on mortalities examined immediately after death, 

 indicating these signs are directly associated with 

 the death of the animal. 



The trends in gas bubble disease signs noted 

 on the steelhead differed from those recorded dur- 

 ing tests with the fall chinook salmon. Two differ- 

 ences were: 



1. Heart and gill emboli occurred in almost 

 100% of the dead steelhead, whereas these signs 

 were rarely noted in dead chinook, suggesting that 

 decomposition of the fall chinook quickly masked 

 these signs. 



2. Incidence rates of certain signs were 

 directly associated with duration of the test. Live 

 and dead steelhead at test termination showed 

 light incidence of exophthalmia, cutaneous bubbles 

 on the head and in the buccal cavity, and no signs 

 of bubbles on the body surface, whereas the chi- 

 nook, subjected to supersaturation for a much longer 

 duration, showed high incidence of these signs. 

 Also, gas bubbles between the fin rays were not 

 prevalent on fall chinook, yet showed very high in- 

 cidence on steelhead mortalities and live subsamples 

 after 7 days exposure to high supersaturation. 



Effects of Water Depth 



When mortality rates in the deep tanks are 

 compared to those in the shallow tanks (Fig. 2 

 and 3), the average depth of the chinook and steel- 

 head groups in the deep tanks appears to have 

 compensated for about 10% and 10-15%, respec- 

 tively, of effective saturation. Fig. 5 shows that 

 the time to 25% mortality of fall chinook at various 

 levels of dissolved gas concentrations in the deep 

 tanks were comparable to time in the shallow 

 tanks at a 9.5 to 10% lower effective saturation (i.e. 

 25% mortality was reached at 30 days of exposure 

 in the deep tanks at 124% and in the shallow tanks 

 at 115%). Also a comparison of incidence and 

 degree of G.B.D. signs on dead chinook between 

 deep versus shallow tanks indicates an effective 

 decrease in supersaturation of 12 to 15% (Fig. 6). 



Vertical distributions of chinook groups for 

 the first 3 days of the test were variable and not 

 significantly different from one gas level to the 

 next. After 3 days, however, the fish groups at 

 higher saturation levels maintained a greater 

 depth than those at the lower levels and main- 

 tained this difference the entire test period (Fig. 

 7). Steelhead freshwater tests showed similar 

 results, i.e, mean depth increased with increasing 

 gas concentrations. 



Night observations showed a depth shift 

 downward of approximately 0.3 m (0.18-0.42 m) 

 for each of the test species at each saturation level 

 (Fig. 8 and 9). The increased depth trend with 



6 Dawley, Schiewe, Monk 



FIG. 5 Exposure times to 25% mortality of juvenile chinook 

 held at various levels of gas in deep (2.5 m) versus shallow 

 water (0.25 m) tanks. 



110 115 120 



TEST LEVEL (% OF SATURATION OF T0GI 



FIG. 6 Frequency (%) of dead juvenile chinook bearing 

 selected gas bubble disease signs from shallow (0.25 m) versus 

 deep (2.5 m) test tanks at various levels of dissolved gas. 



FIG. 7 Mean depth during daylight hours of groups of juvenile 

 chinook held in 2.5 m deep tanks at concentrations of 100, 105, 

 110, 115, 120, 124, and 127% of saturation— averaged for periods 

 of 10 days over 127 days. 



