and salmon fry in the winter, salmon and steelhead fry in the spring and resident fish (mostly sculpin) in the summer, the study says.
Chinook salmon, steelhead and bull trout, all species listed as threatened under the federal Endangered Species Act, coexist in Northern Washington's Skagit River. As the top predator in the river system, 50 percent of a large bull trout's diet is typically made up of salmon eggs and salmon carcasses. The other 50 percent of their diet is juvenile salmon and steelhead, resident fish and immature insects.
While the impact of bull trout on chinook salmon is negligible, the impact on steelhead in the Skagit River is potentially high, according to a study published online June 15, 2015.
"We found that bull trout are very successful at capitalizing on seasonal food resources," said researcher Erin Lowery, who is currently senior fisheries biologist at Seattle City Light. "The annual energy budgets of bull trout in the Skagit were dominated by various energy sources derived from anadromous salmonids. This finding was not surprising."
He added that other salmon and char systems along the west coast have similar food web dynamics. "The players may be different, but the processes are similar," Lowery said.
Among other Washington rivers with the same mix of ESA-listed species are the Nooksack, Snohomish, Quinault, Bogachiel, Deschutes, Elway and many more, Lowery said.
The study, "Trophic Ontogeny of Fluvial Bull Trout and Seasonal Predation on Pacific Salmon in Riverine Food Web," was published online in Transactions of the American Fisheries Society.
Lowery's co-author, David Beauchamp, is Acting Unit Leader at the Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fisheries Sciences, University of Washington, which is associated with the U.S. Geological Survey. Lowery was also with UW at the time of this study.
The Skagit River has three mainstem dams that make up Seattle City Light's Skagit Hydroelectric Project, but the Gorge Dam, which is the most downstream of the three dams, was built at a natural barrier that had historically blocked anadromous fish passage. Most of the spawning for chinook and pink salmon occurs in the mainstem reach from the dam downstream about 40 kilometers (about 25 miles).
Most larger bull trout, larger than 300 millimeters (about 12 inches), reside in the mainstem river and are occasionally found in the tributaries, while most bull trout smaller than 300 millimeters live in the tributaries. This is important because the bull trout diet varies by size, the season of the year and available prey, according to the study.
Bull Trout in the 30 -- 95 mm range (1.2 inches to 3.75 inches) consume mostly aquatic insects. The fish begins to eat other fish when it reaches 100 mm (3.94 inches). Those up to 300 mm in tributaries eat primarily rainbow trout/steelhead and some aquatic insects during the fall, switching to small coho salmon and aquatic insects in other seasons.
Depending on size, but larger than 301 mm, bull trout in the mainstem river consume salmon eggs during the fall, salmon carcasses and salmon fry in the winter, salmon and steelhead fry in the spring and resident fish (mostly sculpin) in the summer, the study says.
Predation of salmon and steelhead in the mainstem occurs mostly in the winter during fry emergence and in the spring during the smolt out-migration. "Steelhead fry and parr were present in the Skagit River year-round and were key components of energy budgets throughout the ontogeny of fluvial Bull Trout," the report says.
"We also found that during our study juvenile steelhead were highly vulnerable to predation during the spring," Lowery said.
That is likely due to a reduction in the number of whitefish, which occupy similar habitats as juvenile steelhead, in the river. Lowery and Beauchamp believe that an increase in whitefish numbers would to some extent divert bull trout away from preying on just steelhead.
In the Skagit River the most abundant prey item for bull trout is currently steelhead. "The hope is that whitefish will rebound and attract bull trout to them rather than steelhead," Lowery concluded.
"The reduction in the whitefish population, a common prey item for predators in these ecosystems, could have increased the pressure on other fishes that exhibit similar behaviors," Lowery said.
The challenge is to balance the needs of the fisheries and the function of the ecosystem, he said.
"Bull Trout are the ecosystem's top predator," he continued. "There are other predators as well. All are using similar resources. These predator-prey systems have evolved over many millions of years and more recently during periods of glaciation. The needs of the ecosystem and management objectives should be linked. They typically are."
There is a balance between the three species -- chinook salmon, steelhead and bull trout -- that is always shifting and "we will likely never see every species maintain the current state for long," he said. "The challenge is to manage this dynamic system effectively."