Marine heatwaves can reduce salmon survival in the ocean because warmer waters are typically less productive. Coho salmon, shown here, have been an exception in some areas since they often stay closer to the coast, taking advantage of cooler and more productive upwelled waters. (NOAA Fisheries)By NOAA Fisheries
A massive marine heatwave is dominating waters off the West Coast since the summer, marking just 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 reported Tuesday.
NOAA Fisheries said the agency and its 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.
Marine heat waves off the West coast, showing departure from normal sea surface temperatures from 1990 through 2025. (NOAA Southwest Fisheries Science Center)The current heatwave has raised the temperature of waters along the West Coast roughly 3 to 4 degrees Fahrenheit above normal.
On Sept. 9, the northeast Pacific reached its highest ever average temperature of 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.
“We have forecast tools that provide some insight, but we are also watching carefully for implications on the marine ecosystem,” said Andrew Leising, a research oceanographer at NOAA Fisheries’ Southwest Fisheries Science Center who runs the California Current Marine Heatwave Tracker.
The tracker has been documenting marine heatwaves off the West Coast through NOAA data from satellites, ships, and buoys since 2019.
The North Pacific has repeatedly hit record or near-record temperatures since The Blob. Like others before it, the current marine heatwave weakened and receded from the coast in October and November 2025. Unlike others, it has since strengthened and returned.
“We’re in La Niña, but water temperatures along our coast look much different,” Leising said. “The conditions are hard to reconcile. We want to be cautious in our interpretation, but at the same time this is not a situation that we have seen before.”
The current heatwave has brought news of species in unusual areas, such as tuna caught in large numbers in Alaska last fall. The Blob and other marine heatwaves have also 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?”
The temperature of the Pacific Ocean’s surface off the West Coast was measuring 1 degree Celsius above normal in mid-February. (NOAA Southwest Fisheries Center)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.
“Although rivaling the Blob in area, the current marine heatwave has had much less ecological impact since so far since it has not penetrated as deeply into the water column, and has not spent as much time near the coast,” said Leising.
NOAA’s marine heatwave forecasts suggest that the warm waters may dissipate in the coming months as warm surface waters mix with cooler water from below, said Michael Jacox, a research scientist at the Southwest Fisheries Science Center. If that happens, however, one key question is whether enough warmer waters will remain to combine with the nutrients to fuel another bloom of harmful algae.
“We are in uncharted conditions, so we need to assess the most likely outcome given what we know,” Leising said. “This is what makes the ocean so interesting. We see different pieces of the picture, but ultimately it’s the marine ecosystem that brings those together and demonstrates where the conditions are leading us.”
