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Study: As River Warms Through McNary-John Day Pools, Juvenile Salmon Change Food-Source To Shad
Posted on Friday, December 08, 2017 (PST)

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.


“Using these empirical rates, bioenergetics models predicted negative growth,” Haskell said. “So then the paper asks the question of how much prey would salmon need to consume to exhibit positive growth at temperatures that we would expect juvenile salmon to encounter while migrating through lower Columbia River reservoirs (16-24 degrees Celsius or 61-75 degrees Fahrenheit).”


Haskell continued, saying that modeling showed that the subyearlings needed to consume 27,000 Daphnia per day to grow, but could not grow above 19 degrees C while solely feeding on Daphnia.


With respect to feeding on juvenile shad, which have a higher energy density, the juvenile salmon would need to consume 20 shad per day at 16 C, 25 shad per day at 18 C, 30 shad per day at 20 C, and 35 shad per day at 22 C. At 24 C or higher, juvenile salmon could not grow irrespective of prey type or amount, he said.


Subyearlings move through the John Day reservoir fairly quickly, the study says, “but compromised growth opportunity there could have lasting effects” as the average subyearling could lose 3.4 percent of its body weight in July and 1.2 percent in August while migrating through the reservoir.


Still, the study says, the body mass loss of just 1.2 to 3.4 percent does not mean that the subyearlings are in immediate danger of starvation.


“Linking functional response and bioenergetics to estimate juvenile salmon growth in a reservoir food web” was published online October 11, 2017 in the journal PLOS one (


Haskell’s co-authors are David Beauchamp, Chief, Ecology Section, USGS Western Fisheries Research Center, and Stephen Bollens, Director of Multi-Campus Planning and Strategic Initiatives, College of Arts and Sciences, and professor, School of the Environment and School of Biological Sciences at WSU.


“This study identifies a threshold (using functional response trials in the lab) at which juvenile fall chinook salmon feeding (consumption rate) is influenced by changes in prey density,” Haskell said in explaining functional response. “There is not a linear increase in consumption with a corresponding increase in prey density – at some point consumption becomes limited by an organism’s ability to handle prey. The resulting relationship is functional response.”


Within the typical range of Columbia River water temperatures, Daphnia generally increase with higher temperature. However, there are other factors (flow, turbidity, algal biomass, and predation) which also influence Daphnia abundance. Both juvenile salmon and juvenile shad are important consumers of Daphnia. But there are other types of plankton, such as copepods, which are also consumed by fish, Haskell said.


Shad may also compete for Daphnia with the subyearlings, consuming Daphnia otherwise available to juvenile salmon.


“Higher numbers of juvenile shad probably result in lower growth of juvenile salmon, but this phenomena occurs mostly at the tail end of the seasonal outmigration of juvenile fall chinook salmon in July- September,” Haskell said. “By this time, most juvenile salmon have migrated seaward.”


The trio of researchers had previously published “Trophic Interactions and Consumption Rates of Subyearling Chinook Salmon and Nonnative Juvenile American Shad in Columbia River Reservoirs” online February 1, 2017 in Transactions of the American Fisheries Society (


That study concluded that subyearlings derive a benefit from the shad presence in the Columbia River.


Still, Haskell had said in March that little is known about Columbia River shad and their life history. They are numerous and have higher run sizes than all salmon combined. While salmon and steelhead runs in the Columbia River approach 1 to 3 million fish each year, shad routinely average that many or more. The annual counts over Bonneville Dam are as high as 4 to 5 million fish, but the USGS has estimated that as many as 10 to 20 million shad enter the river annually.


Shad were introduced to the Sacramento River in 1871 and within five years they began to show up in the Columbia River.


So what did subyearling chinook eat in July and August before shad were present? One answer is that the dams weren’t there and far fewer of the juvenile chinook would be present in the river sections that are now impounded by the dams. With the dams, migration timing today is later.


“Before dams were in place (and shad were far fewer in number), juvenile salmon relied on a food web that was much different,” Haskell had said in March. “Whereas today juvenile chinook salmon consume Daphnia and juvenile shad, prior to dams they would have consumed aquatic insects such as caddisflies and midges.”


Also see:


-- CBB, March 10, 2017, “Non-Native Shad In John Day Reservoir Now A Food Source For Late Migrating Sub-Yearling Chinook,”


-- CBB, July 19, 2013, “Irony: Non-Native Shad Success With Columbia Basin Dams Offers Clues To Recovery In Native Waters”


-- CBB, June 21, 2013, “Shad Don’t Mind The Dams; Over 3 Million Have Already Crossed Bonneville, Extend Range Upstream”

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