50 Years of Environmental Data Can Predict Health and Strandings of Sea Lions Pups in California

For more than 50 years, NOAA Fisheries has conducted vital research on seal and sea lion populations at a remote research station on San Miguel Island, California. The long-term data set provides clues to help us understand what environmental conditions lead to malnourished California sea lion pups and increased stranding levels. This information is important for managing their populations and helping rehabilitation centers prepare for periods when sea lion pup strandings may be elevated.

Our research began on San Miguel Island in 1969 after scientists discovered a colony of northern fur seals breeding on Point Bennett the year before. This amazing uninhabited island lies 26 miles offshore of the California coast. It's part of the Channel Islands National Park and is home to some of the largest populations of land-breeding pinnipeds in the world. Prior to this discovery, northern fur seals were last documented in California during the early 1800s-before their population was wiped out by seal hunters. Researchers began studying the new colony of fur seals along with California sea lions that also breed on the island.

We conduct long-term studies on abundance, distribution, survival, reproduction, foraging ecology, health, and disease of northern fur seals and California sea lions. Our research led to California sea lions being designated a top-level predator and a sentinel species of ocean health. Scientists use this long time series to understand how species respond to changes in their environment.

California sea lions are ecosystem indicators because they respond rapidly to environmental changes. These include increases in ocean temperatures that can affect food availability or the production of algal toxins that can result in death. Starving or sick sea lions often become stranded on public beaches and pose a public health risk because of infectious diseases. When live strandings are high, they can strain rehabilitation facilities, which care for stranded sea lions but operate on limited resources.

Stranding events are often associated with ocean-warming events, like marine heatwaves , that reduce the amount of prey available for sea lions. Today, scientists study these links to help understand, predict, and prepare for large stranding events on the California coast.

In late 2025, a team from NOAA Fisheries and the University of Washington used more than 50 years of California sea lion pup data from San Miguel Island and local ocean conditions. They developed a predictive model linking pup weight and growth to stranding event levels. The preliminary results indicate we can predict the weight and growth rates of California sea lion pups based on sea surface temperatures. This helps provide an early warning of conditions in the California Current Ecosystem that may lead to large stranding events of weaned pups due to poor body condition at weaning.

Having this information months ahead of time allows state and federal managers and stranding network partners to anticipate and coordinate responses to large stranding events of emaciated sea lion pups. Existing regional ocean models that simulate ocean temperatures and other physical and biogeochemical conditions allow us to forecast these predictions months in advance. For more on this story, see this newsletter from University of Washington's Cooperative Institute for Climate, Ocean, and Ecosystem Studies.