acquired LC50 data (that level at which 50% died) 

 for the various saturation levels so that specimens 

 could be obtained which would be in progressive 

 stages of the syndrome. 



Each fish was measured and examined for 

 external gross lesions. A gill arch was cut to observe 

 if gas emboli were present within the conus arteri- 

 osis and the ventral aorta. An entire gill arch was 

 removed and examined under a dissecting micro- 

 scope for evidence of gas emboli in the afferent 

 lamellar gill vessels. The body cavity was opened 

 and the heart, kidneys, liver, and internal vessels 

 were examined for the presence of gas emboli and 

 other pathology. Samples of the fins, gills, various 

 internal organs, and the head were preserved in 

 Bouin's solution for histological examination. These 

 observations are the subject of another report and 

 are not included here. 



At 120% supersaturation, 19 fish died during 

 the 54-hr experiment. Those fish which died during 

 the day were examined in the afternoon of that day. 

 Fish that died during the night were examined the 



following morning. There was no more than a 12-hr 

 maximum time between death and necropsy. All 

 dead fish remained in the supersaturated water 

 until necropsied. Necropsy procedures were the 

 same as for the sampled fish. 



RESULTS 



Occurrence of major gross lesions at selected 

 time intervals is presented in Tables. 1,2, and 3. The 

 lesions in fish still alive when sampled are com- 

 pared to lesions in fish that died. Two fish sampled 

 while still alive after 20-hr exposure were in terminal 

 convulsions. These fish contained gas emboli within 

 the afferent gill vessels, heart and major vessels. 

 Only one other fish sampled while alive in 120% 

 supersaturated water contained gas emboli. This 

 fish did not appear to be in terminal convulsions. 

 Gas emboli were in the gills, heart and major ves- 

 sels in all fish dying of GBD, but were not observed 

 in fish sampled alive except for the three mentioned 

 above. Because mortality did not occur in the groups 

 exposed to 115% and 110% supersaturation (after 



TABLE 1 Occurrence of Lesions in Live Steelhead (Salmo gairdneri) Smolts Exposed 



to 120% Supersaturation Level and Sampled After 8, 20, 30, and 42 hr Compared with Fish Dead 



from the Effects of 120% Supersaturation at 18-, 30-, 40-, and 54-hr Exposure 



Number having gas emboli in: 



afferent gill vessels 



conus arteriosis 



atrium 



other vessels or tissues 



2* 

 2» 



18 



16 



7 



4 



Number with: 



exophthalmia 

 hemorrhage from gill 

 fin hyperemia 

 stomach filled with food 

 intestine filled with food 

 excess mucus in intestine 

 other eye pathology 



•Fish in terminal convulsions when sampled. 



Pathogenesis of Gas Bubble Disease 67 



