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Tuesday, 24 September 2013

Stunted growth of Alaskan crabs

Alaskan crabs are falling victim to acidifying seas.

FOR decades in Dutch Harbor, Alaska, the crab piled up in fishing boats like gold coins hauled from a rich and fertile sea. But the very ocean that nursed these creatures may prove to be this industry’s undoing.

New research earlier this year shows that Bristol Bay red king crab – the super-sized monster that has come to symbolise the fortunes of Alaska’s crab fleet – could fall victim to the changing chemistry of the oceans.

Barring a hasty reduction in carbon dioxide emissions – or evidence that the creatures could acclimate to changing sea conditions – a team of scientists fears Alaska’s US$100mil (RM310mil) in red king crab fishery could crash in decades to come. That grim possibility also raises alarm about the crab fleet’s other major moneymaker, snow crab.

“With red king crab, it’s all doom and gloom,” said Robert Foy, who oversaw the crab research for the National Oceanic and Atmospheric Administration (NOAA) in Kodiak, Alaska. “With snow crab, there’s so little known we just can’t say. But we don’t see anything from our experience that’s good for any of these crab. Some is just not as bad as others.”

The emerging issues with Alaska’s crab underscore the difficulty of trying to comprehend the depth of fallout from ocean acidification. For reasons scientists don’t always understand, similar species, even those living side by side, often respond to changing water chemistry in remarkably different ways.

“The real issue here is unpredictability,” said Richard Aronson, a Florida-based marine scientist who has tracked king crab in Antarctica. “There are all these unanticipated collateral impacts. The problem is, most of them are nasty surprises.”

Certainly the threat to king crab was unexpected. As humans pump CO2 into the atmosphere, a quarter of it gets absorbed by the seas. That lowers the water’s pH and alters the availability of carbonate ions, which crab rely on to build their exoskeletons. Many crab species appear hardy in the face of souring seas, or at least not quite so frail. Exceedingly corrosive waters actually pump up Maryland blue crab to three times their size and turn them into voracious predators. Sour waters kill Dungeness crab, but far less often than Alaska red king crab.

When Foy and his colleagues exposed baby red king crab to CO2 levels expected by mid-century, the young died more than twice as often as crab raised in normal water. When researchers boosted CO2 to levels expected decades later, red king crab died in far larger numbers.

“The overall survival at the larval and juvenile stage is extremely low,” Foy said. “It decreases to a point that is likely to affect the population of the crab.”

Such a loss would exact quite a toll. Red king crab is the showboat of the US Northwest’s billion-dollar fishing industry. It is a television sensation and a marketer’s dream, its image emblazoned on bumper stickers, mugs, caps and T-shirts throughout the Pacific Northwest and Alaska. It is even a tourist attraction: cruise ship passengers stopping in Ketchikan pay US$159 (RM493) for a half-day ride to watch crews haul marine life aboard a 32m crab boat that appeared on Discovery Channel’s Deadliest Catch.

NOAA researchers are using Bob Foy’s research to develop models and a timeline that charts the potential collapse of king crab. But things are changing quickly.

Young red king crabs are being raised in a shellfish hatchery in Seward, Alaska, as part of an experimental effort to provide options for troubled wild populations.
Young red king crabs are being raised in a shellfish hatchery in Seward, Alaska, as part of an experimental effort to provide options for troubled wild populations.

“Bob reared those crabs under conditions that we thought were some time off in the future,” said NOAA oceanographer Jeremy Mathis. “And what we actually found is that at certain times of the year, the conditions near the bottom in the Bering Sea were actually worse than the conditions that Bob was raising his crabs under.”

There’s no evidence that souring seas have yet altered wild populations – the most corrosive seas now occur at times when red king crab aren’t as susceptible. Also, the research comes with plenty of caveats. No laboratory setting can ever properly approximate what happens in nature. Scientists are still conducting genetic tests to see if king crab might have the ability to adapt.

The situation also might be worse than first thought. Souring seas could hit crab at several additional stages of development or attack their food. Ocean acidification is also not the only marine world change underway. Warming seas, also caused by carbon emissions, could compound crab’s troubles.

“Anytime you’re working with an organism at the edge of its threshold and you add another stressor, that’s going to be an issue,” Foy said.

No two Alaskan crab species have responded to CO2 exactly the same way. Golden king crab, for example, live extremely deep, below 333m where waters already are naturally rich in CO2. That appeared to make them highly tolerant of sea-chemistry changes. Meanwhile, baby Tanner crab exposed to high CO2 died at a higher rate than normal – but nowhere near as often as king crab. With snow crab, scientists have struggled to perform extensive tests. The animals are just too hard to keep alive in the lab.

It’s also hard to know how Foy’s results will translate to other species elsewhere. Will more acidic conditions kill these creatures or drive them out? Since they often occupy far deeper water, does that mean they instead might thrive?

The rising threat from acidification has insiders closely watching the work of a shellfish hatchery in Seward, Alaska, that for years has been learning how to raise baby king crab from scratch. Crab are most susceptible to corroding seas as babies, when a mere fraction of young survive even in perfect conditions. In this hatchery, where water can be controlled, survival is up to 500 times higher. Still, no one expects this operation could ever replace wild king crab. The orders of magnitude required to get enough crab to populate the Bering Sea would be ridiculous.

But perfecting the science could provide options, such as the ability to repopulate a few previously devastated areas. The idea that crab might be partially grown in a lab instead of the ocean frustrated Mizrain Rodriguez, a crew of the crab boat Arctic Hunter. But it also saddened him to think that humans could be doing such damage to the sea. “Every single animal on this planet lives in balance with its surroundings except us,” Rodriguez said. “We see it. We understand it. But we don’t want to do anything about it. It seems like we are on this destructive path.” – The Seattle Times/McClatchy-Tribune Information Services

Related stories:

Seas that corrode

Shellfish woes

Tags / Keywords: Environment , ecowatch , climate change , ocean acidification


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