Feeling Crabby

Why are Dungeness crabs washing up on Oregon beaches this summer?

If you’d been strolling along the beach in Seaside at mid-afternoon on Aug. 11, you might have noticed something a little odd. At the high-water mark, along with driftwood, bits of seaweed and toddlers chasing each other with chubby legs, long swaths of Dungeness crab shells stretched along the sand.

Many of the shells were fully intact, heavy with flesh. The cool ocean breeze that day picked up the pungent, briny smell of rot, and seagulls hopped and pecked through the shells. 

Hundreds, possibly thousands, of Dungeness crabs had washed up dead on the beach. The scene puzzled beachgoers. Was this a normal event, such as a seasonal molt, or some sort of catastrophic die-off due to climate change? 

August is molting season for male Dungeness crabs. About once a year, a crab’s old exoskeleton separates from the new one below, allowing the animal to grow. Adult crab populations tend to molt simultaneously, resulting in masses of cast-off shells on beaches in the late spring from females and late summer from males.

In fact, the Oregon Department of Fish and Wildlife (ODFW) often receives calls from worried beachgoers in August reporting dead crabs on the beach that are actually molts.

It’s typical for some dead crabs to be mixed in with molted shells. Molting is stressful for crabs, and some die in the process.

Yet at Seaside that day, not many of the crab shells on the beach appeared to be empty, as molted shells should be. 

Recent reports from the Oregon coast indicate that this type of event — more dead crabs than usual mixed in with seasonal molted shells — might be starting to occur every summer.

More than just seasonal molting?  

According to ODFW, larger than normal numbers of dead Dungeness crabs have been washing up at Seaside Cove for the past four years. 

“This is definitely a trend at Seaside Cove. Since 2015, we’ve been noticing large amounts of crab molts and morts [dead crabs mixed in with molted shells] during the month of August,” says Matthew Hunter, shellfish biologist at ODFW. “Last year, the amount on the beach actually impeded our survey of razor clams. Is this normal? No, because there’s more abundance.” 

On Aug. 12, 2018, Oregon Coast Beach Connection noted “an unusually large amount of [dead Dungeness crabs] on just about every beach on the Oregon coast.” 

And in a Facebook post from Aug. 10, 2018, the Haystack Rock Awareness Program said: “We’re seeing hundreds of dead Dungeness crabs wash in on Oregon north coast beaches, and we’re not sure why! They are not molts.”

Could hypoxia be a culprit? 

It’s too soon for ecologists and others who keep close tabs on Oregon’s oceans to pinpoint a definitive cause of dead crabs on local beaches. But certain factors related to climate change, including hypoxia, could play a role.   

To some extent, hypoxia in coastal regions is typical. Hypoxia, which means a deficiency of oxygen, occurs seasonally near the coast due to factors such as algae blooms and a summer upwelling of oxygen-poor water from ocean depths. 

But hypoxic events — or “dead zones,” where oxygen levels drop below 1.4 milliliters per liter of water — appear to be getting worse and more frequent along the Oregon coast, says Caren Braby, marine resources program manager for ODFW. 

 “Hypoxic events have probably been happening for some time every few years, but all indications are that they’re now a more widespread phenomenon, are happening every summer, and oxygen is severely depleted when they do occur,” she says. 

There’s even a name for it now: Oregon’s summer hypoxia season. The leading hypothesis about why it happens is ocean changes due to climate change. Among other impacts, climate change creates warmer water, which can hold less dissolved oxygen.

Crabs are pretty mobile, so during hypoxic events they likely head to deeper water to get away, Braby says. “But crabs still stuck in the area for some reason, including those in crab pots, die. We have a lot of evidence of that,” she says.  

Francis Chan, associate professor of marine ecology at Oregon State University and an expert on ocean chemistry, says he first became aware of crab mortality events in 2002 with reports from fishermen off the Oregon coast pulling up crab pots with dead crabs inside. But episodes like this used to happen some years and not others.

“Now it occurs every summer. Last year, we saw strong evidence of die-offs: a lot of crab pots that had dead or missing crabs when they should have been caught,” Chan says. 

But what about the crabs washing up at Seaside Cove and possibly other beaches?

“Crabs going through a molting cycle probably aren’t able to get up and out of the way of a hypoxic event, so they may be vulnerable. But that’s just a hypothesis,” ODFW’s Braby says.

Chan agrees that hypoxia could be one factor in why more crabs than usual have been washing up at Seaside Cove. Because hypoxia’s a stressor, it could prove fatal to crabs that are already vulnerable in some way or unable to move to a more oxygen-rich area. 

But, he says, the ocean is changing faster than current scientific knowledge. And changes can compound in new and unexpected ways, resulting in surprises. There’s still much that scientists don’t understand.

A shifting ocean 

Hunter the shellfish biologist recalls walking the beach at Seaside Cove in the early morning in August trying to do his razor clam survey. Along with the large number of dead crabs on the beach, he noticed something even more unusual.

“There were a tremendous number of crabs, and some of them were still alive,” he says. “It was apparent that they weren’t doing too well, that they weren’t in a great habitat. There were obviously some environmental factors that were causing them to be lethargic. Usually crabs in tide pool areas bury fast to get away from predators, but these would not. They were barely able to keep themselves upright,” he adds. 

Hunter can’t speculate on the reason for the lethargic crabs. For now, it’s one of the ocean’s new surprises that scientists are left to decode. 

Focusing on Resiliency

To a casual observer, a beach full of dead crabs might appear catastrophic. To a scientist, it’s at least notable. But at this point, there’s no evidence that Oregon’s Dungeness crab population is currently in steep decline. 

Hunter says that despite the larger than usual number of crab strandings over the past five years, he thinks Oregon’s Dungeness crab fishery is still doing well. “We haven’t seen a change in our harvest, either recreational or commercial,” he says. 

Still, other factors that can co-occur with hypoxia, such as ocean acidification, are shown to be hurting Dungeness crab larvae, Braby says. And if larvae continue to struggle, the crab population will likely suffer over time.

Resiliency will be key going forward, she says. “Problems like hypoxia will be coming our way for decades. We need to figure out ways to help the ocean and our economies adapt.”

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