Crab sit inside a vendor booth outside Alioto's Restaurant at Fisherman's Wharf. (Jessica Christian/S.F. Examiner)

Study predicts decline in Dungeness crab from ocean acidification

SEATTLE — Dungeness crab are forecast to take a hit from ocean acidification driven by fossil-fuel combustion, according to a new study.

Though the populations of the Dungeness crab fluctuate year by year, their overall abundance by 2063 could be about 30 percent lower, according to federal fishery biologist Issac Kaplan, a co-author of the study.

“We think that there will be a moderate decline in a species that is really economically important,” said Kaplan of the Dungeness, which were valued at some $220 million during the 2013 West Coast commercial season.

The study was published in Global Change Biology and had nine co-authors, most of whom work in federal fisheries research in Seattle or at the University of Washington.

Scientists have found that the seawater is growing more acidic due to carbon-dioxide emissions. While most of this carbon dioxide goes into the atmosphere, about a third of it is absorbed by the oceans. As this acidity rises, the water can be more corrosive on some animals’ shells or skeletons as documented in the 2013 Seattle Times series “Sea Change.”

The study published last week relied largely on computer modeling to forecast changes in the West Coast ocean ecosystem as acidity levels — during a peak summer period — are expected to increase by more than 50 percent. It is focused on offshore areas at least 98 feet deep.

“The real challenge is to go from experiments on what happens to individual animals in the lab over a matter of weeks to try to capture the effects on whole population,” Kaplan said.

The study model indicates that the crab, which dwell along the sea bottom, will suffer because of a decline in some of the species that nourish them. Some other bottom-dwelling commercial species, such as thornyhead rockfish and Petrale sole, also are expected to take a hit.

But the study also found that many other species are not expected — at least through the next 50 years — to be significantly affected by the changing ocean acidity. These species include seabirds, marine mammals and many of the commercial fish species that don’t live along the ocean bottom

“There are winners and losers,” Kaplan said.

For scientists, a major concern has been the effects of ocean acidification on tiny sea creatures, such as pteropods, which are a food source for salmon and many other fish. But the computer model indicate that the pteropods, as well as copepods — another staple of many fish — won’t fare too badly. While some earlier studies have shown that increased ocean acidity can harm both of these species, the computer model did not indicate that they would be in serious decline off the West Coast during the next 50 years.

Kristin Marshall, the lead author of the study, said copepods and pteropods benefit from great productivity, so even if some are harmed by acidity the computer model indicates that their overall populations would fare relatively well.

“What stands out is that some groups you’d expect to do poorly don’t necessarily do so badly — that’s probably the most important take-away here,” said Marshall, who undertook the research at the University of Washington and at the federal Northwest Fisheries Science Center.

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