Identifying environmental conditions that trigger coho salmon to migrate and spawn

Each fall, adult coho salmon (Oncorhynchus kisutch) leave the ocean and migrate upriver to freshwater spawning grounds. Coho salmon are a threatened species, protected by the federal Endangered Species Act. Changing precipitation patterns and increasing air temperatures are affecting the seasonal flow patterns in rivers and streams in the Pacific Northwest. It is critical to understand how stream flow and water temperature influence salmon migration and spawning so that fisheries managers can use that information to manage for freshwater habitat.

Using data from the Umpqua River basin in Oregon, scientists, including Rebecca Flitcroft and Gordon Grant with the Pacific Northwest Research Station, found that coho salmon began migrating up the North Umpqua River 7 to 15 days after peak annual water temperature, when mean daily temperatures cooled to 64 °F, but before the increases in discharge that are associated with autumn rains.

Although migration timing appeared to be strongly related to river temperature, spawn timing of coho salmon in tributaries in a subbasin of the Umpqua appeared to respond to a combination of both discharge and temperature thresholds. Spawning occurred after initial annual peak discharge events and when stream temperatures fell below a threshold of 57 °F.

The Oregon Department of Fish and Wildlife is using these findings as it develops a hydrologic model for the state. These findings will inform interpretation of patterns of river discharge and temperature during spawning, as well as the thermal effect of stream restoration actions.