surface and 8 hr/day at 3 to 4 m the first mortality 

 occurred on the 9th day with the cumulative mor- 

 tality reaching 12% by the end of the 20-day test. 

 Mortalities did not occur until the 18th day in the 

 other two intermittent-exposure cages. The cumu- 

 lative mortalities at the end of the test were 4% in 

 the cage spending 12 hr/day at the surface and 1% 

 in the cage spending 8 hr/day at the surface. 



The portion of the survivors showing signs of 

 GBD was related to the length of the surface expo- 

 sure. Signs of GBD were found in 69% of the sur- 

 vivors in the cage spending 16 hr/day at the sur- 

 face, 24% in the 12 hr/day exposure cage, and 15% 

 in the 8 hr/day exposure cage. 



In Test II the 3 to 4 m cage was used to test the 

 recovery of fish from Test I that showed signs of 

 GBD. These fish began to suffer mortalities on the 

 7th day, and had a 10% cumulative mortality after 

 20 days. None of the fish in the recovery cage died 

 after the 12th day. 



These mortalities appeared to be due to severe 

 fungal infections resulting indirectly from GBD. 

 All dead fish had the caudal fin completely eroded, 

 and were covered with fungus as far forward as the 

 anus and dorsal fin. Most of the fish placed in the 

 recovery cage lost all signs of GBD by the end of 

 Test II. Only 9% of the surviving fish in this cage 

 showed any signs of GBD when they were released. 

 The remaining signs were slight cases of exoph- 

 thalmia. Bubbles and hemorrhages were not ob- 

 served in any of these fish at the time of their 

 release. 



Test III 



The supersaturation of the water at the test 

 site in Rock Island forebay at the initiation of the 

 third test was much higher than at the beginning of 

 the two previous tests. During the first 9 days the 

 supersaturation dropped slightly and ranged from 

 120% to 125% at the end of the test. 



During the third test, water temperatures 

 were also higher than they had been during the two 

 previous tests. At the beginning of Test III, the sur- 

 face water was 14.3°C. The temperature continued 

 to show a steady increase throughout the test, 

 reaching 16.1°C at the end. Unlike the previous 

 tests, light penetration showed a steady increase 

 through the third test. Secchi disc readings were at 

 0.76 m at the beginning and reached 1.89 m near 

 the end of the test. 



The initial mortalities occurring in most of the 

 cages during the third test were considerably 

 higher than in the two previous tests. In the to 

 1 m cage 6% of the fish had died with signs of 

 GBD (Fig. 6). Eighty-three percent were dead by the 

 end of the 2nd day, and all test fish had died with 

 GBD lesions by the 3rd day. 



4 6 



10 12 14 16 18 20 

 DAYS 



FIC. 6 Test III, total dissolved gas levels and % cumulative 

 mortalities for the volition and fixed depth cages, June 28 to 

 July 18, 1974. 



Mortalities also occurred in the two fixed- 

 depth cages between 1 and 3 m, but at a much 

 lower rate than in the to 1 m cage. In the 1 to 2 m 

 cage mortalities first occurred on the 9th day of 

 the test (Table 3). In this cage the mortalities then 

 remained constant giving a cumulative mortality 

 of 70% at the end of 20 days. In the 2 to 3 m cage, a 

 single fish died with signs of GBD on the 2nd day. 

 No other mortalities occurred in this cage. Thirty- 

 three percent of the survivors in the 1 to 2 m cage 

 showed signs of GBD, while none of the survivors 

 in the 2 to 3 m cage showed GBD lesions. 



Fish in the shallowest volition cage began to 

 suffer mortalities on the 2nd day (Fig. 6). After a 

 high mortality of 17% between the 2nd and 3rd day, 

 the mortality continued at a relatively constant 

 rate. The cumulative mortality at the end of the 

 test was 61% in this cage (Table 3). In the to 3 m 

 cage, the first mortality did not occur until the 5th 

 day, and only 7.5% had died at the end of the test. 

 No mortalities of any kind occurred in the to 4 m 

 cage during the 20-day test period. Signs of GBD 

 were present in 85% of the survivors in the to 2 m 

 cage, 35% in the to 3 m cage, and in only 8% of 

 the survivors in the to 4 m cage. The 8% with GBD 

 lesions in the to 4 m cage had only slight signs, 

 usually only a few bubbles in a single location. 



30 Weitkamp 



