B^HHD 



FIG. 11 Severe signs of gas bubble disease with exophthalmia 

 of the left eye, accompanied by hemorrhaging and bubbles 

 around the eye. 



exophthalmia was present in about 20% of the fish 

 with GBD lesions. Meekin (1974) found exoph- 

 thalmia in about 5% of the juvenile chinook suc- 

 cumbing to GBD in a similar study. It occurred both 

 as the only sign of GBD, and with other signs of 

 GBD such as hemorrhages and bubbles (Fig. 12). 

 Hemorrhages occurred most frequently at the base 

 of the paired fins, but also occurred around the 

 eyes, in the jaws, gills and opercles, and occasion- 

 ally at the base of the anal and dorsal fins. Hemor- 

 rhage at the base of the caudal fin was not ob- 

 served. Those fish possessing hemorrhages, exoph- 

 thalmia and/or extensive bubbles on the head 

 were considered to have severe GBD lesions. 



Although many of the mortalities displayed 

 severe GBD lesions, it was not uncommon for fish 

 with only slight or moderate indications of GBD to 

 die. It appears that the severity of the external 

 lesions is not a good indicator of the length of sur- 

 vival or of probable mortality. Some fish with 

 extremely severe external signs of GBD survived 

 the 20-day test and recovered. 



Recovery from GBD 



The apparent ability of fish exhibiting GBD 

 lesions to recover has been reported in several 

 studies (Gorham, 1901; Rucker and Hodgeboom, 

 1953; Rukavina and Varenika, 1956, and Ebel 

 et al., 1971). Pauley and Nakatani (1967), however, 

 questioned the apparent ability of fish to recover 

 stating that recovery was probably related to the 

 level of supersaturation, duration of exposure, and 

 water temperatures, as well as the size and species. 

 The destructive tissue changes that Pauley and 

 Nakatani described as part of the early stages of 

 GBD are inconsistent with recovery of the fish. 



Due to the significance of recovery of fish with 

 GBD, the 3 to 4 m fixed-depth cage was converted 

 to a recovery cage in Tests II and III. The fish placed 



FIG. 12 Exophthalmia, gas bubbles, and hemorrhaging around 

 the eye of a juvenile chinook salmon with severe signs of gas 

 bubble disease. 



in this cage during each test possessed a variety of 

 GBD lesions ranging from a few bubbles in one fin 

 to extensive bubbles in most fins and the head, as 

 well as hemorrhages and exophthalmia. Most of 

 the fish placed in the recovery cage displayed at 

 least moderate GBD lesions (bubbles present in 

 several fins). 



The recovery of fish with evidence of GBD is 

 also indicated by the survivors of the to 4 m voli- 

 tion cage in Tests II and III. At the end of Test II, 

 following 10 days of supersaturation between 123% 

 and 126%, GBD lesions were present in 33% of the 

 survivors. During Test III saturations ranged from 

 125% to 128% during the first 10 days and then 

 dropped to between 120% and 125%. Only 8% of the 

 survivors of the to 4 m cage showed evidence of 

 GBD at the end of Test III. If saturations of 123% 

 to 126% for 10 days produced GBD lesions in 33% of 

 the to 4 m cage fish during Test II, it is likely that 

 a similar portion of the fish were affected during the 

 first 10 days of Test III when saturations were even 

 higher. If a third of the fish were affected during 

 the first half of Test III, then most of them subse- 

 quently recovered and lost all external evidence of 

 GBD during the last 10 days of this test when satura- 

 tions did not exceed 125%. 



A few mortalities did occur in the recovery cage 

 during both Test II and III (10% and 13%, respec- 

 tively). The dead fish for the most part did not 

 appear to have died as a direct result of GBD as did 

 the majority of mortalities in the other cages. Most 

 of the mortalities in the recovery cage developed 

 fungal infections of the caudal fin (Fig. 13) which 

 were apparently secondary infections due to gas 

 bubble damage. The remaining mortalities in the 

 recovery cage either showed hemorrhage in the 



Live Cage Bioassays at Rock Island 35 



