A Coho Salmon's Upstream Battle Against Climate Change

A female coho making her difficult way upstream. NPS photo by Jessica Weinberg McClosky.

One fish's journey

In December 2022, a female coho salmon pushes upstream against the current of Olema Creek in West Marin. About 18 months earlier she left this creek for the ocean as a small smolt, letting the current carry her to the estuary and the sea. Now she must fight her way back to spawn — a one-way trip that will use up all her energy.

Coho (Oncorhynchus kisutch) matter to both freshwater and ocean ecosystems: over their three-year lives they carry nutrients from sea to creek and interact with many species, people included. Indigenous peoples of the West Coast have long relied on salmon for food, and some call themselves Salmon People. Because the fish and the creek depend on each other, monitoring salmon also tells biologists how the creek is faring.

Salmon have always faced predators and high death rates, but human disturbance and climate change are making every stage harder. The central California coast population is endangered.

I. Growing up in the creek

She hatched in Olema Creek three years before, and spent more than a year there as a fry. Young coho need cold, oxygen-rich, flowing water and refuges such as piles of fallen wood, where they can hide from predators and avoid being swept away.

Young coho need cold, oxygenated water and fallen wood to hide in. NPS photo by Michael Reichmuth.

• Human changes: people have removed woody debris, rerouted streams, and damaged the streamside plants that keep water cool.
• Climate change: warmer temperatures fuel algae that can lower oxygen and warm the water, and droughts can dry out stretches of creek, killing young fish and trapping smolts trying to reach the sea.

II. Feeding at sea

After a year in the creek, she swam out as a smolt. Coho spend about 18 months at sea, eating fatty zooplankton and then larger prey such as herring, anchovies and squid.

Measuring a coho smolt on its way to the ocean. NPS photo by Maritte O'Gallagher.

Warming seas and ocean acidification leave zooplankton less fatty and less nutritious, and other prey may have moved north to colder water. A salmon that can't store enough energy may be too weak to make it home.

III. Coming home to spawn

Bigger and fueled up, she gathers with others at the creek mouth, waiting for enough flow to swim in. Upstream she'll dig a nest — a redd — in the gravel and lay eggs for males to fertilize, then live as long as her energy lasts.

Erratic rain from climate change hits this stage hard. At Redwood Creek in Marin County, when rain comes late, salmon must wait offshore for the creek mouth to open — easy prey for predators — and if the creek doesn't stay open long enough, many never spawn.

This female was lucky at every stage. Many coho aren't, and the population is suffering.

Protecting coho

Parks can't fix ocean conditions or rainfall — that takes global policy change — so park scientists focus on the freshwater stage, helping young fish grow as big as possible before they leave:

• installing artificial wood structures that create deep pools with cover from predators;
• restoring vegetation along the creek — planting natives, removing invasives — to steady the banks and add shade.

Olema Creek full of young coho in 2022. NPS photo by Michael Reichmuth.

There is hope: in summer 2022, biologists counted a record number of young coho in Olema Creek, which could mean a strong return when they come back to spawn in the 2024–25 winter.

How you can help

• Follow the San Francisco Bay Area Network's salmon monitoring and seasonal updates.
• Practice Leave No Trace, and stay on marked trails — walking creek banks causes erosion that harms salmon habitat.
• Volunteer for restoration with SPAWN or Point Reyes National Seashore.
• Watch spawning coho from safe viewing spots.

Sources

Based on Claire Baker (science communication intern, Point Reyes National Seashore Association), "Coho Salmon's Upstream Battle Against Climate Change," National Park Service, with information from Park Service fisheries staff Brentley McNeill and Michael Reichmuth; published by the National Park Service and in the public domain; rewritten in hubnx's own words. A photo caption on the source dates the young fish's return to the 2023–24 winter and its text to 2024–25; this page follows the text.