HABITS OF YELLOW PERCH. 299 
The greatest difference of biological importance between the two lakes is probably 
the thermal stratification, which is characteristic of Lake Mendota during the warmer 
months, but which is lacking in Lake Wingra. Birge and Juday (1911) have made com- 
plete records of the seasonal changes in Lake Mendota for several years. ‘The lake is fro- 
zen over from three to nearly five months.*. During this period there is little difference 
(o to 2.5° C.) in temperature at different depths, and there is practically no circulation; 
the oxygen in the deepest parts becomes gradually exhausted, and the carbon dioxide 
increases. Soon after the ice goes out in the spring the lake begins to circulate freely 
from top to bottom. The temperature and dissolved gases therefore become uniform 
at all depths and so continue until the temperature reaches 4° C. The water near the 
surface then gradually grows warmer. About the end of June a thermocline, or stratum 
of rapid temperature change, is established, and after that there is no mixing between 
the water in the upper part of the lake and that in the deeper portions. When the 
stratification is complete, the lower, cooler water is cut off from contact with the 
atmosphere, and its oxygen is gradually used up. The plankton organisms leave it, 
but many of the insect larve, molluscs, and other animals which live on or in the soft 
bottom mud remain in the stagnant water (Juday, 1908). The upper stratum circulates 
throughout the summer and, therefore, has abundant oxygen; it also grows warmer and 
may reach a temperature of 25° C. or more. The thermocline is usually established at 
a depth of about 10 m. and moves deeper as the upper stratum of warm water grows 
thicker in latesummer. In the autumn the upper water gradually grows cooler, and, 
finally, about the first of October, the ‘“‘autummnal overturn’’ takes place—the lake 
circulates again throughout; gases and temperatures are again uniform at all depths. 
This condition continues until the lake again freezes. 
Lake Mendota is, then, subject to two periods of stagnation. During the winter 
the water is all cold, and the deepest regions may be without oxygen; during the late 
summer and early autumn the lake is separated into three strata, a warm circulating 
stratum on top, a thin middle region of rapid transition in temperature and dissolved 
gases, and a lower, cool, stagnant region which is without oxygen and contains con- 
siderable carbon dioxide for about three months. During the spring and the autumn 
overturns all parts of the lake have the same temperature and the same dissolved gases. 
Lake Wingra is, of course, subject to the same external seasonal changes as Lake 
Mendota but does not respond in the same way. It is so shallow that the water 
circulates freely from top to bottom while it is not covered with ice. Its shallow basin 
and small total body of water make it more susceptible to short periods of changes in 
temperature. It warms up more rapidly in the spring and cools more quickly in the 
autumn than Lake Mendota. 
This brief summary gives some idea of the great contrasts between the two lakes, 
and there are, of course, many other minor differences. However, perch are the most 
abundant fish in both. The point which first attracted the attention of the writers 
was the fact that the perch in Lake Mendota are generally a third larger than those 
from Lake Wingra. Furthermore, in Lake Monona,” which receives the water discharged 
from Lake Mendota, they are still larger. The questions to which answers have been 
sought may be formulated somewhat as follows: (1) Why are perch the most abundant 
@ The earliest freezing over recorded since 1851 was Nov. 23; the latest, Jan. 14; corresponding dates for opening in the 
spring were Mar. 8 and May 4. 
b Lake Monona has not been investigated to any extent by the writers. 
110307°—21 20 
