EBEL, DAWLEY, and MONK: THERMAL TOLERANCE 



about 17 hr in the shallow tank. Similarly at 

 15° C (5° C increase), the LEso level was never 

 reached in the deep tank but was reached in 

 about 12.5 hr in the shallow tank. No benefit 

 from depth is indicated in the curves at 23° C ; in 

 this comparison, the fact that temperature in the 

 deep tank was 0.8° C higher (23.8° C) than that 

 in the shallow tank could account for the lack 

 of difference. 



Wild juvenile spring chinook salmon from the 

 gatewells at McNary Dam also were tested in 

 the deep and shallow tanks. These fish were 

 acclimated at 10° C and then subjected to a 5° C 

 increase (15° C) with supersaturation of nitro- 

 gen gas at 130 '/f saturation. The fish also were 

 stressed for 12 hr before the test in 10° C water 

 supersaturated at 120% saturation. Again, chi- 

 nook tested in the deep tank survived at a higher 

 rate than those in the shallow tanks; the LEso 

 was never reached in the deep tank, whereas 

 100% mortality was reached in approximately 

 11 hr in the shallow tanks (Figure 6). 



Observations in the deep tank during tests 

 with the coho and chinook salmon indicated that 

 most fish remained between about 1 and 4 m 

 of the surface. Light intensity and turbidity 

 possibly influenced the depth distribution. Dur- 

 ing these tests, artificial light at an intensity of 

 about 100 footcandles was present at the surface 

 of the water. Turbidity in the tank was min- 

 imal; a Secchi disc was visible at the bottom 

 of the tank and the Jackson turbidity unit 

 measurement was 0. 



It is difficult to relate tests in the tank to na- 

 tural conditions because turbidity in natural 

 water varies greatly. In the Snake River, tur- 

 bidity as measured by a Secchi disc varies from 

 0.2 to 8.0 m, depending on season and location. 

 Turbidity usually is high during the spring run- 

 off in both the Snake and Columbia Rivers; read- 

 ings are seldom over 1 m on the Snake River 

 (Ebel and Koski, 1968). This high turbidity 

 limits visible light penetration to a maximum of 

 about 1.5 m (observation verified by scuba div- 

 ing). We therefore believe that juveniles as 

 observed in the tank were at greater depths than 

 they might be in the Snake or Columbia Rivers 

 during the spring migration. Durkin, Park, and 



Figure 6. — Comparison of le,„q curves of wild spring 

 chinook salmon acclimated at 10° C and subjected to a 

 5° C increase (15° C) in tanks 20 cm and 9 m deep that 

 were supersaturated with nitrogen gas at 1307c satura- 

 tion. Oxygen concentrations varied from 115 to 125% 

 saturation. 



Raleigh (1970) found that most juvenile salmon 

 were near the surface as they entered Brownlee 

 Reservoir. Fish in the Columbia and Snake 

 Rivers apparently do not sound to a depth suf- 

 ficient to compensate for nitrogen saturation 

 levels exceeding 130% ; hence the mortalities re- 

 ported herein are probably on the conservative 

 side. We also emphasize that even though the 

 option of having sufficient depth reduced the 

 mortality rate, substantial mortalities occurred. 



TEMPERATURE STANDARDS 



FOR RIVERS WITH 



NITROGEN SUPERSATURATION 



Our test temperatures and experimental de- 

 sign were purposely selected so that these data 

 could be compared with the results reported by 

 Brett (1952). Brett cautions that the informa- 

 tion he presents should not be applied verbatim 

 to other environments. Because of the excel- 

 lence of his work and the lack of later findings 

 concerning temperature tolerance of Pacific 

 salmon, the upper lethal levels established in his 

 paper are widely quoted and used for setting 

 temperature tolerance standards for rivers and 

 streams containing salmon — without regard to 

 other physical and chemical characteristics of 

 the water. The changes in Brett's tolerance 

 curves caused by the stresses of supersaturation 

 of nitrogen gas were obvious. 



841 



