Scientists theorize the problem is occurring in the ocean. A recent peer-reviewed study shows that roughly 85% of the total historical vegetated estuary area for the West Coast has been lost. Losses were highest for major river deltas.

Some scientists have also pointed to the growing numbers of seals and sea lions, which feed on the fish, as behind the declines.

Others say the warming and acidifying of the ocean are killing off the prey that the salmon depend on. There is strong evidence that ocean conditions, including ocean temperatures, are an important predictor of adult salmon returns.

Insights into estuary habitat loss in the western United States using a new method for mapping maximum extent of tidal wetland
12-3-2018 CBC News “More than a dozen B.C. chinook salmon populations in decline, scientists say”
NOAA Northwest Fisheries Science Center Outlook for Salmon Returns