FISHERY BULLETIN: VOL. 69. NO. 4 



Table 9. — Mean values^" of lethal exposure time (in min- 

 utes) for steelhead trout that had been acclimated to 

 10° C water temperature and then subjected to various 

 increases in water temperature and saturation of nitro- 

 gen gas. 



Dead fish 

 in sample 



Level 



of 

 gasb 



Lethol exposure time at 

 various temperatures (° C) 



25 



a Mean of replicated tests. , 



b NN = normal saturation (100%) of nitrogen gas in acclimation and 

 test tanks; 

 NS = normal saturation in acclimation tank and supersaturotion 



(125-130%) in test tanks; 

 SN = supersaturotion in acclimation tank and normal saturation in 



test tanks; 

 SS = supersaturotion in occlimotion and test tanks. 



Table 10. — Mean values^ of lethal exposure time (in 

 minutes) for steelhead trout that had been acclimated 

 to 15° C water temperature and then subjected to var- 

 ious increases in water temperature and saturation of 

 nitrogen gas. 



Dead fish 

 in sample 



Level 



of 

 gasb 



Lethal exposure time at 

 various temperatures (" C) 



26 



28 



SS 



690 



452.5 157.5 225 



a Mean of replicated tests. 



b NN = normol saturation (100%) of nitrogen gas in acclimation ond 

 test tanks; 

 NS = normol soturotion in acclimation tank and supersoturotion 



(125-130%) in test tanks; 

 SN = supersaturotion in occlimotion tank and normal saturation in 



test tanks; 

 SS = supersaturotion in acclimation and test tanks. 



stressed fish were subjecte(i to temperature in- 

 creases in water supersaturateci at 125 to 130%, 

 however, the prior stress significantly (decreased 

 their tolerance at temperatures below 26° C. 

 Figure 1 shows only the data from populations 

 acclimated to 15° C (Tables 3, 6, and 10), but 

 similar cui-ves that clearly show the effect of 

 supersaturation can be constructed from the 

 data on populations acclimated to the other tem- 

 peratures. 



Populations of echo salmon, steelhead trout, 

 and Chinook salmon acclimated to higher tem- 

 peratures were able to tolerate higher temper- 

 atures for longer periods in supersaturated 

 water as well as in normally saturated water. 

 A comparison of our LEso curves for coho with 

 those of Brett (1952) indicates that tolerance 

 to higher temperatures in supersaturated water 



PERCENTAGE SATURATION NITROGEN 



240 360 460 



TirriE (minutes) 



Figure 1. — Comparison of i,E;,(| curves of hatchery-reared 

 juvenile coho salmon, spring chinook salmon, and steel- 

 head trout acclimated at 15° C and stressed at various 

 levels of nitrogen saturation. 



838 



