Crab populations in the Bering Sea are struggling. And researchers say climate change is a big cause. A new tool described in a study published Aug. 3 in the Journal of Geophysical Research Oceans will help researchers predict one of climate change’s biggest threats to crabs: ocean acidity. The study’s authors hope their work can help crabbers and fishery managers.
“I'm excited with this new model and study," said Jamie Goen, the executive director of the nonprofit trade association Alaska Bering Sea Crabbers. "It will help us better understand what could be happening and how that might impact fisheries.”
Darren Pilcher, the study’s lead author and an oceanographer at the University of Washington, said he started the study because of requests from fisheries stakeholders.
The ocean absorbs a quarter of our carbon emissions, which make the water more acidic when the two mix. Acidic conditions are harmful for many different animals. This is especially true for shellfish, like crabs.
“They can eat more, and that helps a little bit, only to a certain point," Pilcher said. "In fact, in some of the previous experiments with red king crab, if you lower the pH to a value of 7.5, the crab just die.”
Scientists have pointed to factors from climate change, like ocean acidification, as the reason the number of Bristol Bay red king crabs has plummeted in recent years. Goen said that other species, like snow crab, have also tanked.
“When I started this job eight years ago, we had 60 vessels in the fleet," she said. "But obviously with the collapse of snow crab and the closure of red king crab, we’re down to 40 active vessels right now.”
Pilcher and his team wanted to help the fishery by creating a tool that could forecast acidity in the Bering Sea. Ocean acidity doesn’t exist in a vacuum, so the first step was to make a computer model of the entire Bering Sea, recreating the water conditions at any given time. They put a lot of existing data into their model on ocean conditions over time, measuring things like temperature, currents, and distribution of phytoplankton.
Then they added in ocean acidity data collected from the region, and tested their new tool by having it forecast the acidity of the Bering Sea over a roughly 40 year period. They compared that forecast to what actually happened each year.
Pilcher said they also compared their new tool to a simple baseline model, which predicts that conditions at the beginning of the year will stay the same throughout the year.
“It’s actually a really good forecast for the ocean because the ocean doesn’t really change that quickly," he said.
They tested both models for both the bottom and surface of the ocean. Pilcher found that they were both equally good at predicting acidity at the bottom of the ocean. But he said their new model did a lot better at predicting surface temperatures than the baseline model.
“It kind of makes sense because the bottom waters, especially during the summer, are more cut off from the atmosphere," he said. "Whereas of course the surface is still very connected to the atmosphere and is responding to it on a quicker time frame.”
Pilcher hopes the information from their experiment, and his new model, can inform future management.
“We could say, ‘Hey, this is what we’re predicting for the upcoming summer,’ so that we can give everybody a little bit advance notice if there are, say, worse acidification conditions," he said.
He said they’ve also discussed partnering with a new king crab mariculture project happening on St. Paul Island. The plan is to raise larval Bristol Bay red king crabs in a lab, and release them back into Bristol Bay when they reach a certain size. But because juvenile crabs are more vulnerable to ocean acidity than adults, Goen said Pilcher’s model could provide important data.
“Where I could see an immediate benefit is knowing if those waters are going to be unusually acidic or higher in temperature in a given year," she said. "Then maybe that's not the best year to put some brood stock back into Bristol Bay.”
Goen said she hopes for more research on the changing ocean chemistry in the Bering Sea, so fishermen can make informed decisions. But for now, she said Pilcher’s results are very promising.
