The Effect of Fluctuating Temperatures and Ration Levels on the Growth of Juvenile Snake River Fall Chinook Salmon.

Determining the fate of juvenile fish that are exposed to elevated temperatures is complicated by the fact that the optimum temperature for the growth and survival of salmonids decreases as the amount of food becomes restricted. In this study, naturally produced juvenile Snake River fall Chinook salmon Oncorhynchus tshawytscha were fed daily ration levels of 1, 4, or 8% of their body weight and exposed to either constant temperatures (10-14°C) or fluctuating temperatures that mimicked the heating rate (1.5°C/h) and maximum daily temperatures (19-23°C) of entrapment pools that form along the shoreline downstream of Hells Canyon Dam when river flows are altered to meet electric power demand. The survival rate for all groups was 99.9%, and there was no evidence that juvenile fall Chinook salmon fed reduced rations and exposed to constant temperatures grew to a greater extent than juvenile fall Chinook salmon exposed to fluctuating temperatures. The only exception was with the 1% ration level in which juvenile fall Chinook salmon that were exposed to a constant 10°C added more weight over the 14-d exposure period (1% of body weight per day [WT/d]) than fish exposed to temperatures that fluctuated from 10°C to 19.0°C (0.3% WT/d) and from 10°C to 22°C daily (0% WT/d). We conclude that the lack of difference in growth rates between the fluctuating and constant temperature regimes at daily ration levels of at least 4% stems from the facts that the rate of temperature change in this study was within the acclimation range for growth, the daily average temperatures were less than thermal optimum values, and the temperature amplitudes were within lethal limits. [ABSTRACT FROM AUTHOR]