An excess of "free" carbon dioxide may have adverse effects on aquatic 

 organisms, ranging from avoidance reactions and changes in respiratory move- 

 ments at low concentrations, through interference with gas exchange at higher 

 concentrations, to narcosis (unconsciousness) and death if the concentration 

 is increased further. According to the Federal Water Pollution Control 

 Administration (1968) respiratory effects seem most likely of these impacts 

 to be of concern. 



Concentration of Salts 



Fresh-water fish have a problem in regard to osmoregulation. Since the 

 concentration of salts is greater in the internal fluids of the body than in 

 the fresh-water environment, then either water tends to enter the body by 

 osmosis if membranes are readily permeable to water or salts must be con- 

 centrated if membranes are relatively impermeable (Odum 1964). Increases 

 in salts (TDS) may create a problem in fish habitat preference. The 

 possibility exists of a barrier to migration being produced by a zone of 

 high TDS concentration. Reduction in flow could produce a TDS problem by 

 concentrating salts and by reducing the diluting force of the stream. 



LIMITING FACTORS UNRELATED TO FLOW 



Several factors not dependent upon discharge may also have an impact on 

 the fishery of the Tongue River. These are: 1) fishing pressure, 2) land 

 use practices, and 3) pollution. 



Fishing pressure is an environmental factor which may be manipulated 

 to increase both fish production and yield. Pressure is often too great 

 or too small, in relation to the productive capacity of a stream, to give 

 maximum yield. Tag returns from the Tongue River suggest that pressure 

 and harvest are light. Therefore it is doubtful that this factor results in 

 a significant impact on the fishery. 



Land use practices such as overgrazing, cultivation of the floodplain, 

 and channel disturbances all can produce a negative impact on the fishery 

 by degrading the habitat. Stream-bank stability can be destroyed through 

 overgrazing and many other agricultural activities, resulting in bank 

 sloughing and siltation. Channel disturbances reduce spawning and food- 

 producing areas, and stream realignment shortens the total length of the 

 river. Land use practices along the Tongue River have historically pro- 

 duced examples of the above impacts. Future practices should not signifi- 

 cantly increase problem areas. 



The final factor, pollution, could possibly result from introduction 

 of an allochthonous, toxic substance, such as an effluent from a steam 

 generating plant containing substances harmful to the fish. Even if the 

 materials were not in toxic concentrations, the basic fertility of the 

 river could be altered, favoring another assemblage of fish species. An 

 alteration of the natural heat budget of the river is another possible 

 impact from coal conversion complexes. Since the Tongue River Basin has 

 been identified as a potential site for such complexes, this impact looms 

 as a threat. 



73 



