Effect Above and Below 125% Saturation 



Although quite constant levels of supersatura- 

 tion were experienced during the three tests, the 

 slight variations in the supersaturation of the 

 reservoir were significant, as demonstrated by the 

 resulting mortalities. During the first test and the 

 first half of the second test, the supersaturation 

 remained below 123%. Almost all mortalities during 

 this time occurred in the to 1 m fixed-depth cage. 

 For the second half of Test II and the first half of 

 Test III, the supersaturation rose to between 124% 

 and 128%, dropping to 123% on only one day. 

 During this period of higher gas content, mortali- 

 ties occurred in most of the cages. Again during the 

 second half of Test III, few mortalities occurred in 

 any cage except the 1 to 2 m fixed-depth cage. The 

 level of supersaturation during this last period had 

 again dropped to between 120% and 125%. 



These results indicate that juvenile chinook 

 remaining below a depth of 1 m can only be ex- 

 pected to suffer significant mortalities over a 20- 

 day period when supersaturation is above 123%. 

 Fish held within 1 m of the surface for no more than 

 16 hr/day also suffered significant mortalities only 

 when the supersaturation rose to about 125% or 

 higher. At supersaturations of about 125% and 

 higher there was a marked increase in mortalities 

 of fish which spent 8 hr/day or more within 1 m of 

 the surface, and also in fish which were held con- 

 tinuously between 1 and 2 m. These mortalities 

 were reduced in fish which received not more than 

 16 hr/day of surface exposure when the super- 

 saturation dropped below 125%. Fish held between 

 1 and 2 m appear to suffer mortalities only after 

 exposures of 9 or 10 days at supersaturations near 

 125%. This is indicated by comparing the mortalities 

 in the 1 to 2 m fixed-depth cage of Tests II and III. 

 In Test II only a single mortality occurred in the 

 1 to 2 m cage on the last day after 10 days of expo- 

 sure to supersaturations between 124% and 126%. 

 In Test III the mortalities in the 1 to 2 m cage began 

 on the 9th day following exposures to supersatura- 

 tions of 125% to 128%. 



The percentage of survivors having signs of 

 GBDin the intermittent-exposure cages at the end 

 of Tests II and III is another indication of the effects 

 of supersaturations below 125% as contrasted to 

 those above 125%. At the end of Test II, GBD lesions 

 were present in 15%, 24%, and 69% of the survivors 

 from the 8 hr/day, 12 hr/day and 16 hr/day near- 

 surface exposure cages, respectively. The super- 

 saturation was low (120 to 123%) during the first 

 10 days of Test II, but higher (124 to 126%) during 

 the last 10 days of this test. At the end of Test III, 

 signs of GBD were absent in the survivors from the 

 8 hr/day and 12 hr/day exposure cages, and signs 

 were present in only 4% of the survivors from the 



16 hr/day exposure cage. The supersaturation was 

 between 125% and 128% during the first 10 days of 

 this test, but dropped to between 120% and 125% 

 during the last 10 days. A comparison of these two 

 tests indicates that fish spending any appreciable 

 time in water less than 1 m deep are affected by 

 supersaturation above 125%, but recover rapidly 

 when it drops below 125%. 



As indicated above, there appears to be a sig- 

 nificant difference in the effects of supersaturations 

 above and below about 125%. Above 125% the 

 effects of supersaturation are much greater pro- 

 ducing GBD raster in a greater percent of test fish 

 and in fish held in deeper water. Supersaturations 

 below about 125% appear to produce GBD only in 

 fish held within 1 m of the surface at least for expo- 

 sure periods of about 10 days. 



Severe Mortality in to 1 Meter Cage 



The severe mortality that occurred in the to 

 1 m fixed-depth cage of Test III cannot be fully 

 explained. This 100% mortality within 3 days 

 occurred at supersaturations (125 to 128%) that were 

 only slightly higher than the supersaturations 

 (123 to 126%) present during the last 10 days of 

 Test II. The final cumulative mortality for the same 

 cage in Test II was 88%. 



There are several factors that may have influ- 

 enced the sensitivity of the Test III fish. They may 

 have been stressed more than the fish used in pre- 

 vious tests due to high atmospheric temperatures 

 during their transport to the test site. There was, 

 however, no indication that the fish were stressed 

 when they were placed in the cages. The fish in 

 Test III were slightly larger than those of Test II. It 

 is doubtful that this slight size difference was suffi- 

 cient to alter the sensitivity of the test fish to pro- 

 duce the great differences between Tests II and III 

 mortalities. 



The only real differences observed between the 

 two tests are the supersaturations at the beginning 

 of each test and the water temperatures during the 

 tests. The water temperatures during the first few 

 days of Test III were less than 1°C higher than dur- 

 ing the last few days of Test II. It seems unlikely 

 that this slight increase in temperature would have 

 been responsible for the much higher mortality 

 rate during the first few days of Test III. Test II had 

 lower supersaturation initially (120%), rising to the 

 higher levels only during the last half of the test; 

 higher levels of supersaturation (125 to 126%) 

 were present at the beginning of Test III. The fish 

 may have been seeking a way out of the cage dur- 

 ing the first few days, and were therefore spending 

 more time near the top of the cage at the time of 

 the high supersaturations in Test III, whereas by the 

 time the highest supersaturations occurred in 



Live Cage Bioassays at Rock Island 33 



