Early runs of wild spring Chinook salmon returning to the Columbia River are bearing the brunt of sea lion attacks, a new study suggests.
The fish arrive in early spring before sea lions have left for summer breeding grounds and when the pinniped population is especially high at the river’s mouth. These salmon see higher mortality rates compared to later runs and the numbers have started to climb even over prior years, corresponding with a growing number of sea lions recorded near Astoria.
The study, published Sunday by researchers with the University of Washington and the National Marine Fisheries Service, looked at the relationship between distinct Chinook salmon populations listed under the Endangered Species Act and sea lions on the river using several different data sets, including information about the sea lions hauling out in Astoria.
What researchers discovered was not necessarily surprising, but it was good information to confirm, said lead author Mark Sorel, with Northwest Fisheries Science Center and the University of Washington.
A line to orcas
Study authors draw a line from the pressure created by a growing sea lion population and predation on salmon to possible negative outcomes for other mammals that rely on a diet of Chinook salmon: endangered orcas. But the study also comes on the heels of increased sea lion management actions by state and federal fisheries managers.
The Oregon Department of Fish and Wildlife estimates several thousand California and Steller sea lions now roam the river waters near Astoria. The population has been growing for several years, with as many as 2,000 sea lions recorded in a single day count at Astoria’s East Mooring Basin. The pinnipeds have resisted numerous strategies to keep them away from places like the basin — from mechanical fiberglass orcas to physical barriers.
In August, the National Marine Fisheries Service issued a permit allowing three states, including Oregon, and six regional tribes to lethally remove sea lions. The Oregon Department of Fish and Wildlife has started to trap sea lions at Bonneville Dam and expects to begin trapping animals at Willamette Falls in November.
The permit allows the applicants to kill up to 540 California sea lions and up to 176 Steller sea lions through 2025 that have been identified as causing a significant negative impact on listed salmon species, that approach those species or that migrate through the Ballard Locks at Seattle in Washington state.
In Oregon, removals in the past have focused at environmental pinch points like Willamette Falls and Bonneville Dam, where sea lions are known to congregate and prey on salmon.
Oregon removed 33 sea lions at Willamette Falls last year, a move fishery managers say reduced predation from a previous high of nearly 25% of the adult spawning run to just 1% to 2% this year.
“We know that this action isn’t going to restore healthy runs, but it is a critical piece of the recovery picture,” said Shaun Clements, senior policy analyst for the Oregon Department of Fish and Wildlife, in a statement this summer after the permit application was approved.
There is enough evidence that sea lions are having a negative impact on salmon to convince managers that some removals are worth trying, said Sarah Converse, another author of the recent sea lion predation study, an associate professor at the University of Washington and leader of the Washington Cooperative Fish and Wildlife Research Unit.
What isn’t clear, though, is how much impact sea lion predation has on salmon across their entire life cycle.
The recent study is “kind of a first step in understanding what the effect is on how many salmon come back,” Sorel said. One motivation behind the study was to better understand the potential risk of this heightened predation to push threatened or endangered salmon toward extinction.
While other types of data and research are needed to further refine the information presented in the study, it is concerning to see such a marked impact on the earlier returning salmon, researchers said. Having viable runs of many different salmon entering the river at different times is key to the health and continued survival of the fish.
Sorel compares it to an investments portfolio. An ideal portfolio has a variety of stocks, so any losses are less disruptive, distributed across a range of possibilities.
Migration corridors like the lower Columbia River are important but risky places. Increased predation here “could affect salmon behavior and life-history expression over time,” study authors wrote. Fish may begin to migrate later to avoid predators like sea lions, but this could cause other issues at spawning grounds and expose fish to warmer water.
“Such changes could affect management of the fishery, which relies on consistent run timing to make in-season assessments of run size and adjust harvest accordingly,” the authors note.
Salmon live in environments that face any number of pressures and shifts every year. As the effects of climate change begin to unfold further, salmon face even steeper challenges to their survival along traditional migratory routes. The systems they rely on are changing rapidly, Converse said.
The variations already found across existing salmon runs may help certain salmon better weather whatever the new normal becomes.
“We’ve got a lot of changes coming up that are not entirely straightforward to predict,” Converse said.