Researchers have identified a strong correlation between toxic levels of domoic acid in shellfish and the warm-water ocean conditions orchestrated by two powerful forces, namely El Niño events and the Pacific Decadal Oscillation.

In addition, researchers from Oregon State University (OSU), the University of Oregon, the National Oceanic and Atmospheric Association (NOAA), and the Oregon Department of Fish and Wildlife have developed a new model to predict with some accuracy the timing of domoic acid risks in the U.S. Pacific Northwest.

The findings were published Monday in Proceedings of the National Academy of Sciences.

Domoic acid, a potent neurotoxin produced by specific types of phytoplankton and ingested by shellfish, can cause serious health effects in humans and some other animals. In recent years, dangerous levels of these toxins have led to the repeated closure of crab and shellfish harvesting in the Pacific Northwest and elsewhere.

Until now, while the problem threatens public health, marine wildlife and inflicts huge costs for coastal economies, its connection to larger climatic forces has been suspected, but not confirmed.

"In the natural world there are always variations, and it's been difficult to connect a specific event to larger forces that operate over periods of years and decades," said Angelicque White, an associate professor and research team leader in the OSU College of Earth, Oceanic and Atmospheric Sciences. "To do so, long observational time-series are crucial. With NOAA's commitment to sponsored coastal ocean research and monitoring, along with state support for monitoring shellfish toxins, we've finally been able to tease out short term variability from natural climate forcing."

Using a combination of time-series data spanning two decades, the new study shows that oscillations to positive, or warm-favorable conditions in natural climate cycles can reduce the strength of the south-flowing California Current. This allows more movement northwards of both warmer waters and higher levels of toxic plankton, and also brings that toxic mix closer to shore where it can infiltrate shellfish.

Beyond problems with domoic acid levels, White said, this correlation appears to mirror problems with green crabs, an invasive species of significant concern in the Pacific Northwest. These same warm climate phases lead to increased numbers of green crabs in Oregon waters, where they compete with native Dungeness crabs. The conditions also deliver communities of lipid-poor "copepods," types of small crustaceans that float with currents, from the south, that are associated with reduced salmon runs.

"Part of the concern is that a large influx of the plankton that produce domoic acid can have long-term impacts," Morgaine McKibben, an OSU doctoral student and lead author on the study, was quoted as saying in a news release. "For example, razor clams are filter-feeders that bioaccumulate this toxin in their muscles, so they take much longer to flush it out than other shellfish. The higher the toxin levels, the longer it takes for razor clams to be safe to eat again, perhaps up to a year after warm ocean conditions have subsided."

Produced by the diatom genus Pseudo-nitzschia, domoic acid enters the marine food web when toxic blooms of these micro-algae are ingested by animals such as anchovies and shellfish. First identified as a public health threat in 1987 and referred to as "amnesic shellfish poisoning," human symptoms can range from gastrointestinal disturbance to seizures, memory loss or, rarely, death.

Domoic acid events have been linked to mass deaths of marine mammals, like sea lions, sea otters, dolphins and whales. And closures of Pacific Northwest beaches to shellfish harvests, such as those that occurred in 2003, 2015 and 2016, can result in large economic impacts to coastal towns and tourism. In 2015, domoic-acid related closures led to a decline in value of nearly 100 million U.S. dollars for the West Coast Dungeness crab fishery, according to the Fisheries of the U.S. Report 2015. Enditem