Maine’s coastal lobsters are finding new digs.

The lobsters living in the shallow waters of the Gulf of Maine are trading in the rocky crevices they have always favored for a life spent out in the open foraging for food on the ocean’s muddy bottom or open ledge, according to a new study by University of Maine researchers.

This discovery upends the traditional understanding of American lobster habitat. It’s in almost any stock assessment or marine biology textbook: American lobsters like rocky areas where they can find shelter. Well, it may be time to order a new marine biology textbook.

Eric Pray unpacks a lobster on a wharf in 2020, in Portland. Robert F. Bukaty/Associated Press, file

“When you consider that this is one of the best studied commercially important marine species in the world, it is stunning that we keep getting surprised by our iconic lobster,” said study co-author Bob Steneck, a retired UMaine professor of oceanography, marine biology and marine policy.

A team of four UMaine scientists reached this conclusion after donning snorkels and fins to survey lobsters at 20 sites between York and Jonesport and compare their findings to historical results dating to 2000. They published their work this month in Marine Ecology Progress Series.

The number of lobsters found in the boulders dropped by 60% between 2000 and 2019, the year the UMaine team did its dive work. Over that same period, the number found on the muddy bottom increased by 633% and the number on open ledge increased by 280%.

OK, but what about their home? They might hang out in the open, but were they opting to live there? The change was not as drastic, but still quite significant: 34% fewer lobsters were living in rock shelters, 161% more were living under algae mats, and 168% more had no shelter at all.

Researchers noted that the algae mats, which had taken over areas where kelp beds had once hidden lobsters from predators and larger lobsters, offered little protective cover for most of the near-shore lobsters, which seemed to move around underneath it rather than seek shelter.

Their findings begged the question: what had caused this fundamental behavioral change?

According to lead author Robert Jarrett, a marine biology Ph.D. student from New Jersey, the team began looking for conditions that might have driven lobsters out of the rocks, such as invasive toxin-producing algae, low oxygen levels, or declining food sources. They crossed each one off.

Warming ocean temperatures may play a role, Jarrett said. The average temperature of the Gulf of Maine at a depth of 32 feet – which is about as deep as the lobster survey was conducted – increased 5.4 degrees Fahrenheit between 1995 and 2021.

That warming had several effects: lobsters spent more time out of their shelter foraging; fewer cold-seeking baby lobsters settled to the ocean floor in warm shallow waters; larger lobsters remaining are more affected by the temperature-limited oxygen concentrations and have greater metabolic demands.

That is one of the other findings of the study: the overall density of lobsters in the shallows is down, and the average size of those that remain is bigger than they used to be – not so much because coastal lobsters are super-sizing as there aren’t a lot of small lobsters dragging down the average.

Even though they’d probably win, Jarrett said the bigger lobsters in Maine’s shallow near-shore waters don’t like to compete with other lobsters, even small ones. Despite a pugnacious reputation, big lobsters were known to seek out deeper, less crowded waters to avoid a fight.

Without as many young ones in the shallows, big lobsters can forage on the muddy bottom or open ledge with little fear of conflict, Jarrett said. Thanks to our legacy of overfishing cod, there are no predators left to eat this new generation of cave-free lobsters.

Max Oliver moves a lobster to the banding table aboard his boat while fishing off Spruce Head in 2021. Robert F. Bukaty/Associated Press, file

The findings will help policymakers assess the survey methods used to set lobstering rules, Jarrett said.

Surveys of rocky coastal areas will catch fewer lobsters than before, while trawl surveys of the near-shore mud flats and ledges will catch more than they used to, he said – the changes could represent a redistribution of lobster habitat, not a change in the overall size of the population.

“Some of the annual lobster surveys used in the (lobster stock) assessment are restricted in the types of habitats that they can sample,” Jarrett said. “These findings about habitat help fill in some information gaps and show that over time the lobsters may be shifting between which surveys catch them better.”

A FOCUS ON COASTAL LOBSTERS

Jarrett cautioned that their findings should not be extrapolated to make any sweeping projections about the overall health of the lobster population. This work focuses on coastal lobster populations, in which there is a noted decline, he said. It does not address the abundance of offshore lobsters.

In fisheries management, population declines often trigger reductions in how many fish can be caught.

But the redistribution of lobsters might warrant a reconsideration of some fishing rules, Jarrett said. If the lobsters are more spread out, roaming the muddy bottom and open ledge looking for food rather than clustered in rocky crevices, lobstermen may need to set more traps farther apart to catch them.

Federal authorities have called for Maine lobstermen to use less rope while fishing to reduce the risk of one of the few endangered right whales that remain becoming entangled in the surface-to-seabed lines that connect a buoy to a string of traps.

The lobster’s huge habitat shift could bode well for its ability to adapt to a changing Gulf of Maine.

“The ecosystem we describe here for the coast of Maine is highly dynamic, particularly with rapid climate change,” the paper concludes. “Past sheltering behaviors do not characterize current behaviors, and behavioral plasticity provides an avenue to adapt to these rapid changes.”