t 

 <? 



60 - 



40 



s 



20 - 



10 12 14 16 18 20 

 DAYS 



FIG. 7 Test III, total dissolved gas levels and % cumulative 

 mortalities for the intermittent exposure cages, June 28 to 

 July 18, 1974. 



two 20-day tests. The presence of nearly constant 

 levels of supersaturation in the 120% to 130% range, 

 combined with the use of healthy test fish, per- 

 mitted the accurate determination of the biological 

 effects of a narrow range of supersaturation, rather 

 than those resulting from a wide range of super- 

 saturation. The gas levels tested were also of par- 

 ticular significance as they were approximately 

 equal to the highest levels suggested in any pre- 

 vious reports as permissible without producing 

 GBD (Egusa, 1959 and Shirahata, 1966). 



This study concentrated on the mitigating 

 effect that a fish's depth in the water column has 

 on the level of supersaturation which it can tolerate. 

 Since most fish in a reservoir occupy depths greater 

 than 1 m most of the time, they should theoreti- 

 cally be able to withstand higher levels of super- 

 saturation than indicated for depths of less than 

 1 m. The increase in pressure associated with this 

 increase in depth of 1 m reduces the in situ super- 

 saturation value by about 10% (Leman, 1971). Thus, 

 fish deeper than 1 m would be subjected to no more 

 than 110% saturation when the surface saturation 

 is 120%. This apparently explains why fish con- 

 tinue to survive in reservoir areas experiencing high 

 levels of supersaturation well above 130% in 

 surface waters. 



Depth Effect 



The distinct mitigating effects of small differ- 

 ences in depth was demonstrated by these three 

 live cage tests. In Tests I and II, at supersaturations 

 of 119% to 123%, only fish restricted within 1 m of 

 the surface suffered mortalities during the first 

 10 days. In Test II the fish held within 2 and within 



3 m of the surface experienced 17% and 3% mor- 

 talities, respectively, during the last 10 days of the 

 20-day test when the supersaturation varied be- 

 tween 123% and 126%. The number of survivors 

 with signs of GBD was likewise directly related to 

 the depth at which the fish were restricted, and 

 thus their exposure to supersaturation. Those fish 

 exposed to effectively decreased supersaturation 

 levels, due to the greater depth of their cage, had 

 both more survivors and a smaller percentage of 

 survivors with signs of GBD. 



It is highly significant that no fish were killed 

 by supersaturation in the to 4 m volition cage 

 during any of the three tests at saturations of 120% 

 to 128%. The number of fish in this cage with signs 

 of GBD was low, ranging from 0% at the end of the 

 first test to 33% at the end of the second test. The 

 lack of mortalities and low incidence of GBD in this 

 cage is of particular importance, as these are the 

 test conditions most representative of what actually 

 occurs in the reservoirs with unrestricted fish move- 

 ment. Yet it should be noted that even in this to 



4 m cage, fish are confined in shallower water than 

 they normally occupy in the reservoirs according to 

 available depth distribution data. If juvenile chi- 

 nooks are able to tolerate supersaturations between 

 120% and 128% in this cage for a period of 20 days, 

 it is highly likely that they would do at least as well 

 without the restraints of the cage during their 

 normal migration. 



Intermittent-Exposure 



The occurrence of GBD was also directly re- 

 lated to the daily duration of exposure to super- 

 saturation resulting from raising and lowering fish 

 in the water column in the intermittent-exposure 

 cages. As with the deeper volition cages, both mor- 

 talities and the percentage of survivors with signs 

 of GBD were lower than those occurring in the to 

 1 m cage. In both of the 20-day tests, the 16 hr/day 

 near-surface exposure cage had cumulative mor- 

 talities and percentages of survivors with signs of 

 GBD that were more than twice those of the fish 

 receiving only 12 hr/day of exposure at the sur- 

 face. Those fish receiving only 8 hr/day of exposure 

 experienced mortalities and showed evidence of 

 GBD only when the supersaturation ranged from 

 123% to 128%, as occurred during the last 10 days of 

 Test I and during the first 18 days of Test II. 



32 Weitkamp 



