260 
BULLETIN OP THE BUREAU OF FISHERIES 
Because of the heavy concentration of the Muskellunge Lake ciscoes within a 
narrow stratum in late summer, it is apparent that the catch of ciscoes in a particular 
net will vary greatly according to the depth at which the net was set. To obviate 
this difficulty the estimate of the relative density of the population of the Muskellunge 
Lake cisco was based only on catches of nets set in the depths at which the cisco is 
most abundant. These depths were 9 to 11 meters in 1930, 9.5 to 10 meters in 1931, 
and 11 to 13 meters in 1932. 
The conditions of life that the Muskellunge Lake cisco finds in middle and late 
summer in the upper strata of the thermocline are probably far from the optimum 
for the species. Not only is the water temperature higher than they would normally 
be expected to select but the crowding itself and the probable competition with the 
perch that inhabit the same region in large numbers (p. 293) may be expected to make 
for quite unsatisfactory feeding conditions. In view of these unsatisfactory condi- 
tions of life the poor condition (p. 248) of the individuals of this population in contrast 
to those of other lakes in which chemical conditions are better, and their short life 
span are not surprising. 
That the existence of conditions that force the cisco into unsuitable strata of 
water may have damaging effects on the population was pointed out by Cahn (1927), 
who stated that when “the cisco is forced into water far warmer than it ordinarily 
selects * * * the result is often disastrous”, and a high mortality occurs. 
Scott (1931) showed that where there is formed a “ thermoclinal notch” in the oxygen 
curve the disappearance of the oxygen from the hypolimnion may force a sudden 
movement of the ciscoes from the hypolimnion to the upper strata of water. His 
observations on the cisco of Snow Lake, northeastern Indiana, showed that no 
great mortality results from this sudden move, but rather that the fish are only 
temporarily incapacitated. After floating for a short time at the surface they make 
a recovery and remain in the epilimnion for the remainder of the season. Such a 
forced removal into an undesirable habitat, either sudden or gradual, doubtless entails 
considerable shock to the individual fish. It is readily conceivable that a temporary 
stoppage of growth might occur, and that the occurrence of accessory annuli such as 
appear in the Muskellunge Lake cisco scales can be thus explained. 
The Silver Lake cisco agrees with the Muskellunge Lake cisco in showing a 
rather sharply delimited vertical distribution. In the summers of both 1930 and 
1931 ciscoes were taken only in nets set in depths between 10.5 and 15.5 meters. 
The dates of collection in 1930 were August 9, 10, and 15; in 1931, July 17 and 
August 22. The reason for this concentration within a 5-meter stratum appears 
from the examination of the data of table 42, which shows the relationship between 
depth and the temperature and 0 2 concentration in Silver Lake on August 28, 1931. 
If the data of table 42 are considered in relation to the known vertical distribution 
of the cisco in Silver Lake, it would appear that a preference for cool water keeps the 
ciscoes from entering the warmer strata less than 10 meters deep, while a deficiency 
of oxygen prevents them from occupying the lower portions of the hypolimnion. 
Thus in late summer the Silver Lake cisco is confined to a narrow stratum in the 
upper part of the hypolimnion and the lower part of the thermocline. The catch of 
nets set between the depths of 10.5 and 15.5 meters can be taken to represent the 
relative abundance of the Silver Lake cisco at that time. 
