In the last few years, researchers have noticed the appearance of an unusual southern species in New England waters, the delectable blue crab.

Populations of the crabs are typically found between the Gulf of Mexico and Cape Cod in Massachusetts, but in 2012, shellfish wardens and wildlife managers started noting sightings of the crustaceans miles north of the cape.

From 2012 to 2014, there were reports of individual blue crabs showing up in parts of the Gulf of Maine from Duxbury Bay and Marblehead, Mass., to New Hampshire and even as far north as Nova Scotia. While it's not unheard of for the crabs to venture so far north, there aren't any established populations in the colder waters.

David Johnson, at the time an assistant research scientist at the Marine Biological Laboratory in Woods Hole, Mass., first heard about the crab sightings 80 miles north of Cape Cod at a dinner party. At first he was skeptical, but after other people backed up the reports, Johnson decided to investigate for himself. A trudge through Plum Island Sound estuary in northeastern Massachusetts yielded four blue crabs in 2012, though he couldn't find any the next two years. Johnson scoured the Internet for other reports of the crustaceans and found several instances of the crabs showing up around the Gulf of Maine.

"Their densities were very low, but they were certainly there," said Johnson, who recently published his findings in the Journal of Crustacean Biology.

Given the crabs' affinity for warmer waters, Johnson hypothesized that increasing ocean temperatures could be facilitating their limited northward migration.

"Basically we have this southern crab found in northern waters. The implication may be a result of climate change," he said.

In 2012, ocean temperatures in Massachusetts Bay were 1.3 degrees Celsius (2.3 degrees Fahrenheit) higher than the average temperature between 2001 and 2013. The following year, temperatures were 0.7 C above the average.

'This is a time-will-tell issue'

The appearance of the blue crabs has sparked interest from researchers, particularly because they are a commercially important species. In 2013, fishermen caught 61,119 metric tons of blue crab, at a value of $192 million. More than 80 percent of blue crabs were caught in Louisiana, Maryland, Virginia and North Carolina, according to the National Oceanic and Atmospheric Administration's most recent "Fisheries of the United States" report.

Johnson said that the recent sightings do not mean the blue crabs have already established a permanent population in the Gulf of Maine. It would take one and two decades to determine whether the crabs are there to stay.

"This is a time-will-tell issue," Johnson said. "If we saw the population [in the Gulf of Maine] completely disappear within a decade, it will be like a crystal ball into the future. They will be back by the time I retire, which will hopefully be in 30 years."

Virginia Institute of Marine Science researchers Rochelle Seitz and Romuald Lipcius were both skeptical of the blue crabs' capacity to establish populations north of Cape Cod. Neither researcher was involved with the study, though Johnson has recently joined the VIMS staff as an assistant professor in the Department of Biological Sciences.

"Because blue crabs are tropical marine species, we don't believe they can breed in cooler waters," said Seitz, a research associate professor.

Lipcius, a professor of marine science, said he wasn't surprised that individual blue crabs had been sighted in the Gulf of Maine.

"[Blue crab] is extremely resilient and opportunistic. It can use different habitats and it can eat a wide variety of prey, it can even be cannibalistic. It's also very tolerant of different environmental conditions, extremely cold temperatures it can't tolerate very well," he said. "To see a self-replenishing population, I don't think the cold temperatures would allow that."

Ocean temperatures below 5 C in the Gulf of Maine would be too cold for the crabs to persist, especially during the winter months. They will continue to show up in the Gulf of Maine as ocean currents can carry larvae into the region, the researchers said.

The crabs may be more likely to flourish farther south near New York City, where water temperatures have noticeably increased in recent years, than near Boston.

"I expect that in time we'll see a persistent population in New York and Long Island Sound. But around the cape it will be patchy," Lipcius said.

Climate puzzle complicated by drought and rain

While the possible expansion of blue crab to the Gulf of Maine is in doubt, there is already strong evidence of a number of species either shifting or expanding their ranges along the Atlantic coastline, said Janet Nye, quantitative fisheries ecologist at Stony Brook University in New York, who was not involved with Johnson's study.

In a survey of 36 fish stocks conducted about five years ago, 24 had either moved northward or shifted to deeper, cooler depths, she said.

As with most species on the planet, it is difficult to predict how climate change will affect the blue crabs' long-term success along the East Coast.

Increases in the length or severity of droughts could make some estuaries, especially those in the southeastern United States, less hospitable to the crabs because of increased salinity. Conversely, more precipitation could make estuaries less saline and could lead to higher numbers of blue crabs, Nye said.

Blue crabs found in warmer southern waters tend to spawn more times in a year than their more northerly counterparts. Females in the Gulf of Mexico can spawn as many as eight times, compared with just once for crabs near Cape Cod. There is speculation among researchers that as ocean temperatures rise, crabs that are farther north will be able to produce more offspring and populations could increase, according to Lipcius.

Even if the crabs themselves are able to adapt, it is unclear what will happen to the crabs' main sources of food, or to their predators. Effects of climate change are further confounded by the simultaneous impact of crab fishing, which can have an even greater impact on populations, researchers said.

As for this summer's crab catch, initial reports suggest that Maryland crabs have experienced some winter mortality from this year's cold winter, particularly among females, but Virginia's blue crabs appear to be doing well.

"We're cautiously optimistic, but we're always concerned about blue crab populations, especially given that there are these other factors beyond our control," Lipcius said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500