146 REPORT OF COMMISSIONER OF FISH AND FISHERIES. * 
fully the natural conditions of the lake in all its main features, a knowl- 
edge of which shows its capacity for fish life, its suitability for the intro- 
duction of new species, and the measures necessary for the protection 
of its fisheries. Most attention in the line of biology was paid to the 
smaller forms of life, such as the larval and adult insects, crustaceans, 
mollusks, and worms, which serve as food for fishes, both adults and 
young. 
In his conclusions, Prof. Forbes remarks: 
It is evident that even in a lake of so moderate sizeas this the smaller inhabitants 
are quite clearly divided into pelagic and littoral groups, the latter containing the 
greater number of species, but the former not less numerous in individuals; and a 
comparison of the results of dredging shows that this difference applies to the ani- 
mals of the bottom as well as to those swimming freely above it. * * * I¢ is 
also apparent, from the product of the towing net in deep water under varying con- 
ditions, that the pelagic entomostraca avoid the surface by day, whether it berough 
or calm, or the weather cloudy or clear; but they do not necessarily withdraw to 
any great depth, hauls 10 feet below yielding “‘ good” or “large” collections when 
the sun was shining. By night, on the other hand, the yield at the surface was 
large, even in a high wind. 
The examination of Lake Mendota was made for the purpose of 
studying a most remarkable mortality among the common perch (Perea 
flavescens) in August, 1884, and was repeated in August of the follow- 
ing year. A study of the lake similar to that of Lake Geneva was also | 
instituted, but upon a less comprehensive scale. The fish mortality 
began early in July of the former year and continued until about the 
middle of August, the quantity of fish dying during that period being 
estimated at above 300 tons. About 90 per cent of the fish which per- 
ished were the common perch, after which in numbers came the lake 
herring, followed by a few other species. Prof. Forbes’s studies of the 
subject were thorough and covered all the questions that could be sug- 
gested, including an examination of all the conditions of the lake, both 
physical and biological, and of the morphological and histological con- 
ditions of both the dead and living fishes. It was impossible, however, 
to arrive at satisfactory deductions respecting the cause of the mor- 
tality, Prof. Forbes not having been apprised of its occurrence until it 
was practically over, and no subsequent opportunity having arisen to 
study similar phenomena. However, all the facts have been carefully 
recorded and discussed, and are available for future consideration. 
The more significant facts connected with the mortality are stated in 
considerable detail, but they are too long to make more than a brief 
reference to them in this connection. 
A diseased condition of some parts of the viscera was noticed, espe- 
cially in the herring, but externally the dying fishes seemed to be in 
good condition. The herring are subject to some mortality of the same 
character every year, but to a much less extent than in 1884. It is 
probable that the causes affecting the two species were the same, and — 
it is also possible that the disease is contagious and was taken directly — 
from the herring. The majority of the dead perch were full grown, © 
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