Blue crabs are becoming more common in Maine. Photo courtesy of the Wells Reserve

The reports have come from up and down the coast of Maine: Blue crabs have been spotted around dock pilings, pulled up in lobster traps and found washed up on beaches.

The crabs, usually associated with coastal Maryland and the Chesapeake Bay, have been showing up more often and in larger numbers in the Gulf of Maine over the past few years. To better understand the shifting population, scientists are now asking fishermen and members of the public to report their sightings through an online survey.

Jessie Batchelder, a fisheries project manager with Manomet, a Massachusetts-based nonprofit focused on conservation and science education that has been studying blue crabs in Maine, said the information submitted by lobstermen, clam harvesters and others who are frequently on the water is key to getting a better picture of the blue crab population.

“They have a vast knowledge of what’s going on and what they’re seeing,” she said.

In the past, researchers would occasionally hear about stray blue crabs being found in lobster traps or by clam harvesters, but the reports started to become more frequent in 2021. That got researchers thinking about studying how blue crabs are moving into the Gulf of Maine as waters warm because of climate change, Batchelder said.

Last year was the second warmest on record in the Gulf of Maine, with an average surface water temperature of 53.66 degrees Fahrenheit. While that temperature fell just short of the 2021 record, it continued the Gulf’s historic trend as one of the fastest-warming ocean areas on the planet.

The Gulf of Maine, 36,000 sprawling square miles stretching from Cape Cod to Nova Scotia, is home to rare whales and seabirds, valuable fish such as cod and haddock, and the $1.5 billion U.S. lobster industry, all of which are affected by warming waters.

The warming waters of the Gulf of Maine are more hospitable for blue crabs than in years past, enabling their northward expansion, researchers say. They are still studying possible negative impacts the crab might have on the local ecosystem and fisheries. But they say there could also be opportunity for a new fishery if the blue crab population becomes large enough.

Dr. Ben Gutzler, a post-doctoral research fellow at the Wells National Estuarine Research Reserve, holds a blue crab fitted with a device that helps scientists track its movements. Photo courtesy of the Wells Reserve

Easy to identify because of their bright blue claws, the crabs are voracious predators and scavengers of other shellfish species, including clams, oysters and juvenile lobsters. It’s important to understand how they might affect local ecosystems and fisheries, said Laura Crane, a research associate at the Wells National Estuarine Research Reserve who has been monitoring blue crabs for the past three years.

Their appearance in the Gulf of Maine follows the invasion of green crabs, a species that arrived from Europe. Green crabs exploded in numbers in recent years and decimated soft shell clam populations in some areas along the coast, a trend also attributed to warming ocean temperatures.

But blue crabs also are highly sought after for their meat, unlike the smaller green crab that has little commercial value. Blue crabs comprise the most valuable fishery in the Chesapeake Bay. Maryland harvesters caught nearly $30 million worth of the crabs in 2021, the most recent data posted by the state.

While blue crabs could have impacts on commercial fisheries here, it’s also possible that blue crabs could take a bite out of the invasive green crab population. The larger blue crabs eat green crabs and have been credited by researchers with keeping green crabs in check in the Chesapeake region.

In the fall of 2020, researchers at the Wells Reserve started finding blue crabs in salt marsh pools they had been monitoring for years, Crane said.

“It was pretty surprising when all of a sudden we started finding live blue crabs,” she said. “We put out traps to see how many we could catch.”

Since then, they have placed blue crab traps in marsh pools in two marshes from April to November of each year to try to get a handle on how big the population is, where they are distributed and if there are seasonal trends. They also collect environmental data in the pools to track various changes, including temperature and salinity, Crane said.

“It’s been really interesting because we’ve had a really big variation in how many crabs we’ve been catching each year,” she said. So far this year, they’ve caught 50 blue crabs – more than all of last year.

Crane said the data collected so far point to the blue crab population in Maine increasing overall, but it’s too soon to say if it is a long-term trend. It’s essential to compare the data collected in Wells with data from others who are trapping and collecting information, she said.

“That will help us really get the big picture of how this population in changing in the Gulf of Maine,” she said.

Manomet started trapping blue crabs in the Casco Bay area in 2022 after hearing from fishermen about their sightings. Batchelder said they quickly realized their traps and those used by other researchers were not covering a wide range of the coast of Maine.

She spent last summer developing a survey that would collect information from fishermen and others all along the coast. The survey, which went live last fall, collects basic information about the sighting: where it was, how the crab was found, how many were seen and the sex of the crab. People are encouraged to submit photos and any additional observations they have.

That information is studied by the researchers and available for the public to view on an interactive map.

While valuable information has been submitted – including observations of mating and molting blue crabs – Batchelder is hoping more people will fill out the survey as they spot blue crabs. She said it has been interesting to see people submit locations and photos of blue crabs spotted as far back as 2018.

“People remember when they see something different that they haven’t seen before,” she said.

The survey is available at manomet.org.