The small screen cages with petri dish bottoms were 

 ideal for Daphnia as they provided minimal current, 

 and food retention was not a problem. Water flow 

 through the cages was sufficient to circulate the 

 test water and maintain gas levels but not disturb 

 the Daphnia. The cages were hung on the outer lip 

 of the tank in the open current (Test 1), sheltered 

 from the current (Test 2), and on the net cages in 

 the center of the tank (Test 3) where the water 

 velocity was less than 5 cm/sec. 



Crayfish Test Methods 



Crayfish used in Test 4 were collected from 

 Beaver Creek in Benton County, Oregon; those in 

 Tests 5, 6, and 7 were collected from the Alsea River 

 in Benton County, Oregon. Before testing, crayfish 

 were held in aquaria with aeration and continual 

 flow-through of water and were fed young salmon 

 that had died in other supersaturation tests. The 

 test gas levels were set at the desired percentage, 

 and the crayfish were taken directly from the hold- 

 ing tanks (11 to 12°C) and placed in the open tanks, 

 in the net cages, in the large screen cages, or in 

 Tank 1a, depending on test needs. 



Observations on the Branchiobdellid Annelid 

 worms living on the crayfish exoskeleton were 

 recorded but no detailed tests were conducted with 

 them. 



Fish Test Methods 



Twenty young steelhead trout (Salmo gairdneri) 

 were tested for 2 weeks with crayfish and aquatic 

 insects at 125%. Time to death was determined, and 

 weight, length, and sublethal signs of GBD were 

 recorded. Five young sockeye salmon were also 

 tested with crayfish at 140%. 



Insect Test Methods 



The stoneflies used were collected from the 

 Calapooia River, 5 miles upstream from Holly, Linn 

 County, Oregon, and were placed in holding tanks 

 with flowing water and adequate water movement. 

 They were acclimated to test water and temperature 

 (12°C) for at least 24 hr before testing. Three 19-C 

 aquaria, 17.5 cm deep, were used as test chambers 

 for Test 8, two containing water supersaturated at 

 125% and one with saturated (100%) water as a con- 

 trol. Siphon lines delivered water to each aquarium, 

 maintaining adequate water movement to simulate 

 stream flow required by stoneflies. Rocks and sticks 

 were placed on the aquaria bottoms for substrate. 



Test 9 was conducted in the large tanks. Two 

 cylindrical stainless steel wire cages (20 cm in diam- 

 eter by 25 cm high) were suspended near the water 

 surface in the open current (21 cm/sec) in each tank 

 so water movement through the cage would be ade- 



quate. The screen served as a suitable substrate so 

 no additional material was placed in the cages. 



RESULTS 



Daphnia Tests 



Test 1 Young non-egg-carrying adults were tested 

 for 96 hr at 140%, 130%, 120%, and 100% total dis- 

 solved gas at 12°C (Table 1) and were held in the 

 20 by 25 cm wire cages. Ten Daphnia were placed 

 in each cage, with one cage immersed in each of 

 the four exposure tanks. 



Time to 50% death (Table 2) determined at 140% 

 total dissolved gas was 71 hr. When the test was 



TABLE 2 Mean Time to 50% and 20% Death for 

 Daphnia, Steelhead, Crayfish, and Insects 



(Steelhead) 



(Crayfish) 



(Crayfish) 



125 



125 



150 



50 



22 



35 



•Not achieved — Insufficient number of deaths. 



'Determined from straight-line plots on log-probit graph paper. 



-'130% animals were accidentally killed. 



54 Nebeker, Stevens, Brett 



