TABLE 5 Daphnia Mortality After 1 Week at 120%, 115%, 



and 110% Total Cas Saturation at 12 C, and Observed Signs of 



Gas Bubble Disease' (Test 3). 



60 80 100 150 200 300 



TIME (HOURS) 



FIG. 7 Time to 50% death of Daphnia magna at 130% and 120% 

 total dissolved gas saturation. 



were suspended in each exposure tank, and five 

 Daphnia, 10 days old, were placed in each cage. 

 About half the Daphnia were beginning to develop 

 eggs in the brood pouch. 



The first death occurred after 48 hr at 120% 

 (Table 5), and air was observed in the gut. One 

 Daphnia, the only one observed during these tests, 

 had a small bubble in the body fluid of the head 

 next to the eye. Fifty percent were dead after 93 hr 

 at 120%; bubbles were observed in the gut and on 

 the carapace of several survivors. The food eaten by 

 Daphnia was unable to pass through the gut when 

 air bubbles were present, and many Daphnia 

 apparently died of starvation. At 115% saturation 

 30% had died when the test was terminated after 

 170 hr of exposure. Eighty-five percent were carry- 

 ing eggs or young after 96 hr. Mean time to 20% 

 death was determined to be 137 hr. 



At 110% the first and only death occurred after 

 48 hr, but there was no evidence of GBD and it had 

 young in the brood pouch. Sixty percent were carry- 

 ing young after 96 hr of exposure. Only 10% were 

 dead when the test was terminated after 170 hr, 

 but one was at the surface with a large bubble in 

 the brood pouch. 



Lethal threshold concentrations for Daphnia in 

 gas supersaturated water were determined by using 

 the time to 50% and 20% death. The saturation per- 

 centage at which 50% were no longer killed was 

 near 128%. The level at which less than 20% of the 

 Daphnia were killed, a possible safe level, was near 

 111% total dissolved gas (Fig. 8). 



Crayfish Tests 



Test 4 The water in Tank 1 was supersaturated 

 at 125% total gas saturation and used both for test- 

 ing and as a water source for siphoning into Tank 1a 



where crayfish were tested with young steelhead. 

 Tank 2 was used as a control and contained satu- 

 rated water. All tanks were maintained at 12°C. 

 Crayfish were placed in the net cages in Tank 1 

 (Fig. 1) and in Tank 1a; water flowing at a rate of 

 9 C/min maintained Tank 1a at 125%. Four large 



58 Nebeker, Stevens, Brett 



