REPORT OF THE COMMISSIONER OF FISHERIES. 21 



In view of the recent and prospective development of hydroelectric 

 projects much interest attaches to the effects of dams and artificially 

 impounded waters on the fisheries of interior waters. When the great 

 dam across the Mississippi River was constructed a number of years 

 ago many persons expressed the opinion that, particularly in the ab- 

 sence of a fishway, the value of the fisheries of the upper river would 

 be seriously impaired. For this reason the bureau has kept Lake 

 Keokuk, the large body of water thus created, under periodical ob- 

 servation, and during the months of July, August, and September, 

 1921, it made an examination to determine the quantity of fish feed 

 developed in this lake as compared with the natural river and Lake 

 Pepin, a natural lake of about the same size, lying in the course of 

 the Mississippi River in Minnesota and Wisconsin. It was found 

 that the content of floating organisms (plankton) per unit of water 

 volume was considerably greater in Lake Keokuk than in the adja- 

 cent part of the river but much less than in Lake Pepin. The yield 

 of the fisheries in the part of the stream covered by Lake Keokuk 

 has increased since the erection of the dam, probably as a result of 

 the increased food supply. 



At very small expense the bureau has been able to continue co- 

 operation with the Wisconsin Geological and Natural History Sur- 

 vey in very important and fundamental investigations of the fish food 

 resources of small lakes. The results indicate a surprisingly high 

 production of plants and animals per unit of water surface and con- 

 firm the opinion long held of the potential importance of lakes and 

 ponds as producers of food. 



FRESH-WATER MUSSELS. 



The propagation of fresh-water mussels, which provide the raw 

 material for the valuable pearl-button industry of the Mississippi 

 Valley, years ago attained large proportions and in the opinion of 

 the industry has achieved results. The bureau has not been satisfied, 

 however, to rest on present accomplishment but is constantly striving 

 to improve the economy and effectiveness of the work and to extend 

 it to species that have not satisfactorily responded to the methods 

 now employed. For these reasons a material part of the activities of 

 Fairport (Iowa) laboratory have been devoted to research and ex- 

 j:>eriment on these river moUusks. If a satisfactory method could be 

 developed for rearing the juvenile mussels to a stage at which they 

 , could be planted directly on suitable bottom a definite advance in 

 mussel propagation in public waters would be achieved, and it would 

 make possible a system of private mussel culture comparable with 

 that extensively practiced with the oyster in coastal waters. 



Some of the experiments in this field at Fairport have been highly 

 successful, while the results of others conducted under seemingly 

 favorable conditions have been insignificant. There are unknown or 

 unrecognized conditions involved, and in addition to the direct ex- 

 perimentation the attaches of the station have undertaken the com- 

 prehensive study of all of the biological and physical factors that 

 may have bearing on the subject. The work as a whole has given 

 such encouragement of ultimate success as to make its continuance 

 imperative. 



In the method of mussel culture now practiced it is necessary to 

 handle large numbers of live fish, and the minor injuries that they 



