24 U. S. BUREAU OF FISHERIES. 



itself both of these major sources of pollution are of critical signifi- 

 cance; on the Flint River the sanitary sewerage (with minor indus- 

 trial wastes) is responsible for a serious case of stream pollution. 



The relation of stream pollution in this region to death of fishes, 

 to angling, to the commercial fishery interests, and to water supplies, 

 health, and recreation was given as much consideration as time 

 allowed. A general report of the investigations was made, including 

 data otherwise obtained by the Michigan Department of Health, as 

 well as those secured in the study here reported. A further report is 

 planned by Messrs. Badger and Hubbs, to be submitted when certain 

 additional data are at hand. 



At the request of the Illinois State Natural History Survey the 

 United States Fisheries Biological Station at Fairport. Iowa, en- 

 tered into cooperation with that organization and Avith the United 

 States Public Health Service and the Chicago Drainage District in 

 a study of the effects of pollution and of reclamation of submerged 

 lands along the Illinois River upon public health and recreation, 

 upon agriculture, and upon the aquatic resources of the river. The 

 special part of the Bureau of Fisheries was to obtain statistical data 

 on the fisheries of the river for 1921. These data, to compare with 

 earlier records, were obtained by H. C. Minch, foreman of the 

 Fairport station, by examination of railroad and steamboat express 

 and freight records of shipments from and to the towns along the 

 river. The survey data are now being summarized for preparation 

 in report form by R. L. Barney. 



• FISH AS AGENTS IN MOSQUITO CONTROL'. 



Investigations of fish in relation to mosquito control were con- 

 tinued from previous years by Samuel F. Hildebrand, scientific 

 assistant, working in cooperation with the United States Public 

 Health Service. The experiments Avere conducted in the vicinity of 

 Augusta, Ga., wdiere there are numerous waters especially suitable 

 for the investigations. The w^ork of previous years was mainly 

 conducted in localities Avhere antimalaria campaigns Avere under Avay 

 and Avhere immediate practical results were the chief aim; but in 

 1921 no antimalaria work Avas attempted by local authorities out- 

 side of the city limits of Augusta so that the iuA'estigator had full 

 opportunity for experiment and observation. 



The highly efficient mosquito control that Gambusia provides in 

 bodies of Avater in Avhich no barriers betAveen the fish and the im- 

 mature mosquito exist has already repeatedly been demonstrated; 

 therefore the season of 1921 Avas devoted mainly to the study of the 

 relation of barriers formed by plants to mosquito control by the use 

 of fish. • 



It was determined that, in the vicinity of Augusta at least, the 

 " silver-leaf grass," HydrochJoa carol inensis^ affords mosquito larvae 

 the most effective proiteftion against fish. Other plants that some- 

 times seriously hinder efficient mosquito control are Myriophyllum, 

 Potamogeton, and some of the algse. Plants of minor importance 

 in relation to mosquito control are cat-tails, burr weeds, arroAvheads, 

 smart Aveeds, and various tall grasses and sedges. Plants that 

 appear to be beneficial are the duckweeds and the bladderAvorts. 



In case of ponds that contain many Gambusia along with effective 

 plant barriers it was learned that the degree of control could be 



