FISHERY BULLETIN: VOL. 69, NO. 4 



Although complete statistical analysis of our 

 data are not presented in this paper, the dif- 

 ferences shown between tolerance cui'ves of fish 

 tested in water with and without supersatura- 

 tion of nitrogen are so great that conclusions 

 concerning the effect of supersaturation can be 

 made with relative confidence. 



Substantial mortalities will occur to migrating 

 juvenile salmon and trout in the Columbia and 

 Snake Rivers — even if no thermal plume or in- 

 crease in temperature is encountered — when- 

 ever the populations must pass through large 

 areas where 125 to 1309^ saturation of nitrogen 

 occurs. Studies of vertical distribution (e.g., 

 Smith, Pugh, and Monan, 1968; Durkin et al., 

 1970) indicate that the majority of migrants 

 are in surface waters, with substantial numbers 

 in waters less than 2 m deep. This is too shallow 

 to compensate for nitrogen levels as high as 

 ISO^/c Surveys of nitrogen levels by Ebel 

 (1969), by Beiningen and Ebel (1971), and by 

 NMFS and State fisheries personnel of Wash- 

 ington and Oregon during the 1970 spring mi- 

 gration, verify that nitrogen in large areas of 

 both rivers exceed 130 9f saturation. Exami- 

 nation of fish in cages suspended on the surface 

 and at various depths revealed that mortalities 

 caused by nitrogen often exceeded 40''/r in a deep 

 (4.5 m) cage where the fish could sound at their 

 volition. Periodic checks of juveniles in the 

 Snake River by NMFS personnel in 1970 indi- 

 cated that 25 to 45 ';r of the chinook salmon and 

 30 to 58% of the steelhead trout migrants arriv- 

 ing at Ice Harbor Dam had external symptoms 

 of gas bubble disease. We made similar obser- 

 vations of migrants at The Dalles and McNary 

 Dams in 1968 and 1969 and recorded similar 

 findings. 



Obviously the migrating juvenile salmon and 

 trout in the Columbia and Snake Rivers are 

 under stress during periods of nitrogen super- 

 saturation. Any increase in temperatui-e over 

 the ambient river temperature, then, will harm 

 these populations. Mortalities already occur- 

 ring will be accelerated even with minimal tem- 

 perature increases. Our data show that LE.-.n 

 levels of temperature (Figures 1-4) are far high- 

 er than could be accepted as standards for ui)per 

 limits of rivers containing trout and salmon even 



at normal concentrations of dissolved nitrogen. 

 The time to first mortality of wild spring chinook 

 salmon, for example, that were acclimated to 

 10° C and tested in supersaturated water at 23° 

 and 25° C was 10 and 3 min, respectively (Table 

 8). Temperatures and temperature increases 

 such as these occur in thermal effluents (Cou- 

 tant, 1969), and substantial mortalities could 

 occur to juvenile salmon and trout passing 

 through thermal plumes. 



During spring and summer when flows are 

 low, increases in temperature of the Columbia 

 River from Priest Rapids Dam to the forebay 

 of McNary Dam have been as high as 2.5° C 

 (Ebel, 1969). Increases in temperature over 

 the acclimated temperature greatly accelerated 

 time to death of juveniles when supersatura- 

 tion of nitrogen gas was present in the test 

 water whether the fish were held in shallow or 

 deep tanks. However, during the low flow peri- 

 ods when temperature increases such as this 

 occur, nitrogen saturation levels are usually low 

 and mortalities such as indicated in the tests 

 would not occur. 



The obvious results of these tests are that 

 supersaturation of nitrogen must be considered 

 when setting temperature standards and that 

 any increase allowed over the ambient temper- 

 ature of the river during periods when the river 

 is supersaturated with nitrogen will be detri- 

 mental to salmon and trout populations. 



CONCLUSIONS 



1. Supersaturation of nitrogen drastically af- 

 fects the tolerance of juvenile coho salmon, chi- 

 nook salmon, and steelhead trout to temperature 

 increases. Tolerance to increases below 26° C 

 is lowered and mortality rates are accelerated. 



2. Acclimation to higher temperatures will en- 

 able the three species to tolerate higher temper- 

 atures longer when nitrogen supersaturation is 

 a factor; however, 50% mortality will be 

 reached in loss than 18 hr at all acclimation 

 temijeratures with supersaturation of nitrogen 

 at 125 to 130%. No temperature is suitable at 

 the 125 to 130''^ level of nitrogen supersatura- 

 tion. 



3. Dei)th is an important compensating factor 



842 



