100 105 110 115 120 



TEST LEVEL (% OF SATURATION) 



124 127 



FIC. 8 Mean depths of juvenile chinook groups in 2.5 m deep 

 tanks at dissolved gas concentrations of 100, 105, 110, 115, 120, 

 124, and 127% of saturation— averaged for 3 segments of the 

 day over 30 days. 



100 105 110 115 120 



TEST LEVEL (% OF SATURATION! 



124 127 



FIG. 9 Mean depth of juvenile steelhead groups in 2.5 m deep 

 tanks at dissolved gas concentrations of 110, 115, 120, and 

 127% of saturation— averaged for 2 segments of the day over 

 15 days. 



higher dissolved gas concentration noted during 

 daytime observations also occurred at night. 



Effect of Gas Supersaturation on 

 Condition Factor 



Weight and length data obtained from the 

 live subsamples and the fresh mortalities were 

 used to calculate a condition factor "K" where: 



w (weight i n grams) x 10 s 

 L 3 (fork length in millimeters) 



These were examined to 1) determine if certain 

 size portions of the population were more suscepti- 

 ble to gas bubble disease and, 2) detect any 

 variations in growth caused by chronic exposure 

 to the various levels of saturation. 



Condition factors of mortalities occurring within 

 a 16-day range of the monthly subsamples (e.g., day 

 of subsample ± 8 days) were compared, by means of 

 a student's T-test, with the mean condition factor of 

 these same subsamples. The mortalities in the 110, 

 115 and 120% saturation shallow tanks and in the 

 124 and 127% deep tanks had condition factors sig- 

 nificantly higher than the live subsamples (t = 3.78, 

 32 df, P < 0.001, t = 3.87, 27 df, P < 0.001; for deep 

 and shallow tanks, respectively). Thus, the larger 

 fish with higher condition factor died at a signifi- 

 cantly higher rate at these concentrations. 



Recovery From Gas Bubble Disease 



At completion of the freshwater phase of testing, 

 chinook and steelhead groups still surviving re- 

 tained signs of gas bubble disease similar to those 

 noted on the monthly subsamples (described earlier). 

 Subsamples of chinook tested at 110% of saturation 

 in shallow tanks and at 110, 115 and 120% in deep 

 tanks were placed in fresh water at 100% saturation 

 for recovery observations. A portion of each of these 

 test groups had sustained significant mortalities 

 from gas bubble disease, other groups not included 

 in the recovery tests had no observable signs of gas 

 bubble disease. All subsamples sustained mortalities 

 from 10-16% in the 2-week recovery period (group 

 size 27-48 fish). However, these mortalities could 

 not be attributed to gas bubble disease. After 2 

 weeks, the survivors no longer exhibited outward 

 signs with exception of one fish with a hemorrhaged 

 eye and another with bubbles in the orbit. 



Eleven steelhead surviving the deep test tanks 

 set at 127% saturation were placed in a shallow 

 tank at 100% saturation. After 3 days, examina- 

 tion indicated that cutaneous blisters had 

 decreased both in size and number, (e.g., 5 mm 

 blisters had decreased in size to 2 mm and 20 

 blisters on the operculum decreased to 4). These 

 fish were subsequently placed in water at 105% 

 of saturation and all signs remained the same after 

 another 4 days, at which time fish were released. 

 Mortality did not occur in the 7-dayrecovery period. 



Effects of Transfer to Salt Water 



Subsamples of survivors from combined 

 replicates of all test groups (chinook and steel- 

 head) were placed into salt water at 25 ppt salinity 

 at 10°C, to determine whether prior exposure to 

 various levels of dissolved gases affected the 



Long-Term Exposure of Salmonids 7 



