TABLE 3 Test III, Percent Cumulative Mortality Due to C-as Bubble Disease in Juvenile Chinook 

 Salmon Exposed to 120% to 128% Supersaturation at Rock Island Forebay, June 28 to July 18, 1974. 



tindccates hr/day at 0-1 m depth, | indicates hr/day at 3-4 m depth 



The fish in the intermittent-exposure cages 

 suffered much higher mortalities during the third 

 test than during the two previous tests. Almost all 

 of these mortalities occurred during the first half 

 of the test. In the cage occupying the to 1 m depth 

 for 16 hr/day, the fish began to die with evidence 

 of GBD on the 1st day (Fig. 7). This mortality 

 reached 45% by the 3rd day and then leveled off. 

 At 9 days the mortality in this cage was 68% and 

 this only increased to 70% at the end of 20 days. 

 The mortality in the 12 hr/day cage was similar to 

 that of the 16 hr/day cage, but at a lower rate. In 

 the 12 hr/day cage, the mortality was 20% by the 

 3rd day and 35% by the 9th day. The final cumula- 

 tive mortality in this cage was 39%. The first death 

 in the 8 hr/day exposure cage occurred on the 3rd 

 day. This cage suffered a much lower mortality 

 than the other intermittent-exposure cages at only 

 7% in the first 10 days. No additional mortalities 

 occurred in the 8 hr/day exposure cage. 



A number of wild fish residing in the area of 

 the live cages were examined for signs of GBD. 

 One or more juvenile chinook salmon about 40 to 

 80 mm long were trapped on top of the deeper live 



cages each day as the cages were raised to check 

 for mortalities. These small chinooks were residing 

 around the live cages from late May to the end of 

 the 3rd test on July 18. Three-spined sticklebacks 

 (Gasterosteus aculeatus) were also common around 

 the live cages. These fish were occasionally trapped 

 on top of the cages, but many were captured with a 

 dip net when they were at the reservoir's surface. 

 As many as 15 sticklebacks were captured at one 

 time in the evening when they were normally found 

 at the surface. None of the juvenile chinooks or the 

 sticklebacks captured adjacent to the live cages 

 showed any signs of GBD. It is uncertain how many 

 fish were checked as the fish were returned to the 

 reservoir and some were undoubtedly captured 

 more than once. 



DISCUSSION 

 Objective 



The major objective of this study was to test 

 the effects of naturally occurring moderate levels 

 of dissolved gas supersaturation (120 to 130%) on 

 juvenile salmonids. This objective was achieved in 

 field experiments consisting of one 10-day test and 



Live Cage Bioassays at Rock Island 31 



