In late 2013, a mass of warm water now known as the Blob appeared in the northeast Pacific—a massive marine heatwave that cooked coastal ecosystems from Alaska to California. Later, bolstered by an El Niño, the vast and potent heatwave wreaked havoc on marine ecosystems: thousands of seabirds died, while blooms of harmful algae poisoned marine mammals and shellfish. The suddenly warmed water also brought an influx of new animals to the northeast Pacific: ocean sunfish appeared in Alaska, while yellow-bellied sea snakes popped up in Southern California.

By 2017, the Blob had waned and many of these more tropical species had retreated. Yet not all. Some of the species that colonized new habitats during the heatwave have stuck around. And now, says Joshua Smith, a marine ecologist at the Monterey Bay Aquarium in California who documented in new research how the Blob triggered a range of subtle yet persistent shifts in the spread of marine species, “I’m starting to sort of question whether those communities will ever look the way they did.”

Historically, it’s common enough that a handful of individuals from warm-water species will make their way north during warmer years, but there wouldn’t be enough of them to sustain a long-term population, says Jenn Caselle, a marine ecologist at the University of California, Santa Barbara, and coauthor of the new paper. But because the Blob was so intense and lasted so long, sizable populations made the move into these normally cooler habitats—populations that were potentially large enough to establish more permanent footholds.

Señorita fish, for example—a bright-orange wrasse that showed up in huge numbers in central California during the heatwave—are still there, Smith says. Ocean whitefish, while historically common around Southern California’s Channel Islands, are now dominant, Caselle says, while California sheephead, a bulbous red-and-black fish, are now also much more abundant near Santa Barbara.

These changes in coastal communities, Caselle says, can have knock-on effects on how these ecosystems function. Sometimes, when one species is extirpated from a community—like a predatory fish that keeps a population of smaller fish in check or a seaweed species that provides a home for invertebrates—the ecosystem loses some kind of important function. But if that lost species is replaced by a new species that does the same thing, that new species could provide some resilience to the ecosystem, Caselle says, even if the community doesn’t look the same as it always did.

People can also adjust to new ecological realities, she says, pointing to fishers’ recently acquired fondness for the now-abundant ocean whitefish.

The Blob was one of the most intense marine heatwaves in recorded history, so it makes sense that it had a big effect on marine ecosystems. But big marine heatwaves have affected the northeast Pacific every year since 2019, including this year. Meanwhile, the current El Niño is further heating the northeast Pacific, and climate change means marine heatwaves will likely continue to be even more frequent.

As oceans continue to warm and the heatwave hits keep coming, William Cheung, a marine ecologist at the University of British Columbia who was not involved in the new research, says fish populations could be in trouble. In his own research, Cheung previously showed how warming and marine heatwaves will stress fish populations in the northeast Pacific. Usually, he says, fish populations can bounce back after a heatwave. But if heatwaves start occurring more frequently, populations will have less time to replenish themselves.

These changes are unlikely to go unnoticed. “The place where humans interact with the ocean the most is right at the coast. It’s where most of the biodiversity lives, and it’s where a lot of the productivity is,” Caselle says. “As these systems change, it can affect our everyday lives.”