over displacement, 

 which must be overcome by muscular exertion, becomes less. The effect to be realized 

 from this cause depends, however, on the volume of air bladder present when the fish is 

 at the surface, for, obviously, when, say, 500 cc. of gas is compressed to 250 cc, a greater 

 difference in specific gravity will result than when 100 cc. of gas is compressed to 50 cc, 

 though the same amount of pressure would be required in either case. Therefore, in a 

 fish whose air bladder had been reduced (if such reduction really occurs) in response to 



