24 
and these are interrelated in so 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 pro- 
duce marked variations from the appearance 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 escape suffocation. At such times they congre- 
gate in the mouths of tributary streams or crowd into "spring 
holes" which do not freeze. Observations 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 slight- 
ly increasing amounts of dissolved oxygen from points downstream. 
In the summer of 1951, 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 embarrassed from lack of oxygen their movements are more 
rapid than at other times. Such a reaction tends to carry them 
over wider areas and wiil 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. 
