People who make their living on the waters off the Oregon coast have long known that ocean acidification is a problem. As carbon dioxide in the atmosphere is absorbed by the ocean, it becomes more acidic and carbonate ions, which many crustaceans use to build their shells, become scarce.
Between 2006 and 2008, the Whiskey Creek Hatchery on Netarts Bay saw massive oyster larvae die-offs and shellfish in some other spots along the West Coast have experienced similar problems. But up until now, ocean acidification has not posed threats to Oregon’s most commercially valuable shellfish: the Dungeness crab. That may no longer be the case.
On Wednesday, researchers from the National Oceanic and Atmospheric Administration released a study that for the first time shows that corrosive ocean waters are causing damage to the shells of larval Dungeness crabs and hampering the development of important sensory organs on the crustaceans. And, as fossil fuel use continues to pump carbon dioxide into the atmosphere, it’s happening decades earlier than researchers had predicted.
“We didn’t think we would see this kind of dissolution until the middle of the century,” said Nina Bednarsek, senior scientist with the Southern California Coastal Water Research Project and lead author of the study. “They are much less protected than previously shown.”
While previous studies conducted in labs have shown that ocean acidification can have harmful effects on Dungeness crabs, this study marks the first to show the effects on larval crabs in their natural environment.
“It is really hard to demonstrate the effects of climate change and ocean acidification in the field like this. That’s the really novel aspect of this work.” said George Waldbusser, an ocean ecology professor at Oregon State University, who was not part of the study. “There’s more work to be done, but this is an important step.”
Previous research has shown that the ocean absorbs as much as 25 percent of human-made carbon dioxide. As the carbon mixes into the ocean, chemical reactions in the water column produce more hydrogen ions and fewer carbonate ions, lowering pH levels and increasing acidity. The problem is often exacerbated in near-shore waters where upwelling brings cold, acidic waters from the depths to the surface. Other federal researchers have found that ocean waters off the West Coast are acidifying at twice the rate of global average.
Bednarsek and a team of scientists collected samples of larval crabs on a research cruise in 2016 that spanned the West Coast from Mexico to the Canadian border, using nets to skim the waters near the surface, where crabs congregate at night to feed. They motored on lines perpendicular to the coast, from the deep ocean far offshore to the shallow waters near the beach where acidification is often more pronounced and where Dungeness crabs grow and mature.
Back in the lab, Bednarsek looked at the crabs’ shells, known as its carapace, under an electron microscope. She saw scars and abnormal ridging on surfaces that were usually smooth, caused by the corrosive chemical makeup of near-shore waters. Bednarsek also saw, to her surprise, that the acidic water was impacting the crabs’ ability to grow hair-like sensory organs called mechanoreceptors. Acidic water caused the receptors, which help the crabs navigate their environment, to fall out in some of the animals collected for the study.
The damage to their shells and the loss of their sensory receptors could alter their swimming behavior and impair their ability to regulate their buoyancy, making them easier pickings for predators.
Dungeness crab is Oregon’s most valuable single-species commercial fishery, said Caren Braby, a marine program manager with the Oregon Department of Fish and Wildlife, which regulates industry. Roughly 16 million pounds of Dungeness crab are pulled from the state’s coastal waters every year, according to the state, and the creatures’ meaty legs and claws are considered a delicacy by many. Crab fishing is also an integral part of coastal culture in places like Newport and Astoria.
“Almost all fishermen in Oregon have some connection to Dungeness crab. It’s a powerhouse of economic value.” she said. “The findings, that there are weak spots in the life cycle connected to changing ocean conditions, are very concerning.”
Ocean acidification is just one of several stressors affecting the ecosystem off Oregon’s coast, Braby explained. Ocean water is getting warmer, increasing the likelihood of toxic algal blooms and seasonal areas of low oxygen, known as hypoxia, are causing changes to the food web, from the smallest species of plankton to top predators like sea lions and whales.
“As management, we look at things like where can we reduce stress,” Braby said, explaining that the state regulates the harvest of species like Dungeness crab in a way that will protect the animals themselves and the livelihoods of those who catch them.
Bednarsek said more research would be needed to fully understand the long-term impacts of ocean acidification on Dungeness crabs. Unlike bivalves, which keep their shells for their entire lives, crabs shed their carapace as they grow so the damage to their shells as larvae may not severely impact their ability to mature. What is well understood, though, is that ocean acidification is likely to increase as long as carbon dioxide emissions remain robust.
“With changes expected, we will see even more effects,” Bednarsek said. “If you don’t know this is happening, you can’t do anything. If you do, you can manage it.”
— Kale Williams