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NW Fishletter #379, March 5, 2018
 Sea Lions, Seals Play Significant Role In Salmon Losses, Federal Biologist Reports
Sea lions and seals below Bonneville Dam are consuming far more salmon than previously thought as the fish swim back to their spawning grounds.
An ongoing study by NOAA's Northwest Fisheries Science Center suggests that each year since 2010, these carnivorous mammals are eating between 11 and 42 percent of the spring/summer Chinook run in the 145-mile stretch of river from the Pacific Ocean to Bonneville Dam, NWFSC fisheries biologist Michelle Wargo Rub told the Northwest Power and Conservation Council Feb. 14.
That translates to between 20,000 and 35,000 Chinook salmon in most of the study years, although losses jumped to more than 100,000 salmon in the high-percentage-loss years of 2014 and 2015.
The losses, and percentages, vary depending on water temperature and timing of the run, flow levels, the spill at Bonneville Dam, and abundance of smelt, among other factors, Rub told the Council. "As much as we're understanding more, the picture's becoming a little more complicated," she said.
Her updated report comes as governors of Oregon, Washington and Idaho are urging Congress to support proposed bipartisan legislation that could help reduce sea lion predation.
State agencies do have some ability to control California sea lions. In 2016, NOAA Fisheries agreed to allow the three states to continue killing these mammals seen preying on salmon, and that do not respond to efforts to scare them away with hazing. The proposed new legislation would give local agencies the ability to act more quickly.
Courtesy Oregon Department of Fish and Wildlife
In her presentation to the Council, Rub said her study's primary goal was to see if seals and sea lions were significantly impacting salmon returns. At the time, she said, there were concerns over increasing numbers of pinnipeds below Bonneville Dam. Since then, she said, "The population basically exploded." NOAA Fisheries reported in January that California sea lions have "fully rebounded," and now number around 250,000 animals. Steller sea lions and harbor seals also dine on returning salmon and steelhead.
To determine their impact on returning salmon, Rub captured, marked and released thousands of fish near the mouth of the Columbia, tracking them as they move upriver to spawn. NOAA Fisheries contracts with commercial tangle-net fishermen in the spring, who catch adult Chinook near Astoria, Ore., during a 25- to 30-day period after Chinook return. Those fish are then pit-tagged, unless already tagged, and returned to the river.
After calculating survival of fish that make it past Bonneville Dam, Rub can determine how many fish died below Bonneville Dam. Once losses from harvest and handling are subtracted, she ends up with "unexplained mortality." Rub said she's confident most of the loss is due to predation.
Those spring/summer Chinook losses ranged from a low of 11 percent of the returning run in 2010, to highs of 43 percent in 2014 and 37 percent in 2015. But although sea lions have continued to come up the Columbia River over the last two years, predator mortality dropped off to 14 percent in 2016, and to 24 percent in 2017. Rub said 2014 and 2015 may have been unusual years, as California sea lions were driven north in search of food due to warm ocean conditions, and a lack of food off the coast of California.
"The relationship between survival and sea lions is complicated. It's not just the absolute number of sea lions" in the Columbia that causes an increase in salmon losses in that stretch of river, she said. The timing of the Chinook run also has a strong connection with predation losses, as some runs arrive after California sea lions have left the area. Fish populations that arrive in the lower river early are "likely receiving the brunt of the predation," she said.
Other factors, too, either contribute or detract from predator success, she said. Data so far suggests that spill over Bonneville Dam helps Chinook survival from sea lions--perhaps because the increased turbulence allows more fish to escape predators--although she hopes to conduct more research to determine why.
A good smelt season hurts chances of survival, she said, which is counter to initial assumptions that more smelt would help, by offering predators an alternative food source. "But if you look at the timing, [smelt] often arrive before the spring run," she said. Scientists realized that a good smelt season tends to draw sea lions into the river, and they continue to feed on Chinook when they arrive.
Rub said her study, for the past two years, has used radio telemetry to determine where the returning fish are being killed. In 2016, she said, 48 percent of the losses were in the tailrace at Bonneville Dam, where large numbers of California sea lions congregate. Many of the remaining losses were in the estuary, and few were lost in the middle portion of the lower river. In 2017, sea lions appeared to shift their position; only 25 percent of Chinook mortality occurred directly below Bonneville, she said, with most occurring in the estuary.
Rub said in future years, she hopes to use radio telemetry to help determine survival and loss in Columbia River tributaries, and strengthen the correlation between other variables that impact salmon losses from the estuaries to Bonneville Dam.
Idaho Council member Bill Booth commended Rub for her work. He said until now, NOAA Fisheries believed predators below Bonneville killed only 3 to 5 percent of returning fish. "Those low numbers were kind of considered insignificant by some," he said, adding, "Now we have this data that shows it's more like 20 to 30 percent, which is very significant." -K.C. Mehaffey
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