HABITS OF YELLOW PERCH. 329 
soon return to deep water and remain there until the lack of oxygen drives them into 
shallower regions. As soon as the autumnal overturn renews the oxygen the perch 
return for the most part to the depths of the lake. Less marked migrations of the same 
general type also take place in Lake Wingra, but there is no stagnation period in the 
summer. Figures 33 and 34 bring out a point which has already been discussed to some 
extent under food and respiration; that is, though the perch are obliged to live above 
the thermocline from August to October, they descend at intervals into the cool, stagnant 
region below, probably to take advantage of the abundant food there. Wells (1915) 
has pointed out that fishes generally prefer water which has a slightly acid reaction to 
: @ 
a ae a 
WT eel) ae 
ENE SLR CO 
123 a A fa 9 ED 19 
AP 2 
feogod | fe | | | 
BRSERHT ith nae hie 
a te EEE EEE EEE EEE EEE 
Fic. 33.—Perch caught in gill nets set at various depths, Lake Mendota, 1915. The curve indicates the thermocline. All nets 
used were r-inch bar mesh. * indicates a net 4 by so feet which caught only one-fifth as many perch as the other nets used, 
which measured 3 by 75 feet. Nets were left in the water about 24 hours. ‘The ice left the lake April 9 to rz; the fall over- 
turn took place October 9 and ro. 
that which is neutral or alkaline. Of course, such behavior would tend to keep perch in 
deep water or near the bottom vegetation. Gurley (1902) is an ardent advocate of 
temperature as the controlling factor in the seasonal migrations of fishes, but in Lake 
Mendota it can have but slight influence. The perch come into shallow water in spring, 
when the temperature is low, uniform at all depths, and the same as that which has 
prevailed for several months; in autumn they descend into deep water when the tempera- 
tures are again uniform throughout the lake. The food and the net and line catches 
both indicate that the perch remain on or near the bottom and in as deep water as 
possible throughout the year. The migrations into shallow water are to spawn and to 
escape stagnant conditions during the summer. 
