Z75 



*» 



150 



£ 100 - 



-<±-4 



END OF TEST 



""«, 



DA PHNIA 



TABLE 6 Time to Death of Young Steelhead Trout Tested 



Concurrently with Crayfish and Insects to Verify Lethality of 



Test Water at 125% Total Gas Saturation (Test 8)' 



110 120 130 140 



PERCENT SUPERSATU RAT I ON 



150 



FIG. 8 Threshold concentration determined for Daphnia magna 

 by using times to 50% and 20% mortality to plot the curves. 



crayfish (30 to 40 g) were placed in net cage C of 

 Tank 1 and 10 medium-sized crayfish (20-30 g) 

 were placed in net cage D. Three medium-sized and 

 11 small crayfish (10 to 15 g) were placed in Tank 1a 

 (with steelhead). They were exposed for 12 days. 



Test 4 was a survey test at 125% total gas 

 saturation to compare the susceptibility of crayfish, 

 insects, and fish. No signs of GBD were observed in 

 crayfish, but the fish died rapidly at 125% (Table 6) 

 and all but one were dead after 12 days. 



Test 5 Crayfish were tested at 150% total gas 

 saturation to determine their tolerance and to 

 expose 10 animals at a high gas level and observe 

 signs of GBD in individual animals. The test expo- 

 sure lasted 6 days, at which time all animals had 

 died. 



One male died after 23 hr and was necropsied 

 to determine signs of GBD. Gills were slide- 

 mounted along with those of a control animal that 

 was sacrificed. Bubbles were readily apparent in 

 the gills of crayfish from 150%, but no bubbles were 

 observed in gills of the control animal. Three more 

 male crayfish (No. 2, 3, 4) were moribund after 40 hr. 

 They were removed from supersaturated water and 

 placed in saturated water to observe them for pos- 

 sible recovery. The others remaining in the tank 

 were sluggish and unresponsive. Bubbles were in 

 the body fluids of the moribund crayfish and they 

 could be seen easily through membranous joints 

 (Fig. 9) and the ventral abdominal sternites. There 

 were no bubbles in the control animals. The three 

 moribund crayfish (No. 2, 3,4) showed no recovery 



'WFTS Test No. 8. 



2 No crayfish or insect deaths occurred during 

 the time period of this test at 125%. 



in saturated water after 24 hr; one was dead, but 

 there were still a few slight movements of abdomi- 

 nal appendages in the other two. 



Crayfish No. 5 was moribund after 47 hr and 

 was sacrificed for necropsy. There were bubbles 

 under the membrane between the carapace and 

 abdominal tergites (Fig. 9). The body cavity was 

 opened, and many bubbles were found in the body 

 fluids, and lodged between the internal membranes, 

 organs, and muscles; there were also bubbles in the 

 heart. Bubbles were very conspicuous in the gills 

 (slide mount) and the main gill rib (Fig. 10). Bubbles 

 were also obvious in the body fluid of the plates 

 protecting and lying with the gills. 



Effects on Freshwater Invertebrates 59 



