Juday and Wagner—The Distribution of Fishes. 21 
the decaying material than the zooplankton forms. The plank¬ 
ton organisms, either directly or indirectly, constitute a very 
important element of the food of most fishes at some stage of 
their development; in fact, it has been asserted that the pro¬ 
duction of fish is correlated with the amount of plankton pro¬ 
duced by a stream, or lake. But the results obtained on a num¬ 
ber of Wisconsin lakes show that, beyond a certain limit, the 
increase in the amount of plankton would tend toward a de¬ 
crease in the production of fish in these lakes, especially of 
some species, rather than an increase, by making conditions un¬ 
favorable for them. This upper limit would vary somewhat 
for different lakes. A very large growth of plankton in a lake 
would furnish an abundance of decaying organic matter which 
would soon use up the dissolved oxygen in much or all of the 
cool, lower water, and this would mean a very considerable re¬ 
striction in the horizontal distribution of the fishes. Under 
these conditions, they could live only in the warm, upper water, 
and a species that required cool water would not be able to live 
in such a body of water. 
So far as oxygen conditions in the lower water are concerned, 
a lake that is poor in plankton would be best adapted to fish 
life in that region, but the question of food now enters as a fac¬ 
tor of equally great importance. As indicated above, plankton 
plays such an important role as a fish food that a scanty supply 
of it would mean a scarcity of proper food. This condition 
would be accompanied by the usual results, few fish and these 
in poor physical condition. Thus the results obtained in these 
studies seem to indicate that the lake that is best adapted to 
fish life is one that contains neither an unusually large amount 
of plankton, which would cause the rapid and early depletion 
of the dissolved oxygen in the lower water in the summer, nor 
an amount so scanty as to endanger the food supply, but a me¬ 
dium amount. However, each lake possesses features peculiar 
to itself, and there are factors involved which make it impos¬ 
sible to give any general rule as to quantity that would apply 
to all lakes. These individual differences must be determined 
for each lake. 
