A new study by Oregon State University researchers and others could help predict the spikes in toxic algae that have led to periodic closures of shellfish harvests on the West Coast.
The study, published Monday in the Proceedings of the National Academy of Sciences, finds a strong connection between warm ocean conditions caused by two major climatic cycles — El Nino and the Pacific Decadal Oscillation, or PDO — and periodic increases in domoic acid in shellfish.
Domoic acid is a powerful neurotoxin produced by certain types of plankton and ingested by shellfish. It can cause serious health effects in humans, ranging from gastrointestinal illness to seizures and, in rare cases, death. It has also been responsible for periodic die-offs of marine mammals.
When toxin levels rise to unsafe levels, fisheries managers announce harvest closures to protect human health, causing millions of dollars in losses to West Coast fishing and tourism industries.
Both El Nino and PDO can raise water temperatures in parts of the Pacific, but they occur on different time scales. When both are in a strong warm phase at the same time, the researchers found, it can interrupt the usual pattern of cold water flowing south along the West Coast of the United States.
“You have a weakening of the California current,” explained Angel White, an associate professor in OSU’s College of Earth, Oceanic and Atmospheric Sciences and a co-author of the PNAS study. “You can get a little slosh-back of that warm California water.”
With financial support from the National Oceanic and Atmospheric Administration, the team used the OSU research vessel Elakha to collect samples of copepods — tiny crustaceans that are carried by ocean currents — from multiple locations off the Oregon coast starting in 2007. Copepods from cold northern waters tend to be rich in lipids, while their southern counterparts are poor in the nutritious fatty acids, so analyzing the relative numbers of the different types provided information about the movement of water masses up and down the coast.
When the conditions are right, the south-flowing California current gets weaker, allowing more lipid-poor copepods to float north — along with more of the toxic plankton that can get into shellfish and cause commercial and recreational harvest closures for razor clams, Dungeness crabs and other species.
“The hypothesis we have is that the (plankton) populations to the south are just more toxic,” White said.
The researchers combined their data with NOAA datasets going back even further to create time series spanning two decades. The information helped fill a data gap for Oregon waters, which had not been as well studied as the seas off Washington and California, and create a clearer picture of long-term patterns affecting ocean conditions along the entire coast.
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“It all suggests that we’re seeing warm water transfers from the south, and that’s only happening in PDO years — when both the PDO and El Nino are strongly positive,” White said.
Spikes in toxic plankton and the possibility of costly shellfish harvest closures aren’t the only harmful effects of these periodic changes in ocean conditions, White and her colleagues noted. The warmer southern waters also bring larvae of the invasive green crab, and the shortage of lipid-rich copepods in the food chain can have a negative impact on salmon runs.
Based on their findings, the researchers created a risk-analysis model for the West Coast which could provide an early warning sign for fisheries managers that shellfish harvest closures may be necessary.
"This is going to be another tool in a resource manager's bag," said co-author Matthew Hunter, who works in the Astoria office of the Oregon Department of Fish and Wildlife.
When the model shows elevated risk factors, Hunter said, it could be a cue to do more sampling to check for toxic plankton in the ocean and increased domoic acid levels in shellfish populations.
"This will allow us to be more proactive than we've been in the past," he said.
The researchers were cautious about drawing connections between their findings and global warming. While generally warmer ocean temperatures could certainly contribute to more frequent outbreaks of shellfish toxicity, White said additional study would be needed to determine causal relationships.
“It’s hard in science to get support to really sustain that kind of long-term monitoring,” White said. “We just now have time series to look at the PDO. I think we would need longer time series to look at climate change.”
OSU doctoral student Morgaine McKibben was the lead author of the study in PNAS. Her co-authors were White, William T. Peterson and Vera L. Trainer of NOAA, A. Michelle Wood of the University of Oregon and Matthew V. Hunter of the Oregon Department of Fish and Wildlife.