124 BULLETIN OF THE BUREAU OF FISHERIES. 



familiar to pilots who find it necessary to ' learn the river" each season. Many of these 

 changes must be catastrophic to mussels in cei tain localities. 



Excessive turbidity with consequent increased sedimentation, when of considerable 

 duration, is no doubt seriously unfavorable to the well-being of mussels. It has been 

 stated that mussels do not feed during periods of high turbidity, but no definite data 

 in support of this can be given. That mussels do not "bite" well on the crowfoot 

 hooks during a rising stage of water is a condition recognized by clammers. Whether 

 the fact that the shells are not generally open and the mussels feeding at this time is due 

 to the turbidity, or to other changing conditions incidental to the rising water, can not 

 be stated. If heavy deposits of sediment are unfavorable for adult mussels, they must 

 be more directly harmful to the young during the early stages of independent life, for 

 the tiny juveniles may be smothered by deposits that would have less disastrous effect 

 upon larger mussels. 



The effects of droughts are ordinarily felt but little by the mussels of the larger 

 streams and lakes. The most unfavorable condition arises when, owing to a prolonged 

 dry season, the water is lowered to such an extent that the mussels fall easy prey both 

 to muskrats and to clammers and pearlers seeking them in the shallow water. Crows, 

 too, are known to pluck out and kill Anodontas when the water over them becomes low 

 and clear. 



In the small streams, lakes, and sloughs, the mussels may be killed by the partial or 

 complete drying up of the water. Certain species of mussels are, of course, more resis- 

 tant to such condition than others. Isley (191 4) states that live specimens of Unio 

 ietralasmus were plowed up in a pond three months after it had become dry. The mus- 

 sels had burrowed down to zones of moisture. 



ARTIFICIAL CONDITIONS. 



Among the conditions imposed by man that may be detrimental to mussel life in 

 our streams may be mentioned the discharge of sewage, industrial wastes, dredging, 

 and the building of wing dams. (See Pis. IX, X, and XL) 



Disposition of the sewage and wastes of large cities without harmful contamination 

 of the rivers presents an issue of growing importance. Portions of streams just below 

 important cities are sometimes veritable cesspools, unsuited to both mussel and fish life. 

 The Illinois River for a considerable distance below its origin is greatly influenced by 

 sewage pollution through the Des Plaines River and the drainage canal; from the head 

 of the stream down to Starved Rock, 42 miles from the source, no mussels are found, 

 and a normal variety and abundance of fishes is not present above Henry, 77 miles from 

 its source (Forbes, 1913, p. 170; Forbes and Richardson, 1919, p. 148). Industrial 

 wastes from pulp and paper mills, tanneries, gas plants, etc., are injurious to fishes, 

 and no doubt harmful to mussels as well. Such unfavorable conditions as arise through 

 the depletion of oxygen supply by the decomposition of sewage are partially or com- 

 pletely corrected by the intervention of rapids or waterfalls. (See Shelf ord, 191 9, 

 p. Ill, and Baker, 1920.) 



River improvement work, such as dredging and the building of wing dams, creates 

 conditions more or less unfavorable for mussels. Hydraulic dredging may destroy 

 mussels, either directly by pumping them up, or by shifting the river channel so that 



