FUNCTIONS OF SWIMBLADDER OF FISHES. 9 1 



not enough to keep the fish alive for more than a fraction of an 

 hour. Some other explanation must be found for the reason 

 why fishes are able to survive for longer periods of time in 

 regions of low oxygen content. 



EFFECTS OF PRESSURE. 



That pressure would affect the percentage of gases in the 

 swimbladder of fishes has been long known. Biot (1807), 

 Provencal and Humboldt (1809) found that the amount of 

 oxygen in the swimbladder varied from i to 87 per cent., the 

 percentage increasing with the depth. The classical work of 

 Moreau (1842) showed that the fish accommodate themselves 

 to changes in pressure very gradually, and can live comfortably 

 at varying depths. 



Only a few experiments are reported here to show that the 

 response of perch to pressure results in the same increase in 

 oxygen as reported by other investigators for other fishes. It 

 was observed that fishes caught in gill nets at a depth of nine 

 meters in Lake Mendota showed a much higher oxygen percent- 

 age than those caught near the surface. The averages of 

 nineteen perch caught at nine meters, during month of August, 

 1921, were: 



Oxygen = 34.7 per cent., Carbon dioxide = 0.60 per cent., 



Nitrogen = 64.7 per cent. 



The averages of twenty-eight perch kept at the surface in Lake 

 Mendota were : 



Oxygen = 19.9 per cent., Carbon dioxide = 0.63 per cent. 



Nitrogen = 79.5 per cent. 



From the data of Birge and Juday (1911) the amount of dis- 

 solved oxygen at this season of the year was less at nine meters 

 depth than at the surface. Thus it appears that the cause of 

 this increase in the amount of oxygen in the swimbladder can be 

 attributed to pressure. 



An apparatus was constructed in the laboratory to study the 

 effects of pressure as a factor in producing increase in the per- 

 centage of oxygen in the swimbladder, other conditions being 

 equal. A steam pipe, eight inches in diameter and thirty-six 



