study conducted in the Rock Island forebay by the 

 Chelan County PUD in August 1974, showed that 

 dissolved gas levels change by about 3% of satura- 

 tion during a 24-hr period. The lowest gas levels 

 during this 24-hr period occurred between 0300 and 

 0900 hr, which includes the time that the gas 

 content was measured during the live-cage tests. 

 Some daily variation in the gas levels observed in 

 the live-cage bioassay is due to differences in the 

 water temperature which varied according to the 

 time of day during which measurements were taken. 



A number of times during the tests, dissolved 

 gas supersaturations were measured inside some 

 of the cages. This was done to detect any measur- 

 able effect of the cages on the gas content of the 

 water as it passed through the cages. The gas con- 

 tent of the water inside the cages was always lower 

 than that of the water outside of the cage by less 

 than 1% of saturation. 



Dissolved oxygen levels were measured by 

 the Winkler method. Water temperatures were 

 measured at the surface using a mercury thermom- 

 eter accurate to i0.1°C. Light penetration readings 

 were taken daily adjacent to the cages using a 

 Secchi disc. 



An attempt was made to record the fish 

 depths inside the to 4 m volition cage. A Benmar 

 DR-681* echo sounder with a minimum range of 

 to 50 ft was used to make the recordings. The 

 DR-681 was the only shallow water echo sounder 

 that could be acquired for these tests. No useful 

 recordings were obtained with the echo sounder 

 due to apparent interference from the cages. 



As the depth distribution of the fish in the 

 volition cages was of considerable importance, an 

 attempt was also made to observe the fish using 

 scuba gear. Even with a strong underwater light, 

 the diver was unable to observe any of the fish due 

 to the turbidity of the river water. 



RESULTS 



Test I 



The fish for the first live cage bioassay were 

 placed into the cages in the Rock Island forebay on 

 May 24, 1974. At this time, the total dissolved 

 gas saturation in the river water was 122%. During 

 the first 4 days of the study the saturations remained 

 near 122%. The saturations then dropped to about 

 120%, and remained near 120% for the remaining 

 6 days of the test. The total dissolved gas levels 

 measured at the study location are shown in Fig. 4. 

 Saturometer readings were also taken inside the 

 cages during all three tests. Saturations inside the 

 cages were not more than 1% of saturation lower 

 than the readings for the surrounding water. 



The water temperatures which were recorded 

 daily during Test I ranged from 10.4°C to 10.9°C. 



DAYS 



FIG. 4 Test I, total dissolved gas levels and % cumulative 

 mortalities for the to 1 m cage, May 24 to June 3, 1974. No 

 mortalities occurred in the other cages. 



These temperatures were recorded between 0800 

 and 1000 hr. Some days surface water tempera- 

 tures were a degree or more higher later in the day. 

 Secchi disc readings taken during this test ranged 

 from 0.91 to 1.22 m. 



In Test I no fish were lost due to handling 

 stress. The first mortaility occurred during the 2nd 

 day in the to 1 m fixed-depth cage. The mortality 

 rate increased somewhat in the to 1 m cage after 

 the 4th day, reaching a cumulative mortality of 53% 

 at the end of 10 days. A single mortality occurred 

 in the 2 to 3 m fixed-depth cage during this test. 

 This single mortality was obviously due to trauma, 

 not to GBD. No mortalities occurred in any of the 

 other cages during this 10-day test. The percent 

 cumulative mortality during this test is shown in 

 Fig. 4, and Table 1 lists the daily cumulative mor- 

 tality for Test I. 



The surviving fish were examined for signs of 

 GBD at the end of the test. Ninety percent of the 

 survivors in the to 1 m cage showed some signs 

 of GBD. These varied from a few bubbles in one fin 

 to bubbles in most fins, the head, and mouth, as 

 well as hemorrhaging at the base of the fins. In the 

 1 to 2 m cage 15% of the fish showed signs of GBD. 

 Only 3% of the fish in the to 2 m cage showed 

 signs. In both cages the signs of GBD were re- 

 stricted to bubbles in the caudal fins. 



Live Cage Bioassays at Rock Island 27 



