Courtesy of the Russian River Salmon and Steelhead Monitoring Program.

Fog & Fish

How the marine fog layer impacts fish in streams
Lena Beck

In parts of coastal California, warm weather days are often balanced out by misty, low-hanging clouds. This marine fog layer hugs the ground and provides coastal ecosystems with a critical buffer from the harsh heat of the summer months.

Coastal communities have a special relationship with fog — formed by a series of interactions between onshore and offshore temperature gradients, atmospheric pressure, upwelling in the ocean and more — but it’s not the first thing anyone thinks about with regard to climate change. However, it is uncertain how the coastal marine cloud layer will be impacted by climate shifts in the future, and how those changes will in turn impact coastal ecosystems. Researchers from California Sea Grant say the cooling marine layer directly impacts stream habitat for fish like salmon and trout.

“Fog might ruin a beach day, but it offsets summertime heat and water stress,” said Sara Baguskas, assistant professor in the Department of Geography & Environment at San Francisco State University and principal investigator in this research. “I think of it as this underdog climate factor that is critical to coastal California.”


Baguskas is an expert in coastal fog and how it affects vegetation. A few years ago, she started wondering how fog influences stream habitat, and what the implications of changing fog patterns are for vulnerable ecosystems. Baguskas received a California Sea Grant New Faculty Award in 2019 to investigate this question.

When coastal fog rolls in, it makes the area cooler and offsets atmospheric stress. Although there had been prior studies showing that fog can impact stream temperatures, Baguskas wanted to know to what extent coastal fog was impacting the stream habitat.

“It was a genuine question as to how much fog matters to the thermal environment in these streams that support salmon,” Baguskas said.

Baguskas and her team focused on two different field sites. The first was Willow Creek, a tributary of the Russian River and a popular recreation spot for locals. Baguskas and her team co-located their Russian River site with the California Sea Grant Russian River Salmon and Steelhead Monitoring program, which helped to share data sets and make the most out of their research.

The second was a tree-lined stretch in the Pescadero Creek watershed between San Francisco and Santa Cruz that provided the stream with lots of cooling shade.

“If you were to walk around the towns near our field sites, the locals would definitely speak to the presence of fog,” Baguskas said.

Fog can be difficult to study. It’s influenced by a combination of large-scale high-pressure systems, sea surface temperatures and inland temperatures. Baguskas and her team — which included a master’s student at San Francisco State University who took a leadership role in collecting, organizing and analyzing the data; and an undergraduate student who conducted independent research as part of the NSF Research Experience for Undergraduate Fellowship — sought to connect the dots between fog events, drought events and stream temperatures. 

The research revealed that extreme temperatures declined in association with fog events following a heatwave. In other words, fog helped relieve the thermal stress in streams. It was surprising to Baguskas that stream temperature and ambient temperature followed each other as closely as they did. In the summer of 2021, California experienced substantial drought. The researchers found that the drought resulted in lower water levels, causing the stream to be disconnected for at least half of the summer months. That means that in parts, the stream dried up, leaving pools isolated from each other. When this happens, the deeper pools become even more important for fish. The researchers were able to compare water levels between a normal flow year in 2019 and a low flow year in 2021. Surprisingly, isolated pools during the drought were cooler than the connected pools in 2019.

“The pools that we focused on were true refugia for the fish,” Baguskas said. As long as the pool’s water temperatures stayed within a favorable range, they were critical to the survival of the salmon population. 

Maintaining that ideal water temperature is a growing concern fueled by climate changes California is already experiencing. Heat waves and drought have pushed water temperatures in the pools toward lethal temperature thresholds. Researchers say that shift is now combined with uncertainty regarding the future of fog frequency and intensity. Fog that helps relieve thermal stress in stream pools is vital, says Baguskas, emphasizing the need for continued research.

“A change in fog will most certainly impact the ecosystems that have evolved with its presence.”

About California Sea Grant

NOAA’s California Sea Grant College Program funds marine research, education and outreach throughout California. Headquartered at Scripps Institution of Oceanography at the University of California San Diego, California Sea Grant is one of 34 Sea Grant programs in the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce.