A toxic algae bloom off the US West Coast that began earlier this year has grown into the biggest and most severe the region has seen in more than a decade.

The bloom and its toxins have thus far been reported from the coast of Santa Barbara, California, all the way up to Alaska, Raphael Kudela, a professor of ocean sciences at University of California Santa Cruz tells Quartz. “For comparison, the other really large event occurred in 1998, and occurred from approximately San Diego to Washington state.”

The algae in the bloom, named Pseudo-nitzschia, produce domoic acid, a neurotoxin, which was originally detected in California’s Monterey Bay in early May. By the end of the month it had reached “some of the highest concentrations… ever observed” in that area, according to UC Santa Cruz. Similar assessments are being made off the coast in Oregon, according to Michael Milstein of the NOAA Fisheries, part of the US National Oceanic and Atmospheric Administration. The agency has brought extra scientists to its Northwest Fisheries Science Center in Seattle to help survey and measure the bloom.

While blooms producing domoic acid are normal in the spring, the size and toxicity of this one are off the charts. Any toxicity above 1,000 ng/L (nanograms per Liter) “is considered an issue,” Kudela says, and this bloom’s levels have now exceeded 15,000 ng/L and maybe gone as high as 20-30,000 ng/L.

Fish like sardines and anchovies eat the algae and the nearby plankton, accumulating the toxin in their bodies. Kudela says researchers are still sorting through the data, but have measured toxicity in shellfish as high as 95 parts per million (ppm), and in anchovies from approximately 100 to 400 ppm. The legal limits are 20 ppm for both. The fish can then pass those toxins up the food chain to the birds and sea lions that eat them, causing neurological problems. The sea lion in this video is experiencing seizures, believed to be caused by eating fish with high levels of domoic acid.

Seafood from the West Coast currently on the market remains safe for human consumption, as it was either caught before the appearance of the algae bloom or came from safe areas. States monitor the levels of the toxin closely and stop the harvesting of “vulnerable” shellfish, Milstein says.

Commercial and recreational shellfish fisheries in Washington, Oregon, and California have all seen closures this spring, including the Dungeness crab fisheries in the south coast of Washington. There are other, open parts of the coast where toxicity is not high and Dungeness can still be harvested, and “some fishers have shifted their operations” to other Dungeness sites or are catching different kinds of fish, says Dan Ayres, a coastal shellfish manager at the Washington Department of Fish & Wildlife. But for the fisherman who rely on these areas for their income and can’t switch locations or catch, the financial impact will be severe. The Dungeness crab fisheries brings in approximately $19.9 million a year to the state.

Higher than usual temperatures are likely partly to blame for the bloom’s vigor, scientists say. El Niño, a cyclical warm weather trend, may be responsible for the temperatures, and climate change may also play a role, though it’s not certain. “These are the kinds of conditions we can expect to see more of in the future because of climate change,” says Milstein, “even though we can’t attribute this [specific one] to climate change.”