VARIATION IN DEPTH AT WHICH FISH CAN LIVE 
7 
dition except two perch. Of those that had been at 60 feet all were in good condition 
except one perch that had died and another perch and a rock bass that were ailing. 
Of those that had been at 62^ feet all were dead except a few bullheads which were 
nearly dead. 
The fish in the 60-foot cage had been confined for over 24 hours, having been at 
the 45-foot level at Station A for 6 hours on the previous afternoon and then trans- 
ferred to Station B, where they were at the 55-foot level for over 14 hours and 
then at the 60-foot level about 4 hours; and yet the majority were in good condi- 
tion, although they had originally been obtained from shallow water. Some of 
the fish in the 62^-foot cage had had an experience similar to that just described, 
and others were fresh specimens put into the cage only about 4 hours before; but 
the difference of 2^ feet meant the difference between life and death. The data 
in the last table furnish the explanation. The fish in the 622-foot cage were in 
the thermocline with reduced oxygen and unfavorable hydrogen-ion conditions. 
A similar change in conditions 20 feet nearer to the surface was found at Sta- 
tion A on August 5, with a corresponding effect on the fish. Fish could live at 
40 i feet but died at 43 feet, just as in the last test they could live at 60 feet but 
died at 62^ feet. 
The results of these observations very definitely answered one question that 
was in mind when the tests were planned. The lowest limit of the body of water 
that supplies the conditions necessary for fish to remain alive and in good condi- 
tion is in the upper portion of the thermocline. In this upper part of the thermo- 
cline there is a marked decline in the amount of available oxygen and a rapid 
adverse change in the hydrogen-ion conditions, indicated by the reduction in the 
numerical value of the pH index. The expression "upper portion of the thermo- 
cline" is somewhat indefinite, and further tests may show that a statement placing 
the lowest limit of the habitable region in the upper quarter or upper third of the 
thermocline layer is warranted. No attempt was made to determine which of 
the various factors involved may be most potent in fixing the lower limit of the 
habitable part of the lake. 
SUMMARY 
The results of a considerable number of tests made in Douglas Lake, Mich., 
show a very definite and close correlation between the lowest limit of the body of 
water in which fish could remain alive and the upper portion of the thermocline. 
During August, 1921, the position of the upper portion of the thermocline at one 
station shifted from a depth of about 40 feet to one of 47 feet, and there was 'a 
corresponding variation in the depth at which fish could live. At another station, 
where the depth of the upper portion of the thermocline shifted from about 50 
to 60 feet, there was a corresponding change in the depth at which fish could remain 
alive. They remained in good condition in cages placed at the depths mentioned, 
while those in cages placed at a level 2^ feet lower down soon died. In each instance 
the fish in the lower cage were in water having decidedly less oxygen and less 
favorable acidity conditions, and these factors, rather than that of mere depth 
alone, determined the lowest limit at which the fish could live. No attempt was 
made to discover which of the unfavorable factors was most potent. 
