estuarine species appear to avoid water containing 

 dissolved oxygen tensions in excess of 110% of 

 atmospheric pressure, while other species do not 

 appear to respond to elevated gas tensions. Simple 

 selection chambers should be employed in addi- 

 tional tests with several species. 



In addition, the following acute tests are 

 needed: 



• Intermittent exposure. Selective spilling 

 such that only a portion of a river system is super- 

 saturated might alleviate the effect of gas super- 

 saturation on diadromous fishes. About 2 yr ago 

 the Nitrogen Task Force recommended that the 

 feasibility of spilling "slugs" of water and the 

 effect of the anticipated intermittent exposure be 



investigated. Additionally, diadromous species 

 are subject to decreases in hydrostatic pressure 

 when they pass over dams. Although some inter- 

 mittent exposure data is being collected, additional 

 studies are needed. 



• Acute tolerance. Data are available des- 

 cribing the tolerance of freshwater fishes to dis- 

 solved gas supersaturation; however, tolerances 

 of aquatic invertebrates and marine organisms 

 to acute exposure are poorly defined at present. 



The session closed with a plea for basic re- 

 search aimed at determining long-term population 

 and ecosystems effects and for some immediate 

 cooperative solution to maintain the Columbia 

 River system's salmonid resources. 



Round Table Discussion 113 



