This recent scientific report explored the change in the effect of predation on chinook salmon since the 1970s along the west coast of North America. It focused on predation by three species of pinnipeds – harbour seals, California sea lions, Steller sea lions – and fish-eating killer whales, and compared these estimates to salmon fisheries.
We find that from 1975 to 2015, biomass of Chinook salmon consumed by pinnipeds and killer whales increased from 6,100 to 15,200 metric tons (from 5 to 31.5 million individual salmon).
The science is pointing to pinniped predation as one of the primary factors in poor ocean survival for many salmonids like Chinook and Steelhead. These fish in particular have larger out-migrations, making them an easier and more desirable target for harbour seals, which are the primary culprit.
The Salish Sea stands out as a growing hot spot for Chinook predation by seals and sea lions. Of an estimated 27.4 million Chinook consumed by harbour seals along the west coast of North America in 2015, 23.2 million were smolts taken in the Salish Sea. The study found that harbour seals grew from a Salish Sea population of 8,600 in 1975 to 77,800 in 2015, and their diet includes a greater percentage of Chinook smolts than seals in other areas. Consequently, harbour seals in the Salish Sea consume about 86 percent of all the Chinook eaten as smolts along the west coast.
Increasing consumption of Chinook salmon by pinnipeds may also be limiting the growth of the Southern Resident killer whale population. Our results suggest that at least in recent years competition with other marine mammals is a more important factor limiting the growth of this endangered population than competition with human fisheries.