THERMAL TOLERANCE OF JUVENILE PACIFIC SALMON AND 

 STEELHEAD TROUT IN RELATION TO SUPERSATURATION OF 



NITROGEN GAS 



Wesley J. Ebel, Earl M. Dawley, and Bruce H. Monk' 



ABSTRACT 



Thermal tolerance of juvenile chinook salmon (Oncorhynchus tshawytscha) , echo salmon (O. kisutch) , 

 and steelhead trout (Salmo gairdneri) that had been held at various acclimation temperatures was low- 

 ered when test water was supersaturated (125-130% of saturation) with nitrogen gas. Increasing the 

 depth of the test tank allowed the fish to compensate somewhat for the supersaturation by sounding, 

 but substantial mortalities still occurred. A comparison of tolerance among the species tested revealed 

 that coho salmon were the most tolerant, chinook salmon next, and steelhead trout the least tolerant to 

 temperature increases in the presence of supersaturation of nitrogen. 



During the past several decades, a number of 

 investigators have examined temperature as a 

 lethal factor by use of the classic pharmacolog- 

 ical assay method. Fry, Hart, and Walker 

 (1946) recognized the importance of acclimation 

 temperature in determining the tolerance of a 

 fish to high and low temperatures and estab- 

 lished upper and lower levels of tolerance at 

 various acclimation temperatures. Brett (1952) 

 listed maximum temperatures for survival of 

 young Pacific salmon (Oncorhynchus spp.) be- 

 tween 23.8 and 25.1° C. In later work (Brett, 

 1958), he emphasized temperatures below those 

 at which a fish dies and constructed hypothetical 

 temperature polygons which described lower 

 levels of temperature tolerance where activity, 

 growth, and spawning would be aflfected. 



More recently, investigators have emphasized 

 the response of fish to temperature changes 

 under multivariate conditions. Many factors 

 such as dissolved oxygen deficits, carbon dioxide 

 increases, and increases in toxic substances all 

 affect an aquatic organism's tolerance to tem- 

 perature increases. Mihursky and Kennedy 

 (1967) stressed the importance of multivariate 

 experiments for establishing more realistic 

 standards for temperature regulation. 



' National Marine Fisheries Service, Biological Lab- 

 oratory, Seattle, Wash. 98102. 



Manuscript accepted June 1971. 



FISHERY BULLETIN: VOL. 69. NO. 4, 1971. 



Several nuclear power plants have been pro- 

 posed for the Columbia River. The National 

 Marine Fisheries Service (NMFS) is particu- 

 larly concerned about the effect that heated ef- 

 fluents from these plants might have on juvenile 

 Pacific salmon and steelhead trout (Salmo gaird- 

 neri) migrating downstream, particularly 

 while they are stressed by supersaturated nitro- 

 gen gas. High levels of nitrogen gas (over 125% 

 saturation) occur within large areas of the Co- 

 lumbia from about early May until mid-August 

 (Ebel, 1969). This period coincides with the 

 downstream migration of most juvenile salmon 

 and trout. Although the effect of supersatura- 

 tion of gas on juvenile salmon and trout has 

 not been examined in great detail, preliminary 

 studies by Ebel (1969) clearly show that Co- 

 lumbia River juvenile salmon have considerably 

 lower tolerance to temperature increases when 

 stressed by supersaturation of nitrogen than the 

 tolerance indicated by Brett (1952). 



The Federal Water Quality Administration 

 (FWQA) recognized that supersaturation of 

 dissolved nitrogen could be a significant factor 

 in establishing water quality criteria for the 

 Columbia River. It therefore contracted the 

 Bureau of Commercial Fisheries (BCF; pres- 

 ently designated as NMFS) to determine the 

 changes in tolerance of juvenile salmon and 

 trout to temperature increases at different levels 



833 



