Low-oxygen levels off coast a concern for fisheries

Sep. 14—Low-oxygen levels off the coast of Oregon and Washington state hit earlier than usual this spring and have lasted longer than any other hypoxic event recorded on the West Coast in the past 35 years.

Exacerbated by human-caused climate change, it has led scientists and fishermen to worry about the formation of a massive "dead zone" and fed concerns about the unknown long-term impacts on ocean ecosystems and valuable fisheries like Dungeness crab.

The Pacific Northwest has experienced hypoxia seasons regularly for two decades. Winds initiate seasonal upwelling, pushing warmer surface water off from shore and drawing up colder, nutrient-rich but low-oxygen water from below. Hypoxia events occur as organisms in the water die and sink, removing even more oxygen.

While fish and crab often flee hypoxic zones, some organisms and animals can become trapped in the middle of these low-oxygen areas and die.

Francis Chan, a marine ecologist with Oregon State University, said scientists expected a bad hypoxia season this year based on weather conditions in April, but they had not expected the low-oxygen levels to persist for so long.

By midsummer, the winds that started the upwelling process were still going strong at a time when scientists would normally expect to see a shift and a corresponding rise in oxygen levels.

At the end of August, oxygen levels in the ocean off Oregon and Washington were as close to zero as scientists had seen all year. The hypoxic zone was approaching 8,000 square miles, still growing just 6 miles offshore. The low-oxygen zone — Jack Barth, an oceanographer with Oregon State University, thinks of it as a ribbon on the sea floor — stretches from Seattle to around the Cape Blanco and Coos Bay area.

Scientists are waiting for the first fall storms to roll through and flush coastal waters and mix oxygen deep down.

"That's when we get to really say, 'OK, finally the hypoxic event is over,'" Chan said.

Detailed mapping

This year's hypoxia season has emphasized the need for more detailed mapping, Chan and Barth said.

Both men happened to be out collecting data in separate areas around the same time this year. They were able to match what they were seeing with additional information collected by the National Oceanic and Atmospheric Administration to show the low oxygen levels extended from Oregon into Washington.

"We just have to be out there all the time mapping this," Barth said. "And we've got to do a good job of communicating where it's not so bad and where it's hurting. That way we can more sustainably use the ocean."

Chan deployed 38 dissolved oxygen sensors with commercial crab fishermen this year to expand tracking of low-oxygen areas. The fishermen zip-tied the sensors to crab pots and sent them down at fishing grounds, providing Chan with up-to-date information about important areas.

He plans to send the sensors out again next season. These kinds of partnerships need to continue and expand, he said.

Even though oxygen levels will jump up with the arrival of fall storms, questions remain about the organisms that had weathered low oxygen conditions for months this year.

In recent years, commercial crab fishermen still saw strong landings after a bad hypoxia season. In waters near the surface, the ocean is still very productive, scientists note, but the long-term effects of the low oxygen conditions on marine life in the bottom third of the water is unknown.

"Dead zone" — a phrase many people use when discussing hypoxic events — is not a scientific term.

"It makes it sound like the Oregon Coast is a dead wasteland," Chan said. "That's pretty far from the truth. There's still a lot of resilience."

Fallout

Still, Chan wonders: Did crab have to spend extra reserves to survive this stress or are they completely fine when oxygen levels rise again? Were animals able to move away and find high-oxygen refuges, or did they get stuck and how did that affect growth and survival?

Land-based communities are experiencing the fallout from the rise of more extreme climate change-related events, Barth and Chan noted. People can see how the summer heat waves hit trees and plants this year. They actively experience the impacts of massive wildfires and long-term drought.

But from a viewpoint along U.S. Highway 101, the ocean looks exactly the same.

What people need to realize, Barth said, is that "all the climate change things we're seeing on land and in the atmosphere — the heat and the fires and the increased hurricanes — the same thing is happening in the ocean: We're getting more extreme events."