Declining crab populations spur search for cause, solution

States on both coasts are struggling to figure out the cause of declining crab populations as they search for solutions to an issue with environmental and economic implications.

From Alaska to Maryland, fishery managers have documented declines as high as 90% among certain crab populations. They are considering factors such as climate change, water contamination, overfishing and flawed fishery management.

The scientific conundrum has left them pushing for more research, as they weigh whether they need to toss out their current playbooks for managing ocean populations.

“We need a smoking gun,” said Tom Miller, director of the University of Maryland’s Chesapeake Biological Laboratory, which monitors Maryland’s blue-crab population, where the number of males was the lowest ever seen in the state’s 32-year history of record-keeping. “And that’s what we’re trying to unravel.”

In Alaska, the state canceled its Bering Sea snow crab season for the first time after the population shrank from around 8 billion in 2018 to 1 billion in 2021. In 2022, the biomass of mature male snow crabs — a gauge of their numbers in the ocean — dropped another 40%.

The state also canceled its Bristol Bay king crab season for the second year in a row.

These fishery manager responses are blunt and economically damaging. Other, more targeted tools include banning certain types of equipment that can disturb the seafloor and the habitats crabs need to recover. Another strategy is reducing bycatch — ancillary creatures such as crabs that are caught during the legal fishing of other types of game.

Scientists are still trying to figure out what happened to the snow crab after several years of robust population figures. Theories include warming water, which increases the crabs’ metabolism and strains the food sources to meet their larger appetites.

“I think broadly what we’re seeing across the board are impacts of climate change,” said Megan Williams, a fisheries scientist with Ocean Conservancy specializing in the Arctic. “And I think crab in particular might be one of the more climate-vulnerable species.”

Warmer water may have also opened up more northern areas to fish, who compete with crabs for food or prey directly on them.

“In the face of climate change, we need to be more precautionary with how we manage some of these stocks,” Williams said.

Fishing bans don’t always yield quick results. The Washington Department of Fish and Wildlife has banned all crabbing in South Puget Sound since 2018 due to low population numbers detected in 2017 test fishing. The numbers remain low.

“It’s abundantly clear to us now that after multiple years of closures where the state and the treaty tribes haven’t fished at all in these areas that they’re not going to bounce back on their own,” said Donald E. Velasquez, a department biologist. “Something is at work besides just the harvest pressure.”

The overall population in Puget Sound remains relatively healthy. Most of the Dungeness crab come from other areas, but the decline in the South Puget Sound population has been a head-scratcher.

Washington State managers will also have to weigh whether to lift the crabbing ban in the area given that other species, such as red rock crab, are abundant.

If Dungeness crabs don’t return on their own, Velasquez said, the question is “then do we do something different to allow at least some harvest of other species that are doing pretty well, that aren’t being affected yet by whatever’s going on.”

Continuing to fund research is also a tool states have. For example, the Pacific Northwest Crab Research Group was formed in December 2018 to help monitor and support the Dungeness crab population in the Pacific Northwest.

“If there’s research funding to attack some of the bigger questions that, for instance, the Pacific Crab Northwest Research Group has about Dungeness crab, [the state is] likely to fund those kinds of things,” Velasquez said

In Maryland, the number of male blue crabs was the lowest seen in three decades of the state’s annual assessment, and the number of juvenile blue crabs was the second-lowest.

Miller, the director of the University of Maryland’s Chesapeake Biological Laboratory, said that theories for the decline include the possibility that scientists gave fishery managers wrong advice and that management practices may have favored females over males.

If this is the cause, he said, certain practices could help put the fisheries back in balance, including changing the harvest rules to restrict harvests of males and increasing the minimum size of crabs that can be caught. As with most animals, the bigger the crabs are, the more offspring they tend to have.

“So that would mean a more even sex ratio and it would mean bigger crabs,” Miller said. “That would be what they’d be trying to achieve.”

Other theories for the decline include more habitat-related changes, such as a new predator, such as the non-native blue catfish, or other environmental changes.

Miller said if those are the cause for the decline, it would require non-traditional fishery management. For example, if the water nutrients are the culprit, addressing the problem could put the state in new territory because the state has never changed water quality standards to benefit a single species.

“They’ve never done that before even though they’ve got the regulatory tools and authority to set water quality criteria,” Miller said. “They’ve not been set for a fisheries management justification.”

Before any such steps are taken, the cause for the decline must still be identified. Miller is studying a fall in crab birth rates over the last five years and how that fits into the picture. The sustainable rate is at least two crabs born and surviving to a certain age for every living crab.

“In five of the last six years, we’ve had a number close to one,” Miller said.