February 22, 2020

Dungeness crabs show indication they may be vulnerable to effects of ocean acidification

MONTEREY — Ocean acidification along the Pacific coast is slowly corroding the shells of young Dungeness crabs floating in the plankton just offshore. Their persistence in the face of ongoing climate change, in which the ocean will continue to acidify, could determine the future of one of the state’s most lucrative fisheries.

A recent study by Dr. Nina Bednaršek, a senior scientist with the Southern California Coastal Water Research Project, examined the shells of larval Dungeness crabs no bigger than a dime and found evidence of pitting and scarring on crabs collected in areas with more acidic water. The results, published in the journal Science of the Total Environment, suggest that ocean acidification is already impacting wildlife.

“It seems like it’s a very easy-to-trigger response, and the first line of evidence that they respond to these negative conditions,” Bednaršek said.

While adult Dungeness crabs are widely monitored, the lives of larval crabs are almost completely unknown. Before settling onto the seafloor, juvenile Dungeness crabs drift about as plankton, pulling free-floating minerals from the surrounding seawater to build a shell like so many microscopic bricks.

Declines and closures of other fisheries have pushed more people to fish for Dungeness, a finding confirmed by California Department of Fish and Wildlife. (Monterey Herald archives)

Bednaršek collected larval crabs off Oregon, Washington and British Columbia, using a high-powered microscope to magnify their shells by as much as 20,000 times. Through computer modeling, she was able to back-trace their likely path along the coast, linking them to areas of upwelling that bring acidic water to the surface.

Ocean water becomes more acidic by absorbing carbon dioxide from the air. While this happens naturally, human activities are also increasing the amount of carbon dioxide in the atmosphere. These gases move throughout the ocean in deep currents, emerging close to shore in upwelling regions such as the west coast of the United States.

Saturated with carbon dioxide, the natural balance of water chemistry shifts to one that eats away at a larval crab’s delicate shell.

“They end up pooled in the coastal waters with extended exposure to more acidic conditions,” Bednaršek said, adding that plankton are generally at the mercy of currents to move them about. “So it becomes a double whammy. They are living and having to mature under conditions that are unfavorable.”

This phenomenon has been observed in other types of plankton, including microscopic animals called foraminifera and swimming snails known as pteropods. Recent research comparing modern-day plankton to samples taken during the famed 19th century Challenger expedition has shown that today’s shells are 76% thinner on average.

What happens at this microscopic scale also matters because plankton form the base of the food web. Even the largest whale is linked through the food it eats to these seemingly invisible organisms. Their proliferation in the waters of Monterey Bay sustains the area’s rich ecosystem.

Monterey Bay sits at the southern tip of the range for Dungeness Crab, and both commercial and recreational fishermen gear up each winter to deploy their crab pots over the bay’s …read more

Source:: The Mercury News – Lifestyle


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