PNNL

Abstract

Fall Chinook salmon (Oncorhynchus tshawytscha) initiate spawning in the Hells Canyon reach of the Snake River, Idaho (rkm 240-397), at water temperatures above 16°C. This temperature exceeds the states of Idaho and Oregon water quality standards for salmonid spawning. These standards are consistent with results from studies of embryos exposed to a constant thermal regime, while salmon eggs in the natural environment are rarely exposed to a constant temperature regime. The objective of this study was to assess whether variable temperatures (i.e., declining after spawning) affected embryo survival, development, and growth of Snake River fall Chinook salmon alevins and fry. In 2003, fall Chinook salmon eggs were exposed to initial incubation temperatures ranging from 11-19°C in 2°C increments, and in 2004 eggs were exposed to initial temperatures of 13°C, 15°C, 16°C, 16.5°C, and 17°C. In both years, temperatures were adjusted downward approximately 0.2°C/day to mimic the thermal regime of the Snake River where these fish spawn. At 37-40 days post-fertilization, embryos were moved to a common exposure regime that followed the thermal profile of the Snake River through emergence. Mortality of fall Chinook salmon embryos increased markedly at initial incubation temperatures >17°C in both years. A logistic regression model estimated that a 50% reduction in survival from fertilization to emergence would occur at an initial incubation temperature of ~16°C. The laboratory results clearly showed a significant reduction in survival between 15°C and 17°C, which supported the model estimate. Results from 2004 showed a rapid decline in survival occurred between 16.5°C and 17°C, with no significant differences in survival at initial incubation temperatures 0.2°C/day following spawning.