100 



«c 80 - 



u 



en 



60 



3 40 



20 



100 



20 



40 



60 80 100 



EXPERIMENTAL DAY 



120 



140 



FIG. 2 Mortality versus time curves (combined replicates cor- 

 rected for mortality of control tests) for juvenile fall chinook ex- 

 posed to various concentrations of dissolved atmospheric gas in 

 shallow (0.25 m) and deep (2.5 m) water tanks at IOC. 



and the two groups at 127% in the deep tanks aver- 

 aged 25% mortality (Fig. 3). Control mortality was 

 2% and was not used to adjust test mortality curves. 



Progression of Gas Bubble Disease 



The frequency of occurrence of most gas 

 bubble disease signs increased with increasing 

 levels of supersaturation. The fall chinook mor- 

 talities incurred the highest incident rate for 

 cutaneous blisters on the head and mouth and for 

 occlusion of gill filaments. Mortalities from 120% 

 and 115% shallow and from 127% deep tanks 

 showed 40-70% incidence of these signs. Other 

 signs which increased with increasing levels of 

 supersaturation but with lower frequencies of 

 occurrence were: heart occlusions (14 to 34%), 

 blisters in the connective tissue surrounding the 

 eye, and blisters between the fin rays (Fig. 4). 



In research by Dawley and Ebel (1974), the 

 appearance of gas emboli in the lateral line was 

 the first external sign of gas bubble disease to 

 develop on spring chinook exposed to various 

 levels of supersaturation. We, however, observed 

 that gas emboli in the lateral line of both fall chi- 

 nook and steelhead trout mortalities were not 

 prevalent within any group, but did appear in high 

 percentage (50-100%) on the biological (live) sub- 

 samples from all test groups. Scattered bubbles (less 

 than 15% of the lateral line) also appeared on most 

 mortalities from the control groups. We cannot 

 account for this observation and for that reason only 



3 4 5 



DAYS EXPOSURE 



FIG. 3 Mortality versus time curves for juvenile steelhead 

 exposed to various concentrations of dissolved atmospheric 

 gas in shallow (0.25 m) and deep (2.5 m) water tanks at IOC. 



100 



52 

 to 



I 



UJ 



u 



cc 



A HEAD 



• GILLS 

 O MOUTH 



* FINS 

 x HEART 

 + EYE 



v ABDOMEN 



A BODY SURFACE 



▼ LATERAL LINE 





 100 



105 110 115 120 



TEST LEVEL (% OF SATURATION) 



125 



FIG. 4 Frequency (%) of dead juvenile chinook bearing gas 

 bubble disease signs from shallow (0.25 m) test tanks at various 

 levels of dissolved gas. 



ascribe this sign to gas bubble disease when more 

 than 15% of the lateral line appears occluded. 



Disease signs on live fish sampled generally 

 appeared in rates and patterns similar to those 

 recorded on dead fish removed from the same test 

 concentrations. However, emboli in branchial 

 arteries, gill filaments, and the heart were rarely 

 observed on live subsamples, but were prevalent 



Long-Term Exposure of Salmonids 5 



