126 BULLETIN OF THE BUREAU OF FISHERIES. 



2. As a fish puts on fat its body specific gravity diminishes, pari passu, and in pro- 

 portion to the amount of fat present (a) its navigation in salt water is more difficult; 

 (b) fresher water is better suited as a physical medium. Until a certain amount of fat 

 is accumulated migration from salt to fresh water must be difficult or impossible. 



3. Reduction of volume of air bladder may possibly be effected by (a) resorption 

 of gas from bladder to blood and expulsion through gills; (b) direct expulsion of gas 

 through pneumatic duct (except in Acanthopteri) ; (c) diving, whereby hydrostatic 

 pressure reduces the volume. The effect produced by diving a given depth is propor- 

 tional to the absolute volume of the air bladder. 



4. Diminishing specific gravity consequent upon increasing fatness probably con- 

 stitutes a strong directive influence governing the movements of fishes, both marine 

 and anadromous. 



LITERATURE CITED. 

 Bull, Henrik. 



1896. Enkel Methode til Bestemmelse af den ferske Silds Fedtholdighed. In Aarsberetning fra 



Fors0gsstationen og Laereanstalten for Tilvirkning af Fiskeriprodukter i Bergen for Aaret, 

 1895-1896. Norsk Fiskeritidende, isde Aargang, 1896^.549-557. Bergen. 



1897. Om Egenvaegt og Fedt i Sild, Torskelever, m. m. Ibid., i6de Aargang, 1897, p. 294-300. 

 Goodrich, E. S. 



1909. A treatise on zoology, edited by Ray Lankester, Part IX, Vertebrata Craniata (First fascicle: 

 Cyclostomes and Fishes) (p. 223 and 359-362). 518 p. Adam and Charles Black, London. 

 Greene, Charles W. 



1914. The storage of fat in the muscular tissue of the king salmon and its resorption during the 

 fast of the spawning migration. Bulletin, U. S. Bureau of Fisheries, Vol. XXXIII, 1913 

 (1915), p. 69-138, XI pis. Washington. 



1919. Biochemical changes in the muscle tissue of king salmon during the fast of spawning migra- 

 tion. The Journal of Biological Chemistry, Vol. XXXIX, 1919, p. 435-456. Baltimore. 

 Gunther, Albert C. L. G. 



1880. An introduction to the study of fishes (p. 142-149). 720 p. Adam and Charles Black, 

 Edinburgh. 



HjORT, JOHAN. 



1914. Fluctuations in the great fisheries of northern Europe, viewed in the light of biological 

 research. Conseil Permanent International pour 1' Exploration de la Mer. Rapports et 

 Proces-Verbaux, Vol. XX, 288 p., 3 pis. Copenhague. 

 Miescher-Ruesch, F. 



1880. Statistische und biologische Beitrage zur Kenntnis vom Lcben des Rheinlachsos im Siiss- 

 wasser. Schweizer Katalogue der Fischerei-Ausstellung zu Berlin, 1880. p. 154-232. 

 Leipzig. 

 Paton, D. Noel [editor] and OTHERS. 



1898. Report of investigations on the life history of the salmon in fresh water: From the Research 



Laboratory of the Royal College of Physicians of Edinburgh. Fishery Board for Scotland, 

 Salmon Fisheries, iv+176 p., 14 pis. Glasgow. 

 Tower, R. W. 



1902. The gas in the swim bladder of fishes. Bulletin, U. S. Fish Commission, Vol. XXI, 1901, 

 p. 125-130. Washington. 



