24 
complex a manner, and exist in such a constant state of flux, that 
no one factor is of sufficient importance or persists over a long 
enough period of time to produce marked variations from the aprear- 
ance of randomness, 
For example, it is well known that fishes move upstream in 
spring during times of heavy rainfall and floods. Conversely, they 
move downstream during summer and fall when streams are shrunken and 
suffer from drouth. This movement exists because most kinds of fishes 
prefer a stream of a certain size, some great and some small. When 
rains cause an increase in the volume of water, the fishes tend to 
move upstream until they find their optimal stream size. When drouth 
causes a shrinkage they tend to drift downstream until they find this 
same optimal stream size. 
In winter, the Illinois River is occasionally covered with 
ice for periods of several weeks which halts the natural reaeration 
of the water and in the presence of pollution causes a deficiency of 
dissolved oxygen. Under such conditions fish are seldom killed unless 
they are caught in nets or are otherwise trapped and cannot escare 
suffocation. At such times they congregate in the mouths of tributary 
streams or crowd into "spring holes" which do not freeze. Observa- 
tions at such times indicate that the fish do not blunder into the 
mouths of these tributaries and into "spring holes" but are guided to 
them by following slightly increasing amounts of dissolved oxygen 
from points downstream. 
In the summer of 1931, while fishing in Meredosia Bay with 
fyke nets, near the mouth of a long narrow slough, we learned that on 
a certain night the dissolved oxygen in this water fell below the 
critical concentration and the fishes of this slough moved out into 
the bay in a body. We learned also that when fishes become embar- 
rassed from lack of oxygen their movements are more rapid than at 
other times. Such a reaction tends to carry them over wider areas 
and will more probably bring them into higher concentrations of oxygen 
if such exist in the vicinity. 
During autumn the fishes of certain of our swifter streams, 
such as Rock River, gradually accumulate in the eddies and quieter 
pools. By midwinter almost the entire fish population of the stream 
may be found in pools and eddies which make up only a few per cent of 
the total area of the stream. When the water warms in spring they 
leave these quiet spots and scatter until they may be found in about 
equal abundances in all velocities of water. This behavior, too, is 
essentially random in nature, since as the water cools the swimming 
movements of the fishes become so slow that they can no longer stem 
the current. Then they drift tail foremost downstream until they find 
water so quiet they can hold their position even though numbed by ice 
cold water. 
