Record Rain Filled Bay Area Creeks in Winter 2022–23. So Where Were the Coho Salmon?

The storms of the 2022–23 winter gave coho salmon everything they needed to reach their spawning streams in the San Francisco Bay Area. Yet after more than twenty spawner surveys since November, the National Park Service's monitoring crew had found almost nothing: one mostly eaten coho carcass and what was probably one redd, both in Olema Creek. The explanation lies in counts taken nearly two years earlier.

A coho salmon head recovered from Olema Creek in December 2022. Credit: Catherine Masatani, Watershed Stewards Program / NPS.

Rivers in the sky

Atmospheric rivers — "rivers in the sky" — carry columns of water vapor out of the warm tropics and drop it as rain or snow over cooler land. Big ones can bring extreme rain and wind, flooding vulnerable watersheds, toppling trees and causing erosion and runoff. On the California coast they are common in winter and supply a large share of the year's precipitation and fresh water. Bomb cyclones, which come with them, form when cold and warm air collide, creating a low-pressure system whose pressure drops suddenly as the storm strengthens.

What the floods did to the monitoring

• Equipment came out. The crew pulled a DIDSON sonar camera from Lagunitas Creek — deployed with the California Department of Fish and Wildlife to count fish passing — and a PIT tag array from Olema Creek, which electronically detects tagged salmon. Both were to go back once flows dropped.
• Surveys stalled. High water made fieldwork unsafe and too murky to see into.

The DIDSON sonar camera on Lagunitas Creek, November 2019. Credit: NPS.

Why so few fish

Coho live on a three-year cycle, and this winter's spawners were the smolts that left for the ocean in spring 2021. That year's counts were below average: over 700 smolts from Olema Creek and fewer than 80 from Redwood Creek. Only about 2–3 percent of smolts typically survive to return as adults — so the crew was looking for about 20 adults in Olema Creek and just 1 or 2 in Redwood Creek.

A small consolation

Any coho that arrive after the storms may escape one danger of high water: redd scour, when floods lift the gravel protecting salmon eggs and wash the eggs away. The team planned to return to the creeks once the water receded, to see how dire the run really was.

Sources

Based on "Historic Rainfall Surges Bay Area Creeks, but Where are the Salmon?" by Watershed Stewards Program Corpsmembers Catherine Masatani and Elizabeth Bear, San Francisco Bay Area Network Salmonid Monitoring Program (January 2023), from the National Park Service; a work of the United States government in the public domain.