Commercial and recreational fisheries up and down the West Coast have been forced to close as a result of a massive toxic algal bloom, which scientists are describing as one of the largest in history.

“We have received reports of this particular bloom causing problems as far south as Monterey Bay and we’ve heard from our colleagues in Homer, Alaska that they’re seeing these cells,” Vera Trainer, manager of the Marine Biotoxin Program at NOAA’s Northwest Fisheries Science Center, told ThinkProgress. “It’s geographically very widespread, more so than we’ve seen in the past.”

The last time an algal bloom of comparative size occurred on the West Coast was in 1988. That bloom stretched from San Diego up to Washington.

Algal blooms happen when microscopic marine algae — also known as phytoplankton — proliferate in huge numbers. This proliferation results in a buildup of toxins such as domoic acid, a powerful and fatal neurotoxin. High concentrations of algae — or domoic acid — aren’t uncommon, occurring in the Pacific primarily in the fall, when ocean temperatures tend to be at their warmest. But according to Dan Ayers, coastal shellfish manager with the Washington Department of Fish and Wildlife, to see such an intense and extensive concentration of toxic algae in the late spring and summer months is more rare.

“The thing that is so significant of this bloom is its timing,” Ayers told ThinkProgress. “In the past, these blooms have occurred in the fall just prior to change of ocean conditions to a winter regime.”

Scientists are unsure exactly what is causing the historic bloom, though Trainer said that it is likely related to unusually warm ocean temperatures.

“We’ve had an unusually warm and sunny spring, and that not only affects our land plants, but the sea plants,” she said.

Scientists first noticed a patch of unusually warm water off the West Coast in the fall of 2013, and have since attributed everything from the California drought to the recent swarm of red crabs on California beaches to the so-called “blob.” Though the blob has since dissipated, ocean temperatures off the Pacific coast remain about 2° Celsius warmer than normal.

“Essentially what we’ve got is just perfect plankton growing weather,” Ayers said.

Domoic acid can end up in anything that feeds off of algae, from filter feeders like shellfish to small fish like sardines. The animals that feed on those organisms can also become poisoned by the toxins; Trainer described a sea lion observed having seizures early this week off the Washington coast as a result of eating contaminated fish. The sea lion had to be euthanized.

“It has really disruptive impacts to the ecosystems,” Raphael Kudela, professor of ocean studies at the University of California, Santa Cruz, told ThinkProgress. “The sea lions and the birds don’t pay attention to the warnings, so they’re the ones that are really being impacted. It’s much more a wildlife concern at the moment than a human health concern.”

The bloom isn’t without its impacts to humans, however, especially for small coastal communities. All beaches along the Washington coast have been closed to razor clamming, a closure that comes at a critical time for coastal towns that were expecting tourism revenue from recreational clammers. The state has also closed half of its coast to commercial Dungeness crab fisheries, a move that Ayers called “unprecedented.”

“We’ve never closed [Dungeness fisheries] because of harmful algae blooms,” he said, adding that while the large part of the commercial season is over, there are still some 30 to 50 small crabbing boats active.

“Each of those boats have three to four guys that depend on that income,” Ayers said. “It’s not a huge impact for the wider economy, but to the local economy it is a huge impact.”

Trainer also explained that the algal bloom can have an outsized impact on tribal communities, which tend to live far from grocery stores and depend more on shellfish and crabs for both food and religious ceremonies.

“It’s not only an economic impact, it’s a social impact,” Trainer said.

“Washington isn’t the only state whose fisheries have been impacted by the bloom. Oregon has stopped all shellfish harvesting from the Columbia River to just north of Cannon Beach, and has closed the entire coast to razor clamming. In California, coastal fisheries are keeping a close eye on the toxin levels in their shellfish — last week, mussels in Santa Barbara surpassed the threshold for toxicity, but this week have returned to safe levels.

On Monday, NOAA deployed a research vessel to better understand the ocean conditions that are causing the bloom. The vessel will collect water and algae samples from areas off the Mexico border to the tip of Vancouver Island during its three-month journey, slated to end September 11.

But Trainer and Kudela hope that the samples collected from the ship will tell scientists about more than just the present bloom. They hope that by providing a more complete picture of the physical properties that encourage algal blooms, scientists will gain a better understanding of how climate change might impact those blooms in the future.

“The conditions this year are sort of a window to the future, they’re a sign of things to come,” Trainer said. “We want to know if that is indeed the case. Is this what we’re going to be seeing more of as our oceans continue to warm?”

Scientists worry that algal blooms will get worse with climate change. Algae tends to grow better in warmer waters, both because some species prefer warmer waters and because warm waters are less prone to mixing, which impedes algae from growing. Changes in rainfall patterns could also make algal blooms worse, with large downpours creating more of a chance for nutrient runoff, which helps feed algal blooms.