EBEL, DAWLEY, and MONK: THERMAL TOLERANCE 



was considerably lower at exposures over 100 

 min for the same acclimation temperature than 

 at normal saturation levels (Figure 2). 



A comparison of the NN and SN curves (Fig- 

 ure 1) indicates that the prior stress of super- 

 saturation of nitrogen had little effect on the 

 fish when they were subjected to test water that 

 was not supersaturated. This suggests that mi- 

 grating salmon and trout under stress from su- 

 persaturation of nitrogen gas could recover from 

 the effects of supersaturation if there were 

 river areas where water would equilibrate. 

 These data also show that salmon and trout pop- 

 ulations acclimated at 15° C and subjected to 

 nitrogen saturation of 125 to 130% will probably 

 have about 50% mortality in less than 360 min 

 with no temperature increase when stressed for 

 12 hr before testing and that subjecting the pop- 

 ulations to temperature increases merely re- 

 duces the time to death. 



In comparing the tolerance to temperature in- 

 creases between coho and spring chinook salmon, 

 we found that the results from the control tests — 

 where both acclimation and test water were at 

 100% saturation — were similar to the results of 

 Brett (1952). That is, coho were more tolerant 

 than chinook and the respective upper lethal tem- 

 peratures were 25 to 26° C. Brett did not study 

 steelhead trout. We found that steelhead trout 

 were nearly identical to coho in their tolerance 



200 300 



TIME TO DEATH (minutes) 



Figure 2. — Median-resistance-time (LE50) plotted from 

 temperature tolerance tests with coho salmon fingerlings 

 acclimated at 5°, 10°, and 20° C and stressed for 12 hr 

 at 115-120% saturation of N2, then subjected to temper- 

 ature increases in water supersaturated at 125-130%. 

 Brett's LE51) curves for fish acclimated at 5° and 20° C — 

 without N2 stress — are shown for comparison. 



to temperature increases when supersaturation 

 of nitrogen was not present (control tests) but 

 were the most vulnerable species when super- 

 saturation was entered as a factor. Figure 3 

 compares the tolerance of three species to tem- 

 perature increases when acclimation water and 

 test water were supersaturated. Coho were the 

 most tolerant, wild spring chinook next, and 

 steelhead the least tolerant. 



Figure 3. — Comparison of LE50 values between species 

 of salmonid juveniles (acclimated at 10° C) in tolerance 

 to tempei-ature increases when stressed by 115-120% 

 saturation of nitrogen gas for 12 hr, then subjected to 

 temperature increases in supersaturated water at 125- 

 130% nitrogen. 



Size of fish at time of testing could influence 

 the comparison between species. Coho and wild 

 spring chinook acclimated at 10° C were nearly 

 the same size at time of testing (Table 1), but 

 steelhead acclimated at 10° C were larger than 

 the other species when tested. There is evidence 

 that extremely small chinook fry are more sus- 

 ceptible to nitrogen supersaturation than finger- 

 lings; then, as the fingerlings increase in size, 

 they become more susceptible than the small 

 fingerlings but less susceptible than the fry 

 (Meekin, 1969).' If this occurs with steelhead 

 also, it could account for their lower tolerance. 

 During one test with coho acclimated at 15° C, 

 we found no differences in susceptibility within 

 the size range tested (101-151 mm) at 125 to 

 130% saturation. 



' Personal communication, Thomas Meekin, Washing- 

 ton State Department of Fisheries. Experiments at 

 Priest Rapids Dam. 



839 



