FISHERY BULLETIN: VOL. 69, NO. 4 



Chinook salmon) at 10° C acclimation temper- 

 ature. 



EFFECT OF SUPERSATURATION OF 

 NITROGEN GAS ON TOLERANCE OF 

 FISH TO TEMPERATURE INCREASES 



Supersaturation of nitrogen (125-130%) in 

 the test tanks lowered the tolerance of all fish 

 to temperature increases at all acclimation tem- 

 peratures when tested below 26° C (Tables 2- 

 10). More than 50% mortality occurred within 

 18 hr even with no temperature increase when 

 the test tanks were supersaturated at this level. 

 Time to death of fish was accelerated regardless 

 of acclimation temperature. 



A comparison of LE.^^o (e.xposure time when 

 50% of the population is dead) curves developed 

 under the four treatment levels indicates that a 

 prior stress of 115 to 120% saturation for 12 hr 

 did not greatly affect the fish when they were 

 subjected to temperature increases in water sat- 

 urated at 100% (Figure 1). When these 



Table 3. — Mean values" of lethal exposure time (in min- 

 utes) for coho salmon that had been acclimated to 10° C 

 water temperature and then subjected to various in- 

 creases in water temperature and saturation of nitrogen 

 gas. 



Dead fish 

 in sample 



Level 



of 

 gasb 



Lethal exposure time at 

 various temperatures (° C) 



27 



One fish 



Three fish 



Half of sample 



Ail of sample 



NN 

 NS 

 SN 

 SS 



NN 

 NS 

 SN 

 SS 



NN 

 NS 

 SN 

 SS 



NN 

 NS 

 SN 

 SS 



.. 1,085 

 960 480 



130 

 130 

 135 

 125 



100 

 152 

 67 

 95 



167 

 192 

 232.5 

 480 282.5 193.5 



215 

 255 

 255 



540 



550 



1,050 750 395 



4 

 6 

 4 

 12 



< 5 



16 

 7.5 

 15 



15 

 23 

 16.5 

 35.5 



32.5 

 72.5 



35 

 70 



a Mean of replicated tests. 



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

 test tanks,- 

 NS = normol saturotion in acclimation tank and supersaturation 



(125-130%) in test tanks; 

 SN = supersoturotion in acclimation tank and normal saturation in 



test tonks; 

 SS = supersaturation in acclimation and test tanks. 



Table 2. — Mean values'" of lethal exposure time (in min- 

 utes) for coho salmon that had been acclimated to 5° C 

 water temperature and then subjected to various in- 

 creases in water temperature and saturation of nitrogen 



Table 4. — Mean values" of lethal exposure time (in min- 

 utes) for coho salmon that had been acclimated to 15° C 

 water temperature and then subjected to various in- 

 creases in water temperature and saturation of nitrogen 

 gas. 



a Mean of replicated rests. 



b NN = normol saturation (100%) of nitrogen gas in occlimotion and 

 test tanks; 

 NS = normal soturotion in acclimation tank and supersaturation 



(125-130%) in test tanks; 

 SN = supersaturation in acclimation tonk ond normal soturotion in 



test tanks; 

 SS = supersaturation in acclimation and test tonks. 



a Mean of replicated tests. 



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

 test tanks; 

 NS = normal saturation in acclimation tank and supersoturotion 



(125-130%) in test tonks; 

 SN = supersaturation in acclimation tank and normal saturation in 



test tanks; 

 SS = supersaturation in acclimation and test tanks. 



836 



