TABLE 2 Test II, Percent Cumulative Mortality Due to Gas Bubble Disease in Juvenile Chinook 

 Salmon Exposed to 120% to 126% Supersaturation at Rock Island Forebay, June 5-25, 1974. 



Holding 

 time 



(days) 



Cage depth (meters) 



0-4 m 0-3 m 



0-2 m 



0-1 m 



1-2 m 



2-3 m 



Recovery 

 3-4 m 



0-1 m and 3-4 m' 



t" ; { 



12 112 



t 12 + 



t» I 16 



1 



2 



3 



4 



5 



6 



7 



8 



9 



10 



11 



12 



13 



14 



15 



16 



17 



18 



19 



20 



Survivors 

 with CBD 

 lesions (%) 



38 



64 



77 



91 



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

 •Approximately 1/2 of the fish lost through small tear in cage. 



69 



24 



15 



days 5 and 14 during Test II was very close to that 

 of the same cage between days 2 and 10 during 

 Test I. The mortality rate in the to 1 m cage of 

 Test II appeared essentially the same as that of the 

 same cage in Test I, except that it began about 

 3 days later. A cumulative mortality of 53% was 

 reached on day 10 of Test I, and a similar mortality 

 of 54% was reached on day 13 of Test II. 



The mortality rate in the to 1 m cage de- 

 creased during the last 6 days of Test II. From 

 day 14 to the end of the test, the cumulative mortal- 

 ity only increased from 72% to 88% All but one of 

 the fish remaining in the to 1 m cage at the end of 

 the test showed signs of GBD. 



Unlike the first 10-day test, mortalities oc- 

 curred in a number of the other cages during Test II. 

 These mortalities began to occur prior to 10 days, 

 but at a very low rate (Table 2). The to 3 m cage 

 lost one of 200 fish on the 8th day. No other mor- 

 talities occurred in this cage until the 16th day. No 

 mortalities occurred in the to 4 m cage during 

 Test II. The size of the test group in the to 4 m cage 

 was reduced around the 18th day when slightly 



more than half of the group was lost through a 

 small tear in the netting. The to 3 m cage fish 

 experienced a few mortalities during the last 

 5 days with a final cumulative mortality of 3%. 

 Mortalities began to occur in the to 2 m cage on 

 the 14th day and increased to 17% at the end of 

 Test II. 



A portion of the survivors in each of the three 

 volition cages showed signs of GBD. In the to 4 m 

 cage 38% of the survivors showed indications of 

 GBD, as did 64% of those in the to 3 m cage and 

 77% in the to 2 m cage. 



Only a single mortality occurred in each of the 

 two fixed-depth cages between 1 and 3 m. In the 



1 to 2 m cage a fish died on the last day, and in the 



2 to 3 m cage a mortality occurred on the 13th day. 

 No survivors in the 1 to 2 m cage showed signs of 

 GBD; however, 2% of those in the 2 to 3 m cage had 

 GBD. 



Some mortalities occurred in each of the 

 intermittent-exposure cages which were raised to 

 a depth of to 1 m in Test II rather than to 1 to 2 m 

 as in Test I. In the cage spending 16 hr/day at the 



Live Cage Bioassays at Rock Island 29 



