The Columbia Basin Bulletin –
Subyearling fall chinook salmon lose weight as they migrate downriver through the McNary and John Day dam pools. Although their preferred food through these still waters is Daphnia, a naturally occurring small planktonic crustacean, warmer water requires more energy and the fish in August will turn to non-native juvenile America shad as a food source.
According to a recent study, the subyearlings generally lost weight in July and August when water temperature in the river typically climbs to 20 degrees Celsius (68 degrees Fahrenheit) and at 22 C it was not at all possible for the subyearlings to gain weight on average.
By switching food sources from Daphnia, which are prevalent in high densities in July and August in the lower Columbia River, to juvenile American shad, a non-native but abundant food source with a higher energy content, the subyearlings can partially compensate for the effects of high water temperature.
The study estimates juvenile salmon growth based on changes in consumption in reservoirs of subyearling fall chinook salmon’s primary prey – Daphnia and juvenile shad – in the two reservoirs using consumption rates measured in the field, according to researcher Craig Haskell, fishery biologist at the U.S. Geological Service’s Western Fisheries Research Center.