VARIATION IN THE MAXIMUM DEPTH AT WHICH FISH CAN 
LIVE DURING SUMMER IN A MODERATELY DEEP LAKE 
WITH A THERMOCLINE 
By FRANK SMITH 
Professor of Zoology, University of Illinois 
Contribution from the University of Michigan Biological Station and the Zoological Laboratory 
of the University of Illinois 
During several summer sessions the writer served on the staff of the Univer- 
sity of Michigan Biological Station, which is on the shore of Douglas Lake, about 
17 miles south of the Straits of Mackinac. During the last few years of service he 
was responsible for instruction work, which included among other things a study 
of the habits and distribution of fishes in the lake. This led to a desire for a more 
precise knowledge concerning the vertical distribution of fish in places where the 
depth is sufficient to permit the establishment of a definite thermocline. During 
the latter part of the sessions of 1920 and 1921 attempts were made to determine 
the maximum depth at which fish could remain alive for any considerable length of 
time in such places and to ascertain what relation such maximum depth might have 
to the position of the thermocline and to the correlated oxygen and hydrogen-ion 
conditions. These attempts led to results sufficiently definite and interesting to 
make publication seem worth while. It became evident that one could predict, 
within 2 or 3 feet, what such maximvun depth would be if furnished accurate data 
on the temperature, amount of dissolved oxygen, and the acidity conditions exist- 
ing at levels having 2 or 3 foot intervals in the upper part of the thermocline. From 
the temperature data alone one could predict nearly as closely. Variations in the 
depth at which the thermocline is established are accompanied by similar varia- 
tions in the maximum depth at which fish can remain alive. 
Douglas Lake is approximately 4 miles in length, with its long axis extending 
in a northwest-southeast direction, and the average width is somewhat less than 
half as much. With the exception of about a half dozen small isolated areas, in 
which the water is 60 to 90 feet in depth, the water of the lake is too shallow to 
have a thermocline established. In the deep places or holes, as they are termed 
in station parlance, a thermocline is ordinarily present during July and August, 
when the station is in operation. The depth at which the thermocline is established 
varies considerably in the different holes and in any given one of them is subject 
to some fluctuation. The depth is apt to be greater in the latter part of the season, 
and long-continued, strong winds tend to depress the level of the upper part of the 
thermocline, if not of the entire stratvmi. For the observations described in this 
103591°— 24t ' (I) 
