A massive marine heatwave has dominated waters off the West Coast since last summer. This marks only the third time on record that such a large section of the coastal ocean has remained so warm for so long—particularly into winter months—without it being an El Niño, NOAA scientists report. NOAA Fisheries and our partners are tracking possible heatwave impacts, which can include harmful algal blooms that can sicken marine mammals and close shellfish fisheries.
At one point last September, the current marine heatwave rivaled the enormous 2013–2016 marine heatwave known as “The Blob” in terms of size and surface temperatures. The current heatwave has raised the temperature of waters along the West Coast roughly 3 to 4 degrees Fahrenheit above normal. On September 9, 2025, the northeast Pacific reached its highest ever average temperature of 20.6 degrees Celsius, or about 69 degrees Fahrenheit. That’s almost a half-degree warmer than ever before. Past marine heatwaves shook up marine ecosystems that drove shifts of species, die-offs, and other disruptions of ecosystems in the Northeast Pacific Ocean.
In addition, the unusual warmth could begin to set the stage for a repeat of last year’s harmful algal bloom off Southern California. It hit unusually early in 2025 and killed hundreds of California sea lions, dolphins, and seabirds. Harmful algal blooms can also close fisheries, especially recreational fisheries involving shellfish that concentrate the toxins, affecting the coastal economy.
www.fisheries.noaa.gov
At one point last September, the current marine heatwave rivaled the enormous 2013–2016 marine heatwave known as “The Blob” in terms of size and surface temperatures. The current heatwave has raised the temperature of waters along the West Coast roughly 3 to 4 degrees Fahrenheit above normal. On September 9, 2025, the northeast Pacific reached its highest ever average temperature of 20.6 degrees Celsius, or about 69 degrees Fahrenheit. That’s almost a half-degree warmer than ever before. Past marine heatwaves shook up marine ecosystems that drove shifts of species, die-offs, and other disruptions of ecosystems in the Northeast Pacific Ocean.
Heatwaves Shift Species
The current heatwave has brought news of species in unusual areas, such as tunas caught in large numbers in Alaska last fall. The Blob and other marine heatwaves have been shown to reduce the survival of salmon in the ocean, leaving fewer fish to catch and to return to rivers to spawn. “We know these marine heatwaves alter ecosystem conditions, which affects fish and other marine life,” Leising said. “We’re very interested in what the fishing fleet and others who are out on the water are seeing and are looking into new ways to collect this information. Are there species in unusual places, or what might be changing?”In addition, the unusual warmth could begin to set the stage for a repeat of last year’s harmful algal bloom off Southern California. It hit unusually early in 2025 and killed hundreds of California sea lions, dolphins, and seabirds. Harmful algal blooms can also close fisheries, especially recreational fisheries involving shellfish that concentrate the toxins, affecting the coastal economy.
West Coast Waters Experiencing Another Large Marine Heatwave
NOAA scientists track growth of heatwaves that can affect fisheries.
