Back in 1998, something strange started happening to California sea lions. Over the course of just a few months, hundreds of them hauled themselves onto beaches near Monterey Bay and began convulsing uncontrollably. They were having seizures; many died right there on the sand. “It was a mystery,” remembers Kathi Lefebvre, who was then a grad student at UC Santa Cruz. “No one knew what was wrong.”

But Lefebvre had a suspect: domoic acid. A powerful neurotoxin, it’s produced by algae in the genus Pseudo-nitzschia—and gobbled up by krill, anchovies, sardines, and other small fish that eat the algae. When there’s a lot of Pseudo-nitzchia around, these algae-feeders turn into little domoic acid bombs, waiting to deliver a concentrated dose of the toxin to their predators. Lefebvre was studying domoic acid because it was already known to cause brain damage and death in humans. The disoriented, seizing sea lions reminded her of those cases. Tests of dead sea lions and their prey proved Lefebvre right: They were both packed with the toxin.

“After that event, it started happening all the time in California,” says Lefebvre, who now runs the Wildlife Algal Toxin Research and Response Network at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center. Clusters of seizing sea lions would show up on the beaches anytime there was an algal bloom in the Pacific—a distressing event, certainly, but not necessarily cause for alarm. “The blooms do wax and wane,” says Claire Simeone, a conservation medicine veterinarian at the Marine Mammal Center in Sausalito, California. A spike in poisoned sea lions meant a lull was on its way.

At least until this year. An unusually deep layer of warm water in the Pacific has fueled a Pseudo-nitzchia bloom like scientists have never seen. Usually algal blooms last for a few weeks, maybe a month, Lefebvre says. “This has been for the entire summer.” Plus, “this bloom has been massive,” stretching from California to at least Washington, and potentially as far north as Alaska. This week, the California Department of Public Health warned human diners to avoid Dungeness and Rock crabs caught off the coast because of dangerous levels of domoic acid. But sea lions eat crabs, too, in addition to all those little algae-feeders like sardines and anchovies—all packed with domoic acid because of the monster bloom. And there’s no way to warn them of the poison lurking in their meals.

Some sea lions had such extensive brain damage that Simeone and her team decided it would be more humane to euthanize them.

At the Marine Mammal Center, Simeone has already seen 213 sea lions affected by domoic acid poisoning this year. That’s compared to 233 cases in all of 2014, and up from only 63 cases in 2013. Even worse, this year is the first time Simeone has seen poisoned Guadalupe fur seals, an endangered species. “There’s really no antidote for the toxin itself,” she says. And although domoic acid is usually flushed out of the body within 24 to 48 hours, that’s plenty of time for it to cause permanent brain damage. Eighty percent of Simeone’s domoic acid poisoning patients have died this year. Some of those were direct deaths from poisoning; others had such extensive brain damage that Simeone and her team decided it would be more humane to euthanize them.

In both sea lions and humans, domoic acid targets the part of the brain called the hippocampus, and particularly affects memory and spatial processing. That’s particularly bad news for top predators like sea lions, who need those skills to hunt and migrate. “With more chronic exposure to the toxin, animals will seem to lose their memory,” Simeone says. “We find them in very abnormal places, like up a river, in an orchard, on a police car.”

Those chronic effects are particularly worrisome to Lefebvre: “You can’t navigate well, you’re not feeding properly, you’re going to the wrong place.” Sea lions play a vital role in food webs by regulating populations of all kinds of other species. If enough of them were to succumb to domoic acid poisoning—or even just lose their hunting skills—that could be devastating for entire ecosystems. What’s more, the affliction is spreading. This is the first year that poisoned sea lions have shown up in Washington, Lefebvre says.

For now, the huge bloom in the Pacific shows no signs of dissipating. And it’s likely just the first sign of things to come. As ocean temperatures warm as a result of climate change, scientists fully expect to see more and more blooms of this magnitude. That will probably increase both the number of sea lions that die from acute domoic acid poisoning and those that suffer chronic brain damage. The repercussions will be felt throughout marine food webs. “We don’t know what we’re going to be seeing in the next five to ten years,” says Simeone. This year’s bloom and subsequent marine mammal massacre, she says, could be “a new normal.”