FISHERY BULLETIN: VOL. 69, NO. 4 



of temperature acclimation and supersaturation 

 of nitrogen. In the experiments described in 

 this report, BCF personnel sought to determine 

 the change in tolerance of juvenile salmon and 

 trout to temperature increases when stressed by 

 supersaturation of nitrogen and to determine 

 possible changes in tolerance if they had the 

 option to sound to different depths. Ebel (1969) 

 reported that the depth at which fish migrate 

 influences the effect of supersaturation of nitro- 

 gen because the gas remains in solution at a 

 much higher concentration when under pres- 

 sure; we therefore considered depth as well as 

 temperature increase and supersaturation of 

 nitrogen to be important. 



Our first series of experiments describe the 

 effect of supersaturation of nitrogen and tem- 

 perature increases at surface pressures. Later 

 experiments show how depth changes the above 

 effect. 



METHODS 



The general approach used to determine the 

 effect of nitrogen supersaturation on the tol- 

 erance of juvenile salmon and trout to increased 

 temperature was similar to that used by Brett 

 (1952). Stocks were acclimated to tempera- 

 tures identical to those used by Brett; test tem- 

 peratures encompassed the ranges Brett used 

 in his lethal temperature determinations. These 

 test and acclimation temperatures were purpose- 

 ly selected so that changes in tolerance caused 

 by the stress of supersaturated nitrogen could 

 be compared with Brett's well-established levels 

 of temperature tolerance. 



Groups of 20 test fish, each acclimated to a 

 given temperature, were placed simultaneously 

 in control and test situations involving identical 

 test temperatures at treatments of high (125- 

 130 o; ) and normal (100';; ) nitrogen saturation. 

 Two acclimation groups were used in each set 

 of tests — one that had a normal acclimation 

 history and one that had been exposed to super- 

 saturated nitrogen for 720 min. Observations 

 of behavior and mortality were made continu- 

 ously for the first 6 hr, then every hour for the 

 remainder of an 18-hr period. Events recorded 

 were times to first indication of stress, to loss 



of equilibrium, and to death. The numbers of 

 live and dead fish with obvious external symp- 

 toms of gas bubble disease were recorded at the 

 termination of tests. All tests were then re- 

 peated (most tests were duplicated, some were 

 done three times) ; the data given in this report 

 are derived from the average value of the du- 

 plicated tests. 



Hatchery and wild stocks of fish were tested. 

 Hatchery fish were from the following stocks: 

 coho salmon (0. kisutch) reai-ed at Leavenworth 

 National Fish Hatchery, Leavenworth, Wash.; 

 spring Chinook salmon {0. tshairytscha) reared 

 at Little White Salmon National Fish Hatchery, 

 White Salmon, Wash.; and steelhead trout 

 reared at the Washington State Fish Hatchery 

 at Green River, Cumberland, Wash. The wild 

 fish (spring Chinook salmon) were collected from 

 the turbine intake gatewells at McNary Dam on 

 the Columbia River. Because of time limitations 

 and lack of sufficient populations of fish, only 

 the coho were tested through the entire range 

 of acclimation temperatures (5, 10, 15, and 

 20° C). Hatchery-reared steelhead trout and 

 hatchery and wild spring chinook were tested 

 only after exposure at selected acclimation tem- 

 peratures — steelhead at acclimation tempera- 

 tures of 10° and 15° C and wild chinoOk at 10° C. 



The experimental temperatures (test and ac- 

 climation) , lengths of time that the various 

 groups of fish were in holding and acclimation 

 tanks, and size of fish at time of testing are 

 summarized in Table 1. Water in test tanks 

 was adjusted to the appropriate test tempera- 

 tures established for each acclimation temper- 

 ature. Temperatures were maintained within 

 ±0.2° C in both test and acclimation tanks. 

 When the test series required stressing of the 

 acclimated fish with supersaturated nitrogen, an 

 acclimation tank at the appropriate temperature 

 was saturated at 115 to 120*;; nitrogen and about 

 114 to 120' ; oxygen. The fish were then trans- 

 ferred from the normally saturated tank ( 100 "^r ) 

 to the supersaturated tank and stressed for 720 

 min. When supersaturated water was needed 

 in the test tanks, the supersaturating equipment 

 was activated, and each tank was adjusted to 

 maintain between 125 and ISO'/r nitrogen. To 

 ensure stability, the temperatures and satura- 



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