431 



considered a disadvantage of hoop-nets is their selcctiveness. Sluggish 

 fish are not caught as often as active fish. Carp avoid small-mesh 

 nets and traps, but they are readily taken in large-mesh nets. Chan- 

 nel cat and bullheads seem to go most readily into small-mesh nets 

 and basket traps, probably because, being largely nocturnal and hifl- 

 ing during the day, they go into the nets for concealment. 



Table II gives the winter data showing the relation of fishes to 

 different conditions of ice and dissolved oxygen. It seems quite cer- 

 tain that dissolved oxygen concentrations between zero and two 

 parts per million will kill all kinds of fishes. Carp and buf?alo have 

 been found living in water showing as low as 3.5 parts per million. 

 As a rule, a variety of fishes were found only when they were four 

 or more parts per million, and the greatest variety of fishes was taken 

 when there were nine parts per million. This catch was made on 

 February 2G-27 in Vette Swale when at least twenty-two and ]ier- 

 haps as many as twenty-six species of the larger fishes were taken 

 in nine 2y>-mch mesh four-foot hoop-nets. 



It was noticed a number of times that carp and buffalo taken 

 from oxygen-deficient water were very light in color and sluggish 

 in their movements, while the same kinds of fishes taken from well- 

 aerated water were quite darkly pigmented and very active when 

 disturbed. 



Mention was made by fishermen, from time to time, at several 

 points on the Illinois River, of the inferior value of certain fish be- 

 cause they were "gassy" — their expression for a taste and smell of 

 the flesh of the fish, like the smell of coal gas. Others have described 

 it as like the taste of kerosene or of tar ; but to the writer it seems 

 more than anything else like the smell of the putrifying mud on the 

 bottom of the river, which is very like that of coal gas. This taste is 

 often so pronounced as to make the fish very disagreeable as food. 

 "Gassy" fish are definitely associated with periods of prolonged ice 

 on the river, mortality in nets and traps, and other indications of a 

 .scanty supply of dissolved oxygen. The species which most com- 

 monly have this taste are carp, buffalo, channel cat, and bullheads, and 

 these are the species which are most often found living in water with 

 a low oxygen concentration. If these fishes are kept in well-aerated 

 M'ater for a few days, they lose this "gassy" taste. 



During the winter, except when the river is frozen over, fishes do 

 not suffer directly from lack of dissolved oxygen. From November 

 to May, in open water, the rate of aeration exceeds the rate of oxygen 

 consumption, so that there is usually a sufficiency of dissolved oxygen. 



In midsummer, down as far as Peoria and .sometimes below, except 

 during high water, lack of dissolved oxygen excludes fishes from large 

 feeding grounds and destroys large amounts of fish food. For example, 

 Mr. Richardson observed a heavy mortality of snails in Peoria Lake in 

 August, 1917. As increase in temiterature accelerates the life processes 

 of fishes, lack of dissolved oxygen can not be endured in warm water as 

 long as in cold. 



