NORTHWESTERN LAKES OF THE UNITED STATES. 
73 
and cool. The tilling of the drainage basin and the increase of population increased 
the nitrogen content of such lakes, and this with the warmer water caused the 
vegetable plankton to increase. The growth of this plankton used up the carbon 
dioxide from the surface water, leaving it alkaline. Later the decay of the same 
plankton in the hypolimnion used the oxygen and produced carbon dioxide, making 
this water acid, and in some cases all or nearly all of the oxygen below the thermocline 
or in the cold water has been used. So the trout have been forced up into the warmer 
water. The curves of Clear and Silver Lakes show this best, while Cottage Lake 
is similar, except for the alkaline surface water. The others show similar conditions 
varying with the lakes and the date of testing. 
MACKINAW TROUT. 
One of the objects of the work on the lakes in Washington and Idaho was to 
determine those best suited for Mackinaw or Great Lake trout ( Cristivomer 
namaycusli). 
The exact conditions necessary to the welfare of these trout are not definitely 
known, but, in general, we know that they require deep, cold water containing 
oxygen. That is, the lakes of Group II, division 1, which have oxygen in the 
bottom water during the whole summer, seem to be best adapted for Mackinaw 
trout. 
The amount of oxygen necessary to allow the trout to live in this cold water, 
the hypolimnion, has not been definitely determined, but the work on the Wis- 
consin lakes (Birge and Juday, 1911) has shown that they can live in water con- 
taining 0.9 cc. of dissolved oxygen per liter. 
A list of the lakes in which lake trout have been planted shows that the Bureau 
of Fisheries has furnished many fry that have been planted in the shallow lakes 
of Group I. It may quite safely be assumed that all of these fry have been wasted 
or at most have furnished a little food for the larger fish. 
Many of the larger lakes have oxygen and temperatures similar to the Great 
Lakes, and lake trout would be expected to thrive in such. In some of these lake 
trout have been planted. They were reported as fairly plentiful in Fallen Leaf 
Lake and Lake Tahoe, Calif., and a few have been caught in Deer Lake, Wash. 
Beyond this all of our efforts to get information concerning catches were without 
results. Our time was too limited to fish for them ourselves, but sportsmen at many 
of these lakes promised to try for them and report. Several of them have written 
since, but with the exception of Deer Lake, Wash., all the results have been negative. 
In some of these deep lakes it appeared from stories told us by the fishermen 
that one reason the plants of lake trout did not succeed might be the careless way 
in which they were planted. At two of the important large lakes, we were told that 
the cans of fingerlings were emptied off the wharves where the perch were the most 
plentiful. 
A more careful comparison of the food to be found in the lakes in which they 
thrive with that in the lakes in which they do not may be of interest, likewise a 
comparison of the spawning grounds. 
From this it may be concluded that all of the shallow lakes and some of medium 
depth, which do not have oxygen in the hypolimnion all summer, are not suitable 
