HABITS OF YELLOW PERCH. 323 
the surrounding water has a very low salt content. Garrey (1905) found the osmotic 
pressure of blood of marine fishes was about half that of sea water. 
With this brief review of facts gleaned from literature on the respiration of fishes, 
the discussion will now be turned to the discoveries made during the present investiga- 
tions in regard to the respiratory activities of the perch in Lake Mendota. 
During the summer of 1915 it was noticed that many fish caught in gill nets in 
Lake Mendota were dead, and that there was a greater mortality in the region of the 
thermocline and below it than above (fig. 31; Table 2). Of 2,194 perch caught, 343 
were dead and 1,154 alive above the thermocline; 610 dead and 87 alive below. This 
‘101724 317 '14'21'28 4 ‘NN I8'25° 2° 4° 
JULY AUG. SEPT. OCT. 
Alive 
Dead 
100. °° 
Fic. 31.—Percentage of perch caught alive or dead in gill nets above and below the thermocline, Lake Mendota, sors. 
, above thermocline; ------ , below thermocline. 
indicated that perch commonly entered water which contained too little oxygen for 
respiration. During the summer of 1916 careful observations were again made, and 
the same results were obtained. Though perch were usually most abundant imme- 
diately above the thermocline, large catches often occurred just below it, where there 
was no oxygen. 
The next problem was to discover how long perch could live in the oxygen-free 
water below the thermocline. Accordingly, from August 30 to September 4, 1916, 
when the amount of oxygen at a depth of 13.5 m. was 0.05 c. c. per liter and the carbon 
dioxide was 5 c. c. per liter, perch were let down into the stagnant region and left for 
various periods of time. The results of the experiments are shown as follows: 
