These bioassays have shown that although 

 both the fall chinook and steelhead tend to remain 

 at greater depths with increasing levels of super- 

 saturation, they are unable to totally compensate 

 for dissolved gas concentrations above 120%. 

 Therefore, it is imperative that corrective measures 

 to reduce supersaturation be implemented as soon 

 as possible to reduce mortality. 



SUMMARY AND CONCLUSIONS 



Bioassays in shallow (0.25 m) and deep (2.5 m) 

 tanks with dissolved nitrogen and argon gas con- 

 centrations ranging from 100 to 127% of saturation 

 were conducted to determine lethal and sublethal 

 effects on juvenile fall chinook salmon and steel- 

 head trout. Throughout the test, mortalities and 

 live subsamples were weighed, measured, and 

 examined for signs of gas bubble disease. After 

 exposures of 127 days (fall chinook) and 7 days 

 (steelhead), remaining groups of fish were: 1) 

 put into saltwater tanks to determine the ability 

 to transfer to salt water; or 2) put into equilibrated 

 water (100% T.D.G.) to determine the ability to 

 recover from gas bubble disease. 



We concluded from these experiments that: 



1) Significant mortality of juvenile fall chi- 

 nook commences at about 115% of supersaturation 

 (T.D.G.) in shallow tanks where hydrostatic com- 

 pensation is not possible and at about 124% in deep 

 tanks where compensation is possible. 



2) Significant mortality of juvenile steelhead 

 commences at about 115% in shallow tanks and at 

 about 127% in deep tanks where hydrostatic com- 

 pensation is possible. 



3) Tolerance to supersatuation of atmo- 

 spheric gas of both fall chinook and steelhead 

 decreases with age and growth. 



4) Emboli in branchial arteries, gill fila- 

 ments, and the heart were rarely observed on live 

 subsamples, but were prevalent on mortalities 

 indicating these signs are directly associated with 

 the death of the animal. 



5) The average depth maintained by chinook 

 and steelhead groups when allowed to sound com- 

 pensated for about 10% and 10 to 15% (respec- 

 tively) of the saturation value measured and com- 

 puted on the basis of surface (760 mm) pressure. 



6) Both fall chinook and steelhead with 

 signs of gas bubble disease are able to recover 

 from exposure to supersaturation. 



7) Exposure to various levels of supersatura- 

 tion does not seem to affect the ability of steelhead 

 to transfer to salt water; data on effect of expo- 

 sure to supersaturation on ability of fall chinook 

 to transfer to salt water were inconclusive. 



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10 Dawley, Schiewe, Monk 



