ELLSWORTH — It is not like southern New England, where as much as a third of lobsters have been affected in recent years, but fishermen and scientists say there are signs that lobster shell disease is increasing in the Gulf of Maine.

Water temperatures in the gulf historically have been colder than the coastal waters south of Cape Cod and have fostered a greater degree of marine biodiversity than other parts of the East Coast. The gulf is where the vast majority of American lobster is caught, with a record amount of more than 126 million pounds of lobster brought ashore in 2012 by Maine fishermen who cumulatively earned nearly $339 million for their catch.

The lobster population south of Cape Cod has not fared well in recent decades, and the spread of lobster shell disease along the southern New England coast during that time has been one of the problems the crustaceans have faced. Increased water temperatures, pesticides, ocean acidification and other types of runoff have been cited as possible factors in the decline of the southern New England lobster population.

In the Gulf of Maine, instances of shell disease historically have been rare — maybe one or two cases for every 10,000 lobsters, according to scientists. This year, however, there have been more reports about lobsters with the disease being caught in the gulf, though the apparent increased rate of occurrence still lags far behind the disease’s frequency south of Cape Cod.

The disease manifests when naturally occurring bacteria consume the shells of living lobsters, making the lobsters unsightly and possibly weak but still safe to eat. Though the lobster meat is unaffected in appearance and flavor, the disease can depress the value of Maine’s lobster fishery catch because the scars and lesions on affected animals make them unsuitable for the lucrative restaurant market.

Why some lobsters are affected by the bacteria and others are not is unknown. Scientists have debated whether water temperature, relative acidity, food availability and population density might be factors. Some scientists believe something internal in the lobster — its genetic makeup, a pre-existing condition or another type of stress — could trigger the disease, but they have not come up with an answer.

“We have yet to figure out what that weakness is,” Bob Bayer, executive director of the Lobster Institute at the University of Maine, said Thursday. “It’s so hard to know.”

Bayer said scientists have been interested in finding out the cause of shell disease for “well over 20 years.” Researchers have been unable to transmit the disease from one lobster to another so, though it can be fatal, it is not contagious, Bayer said.

Lobsters seem to rid themselves of the disease when they molt and shed their shells, which grow back without any trace of pitting or rot, he added, but the same lobster can contract it again. Egg-bearing females, which retain their hardened shells longer than other lobsters, are most susceptible to the disease.

Attempts this week to contact scientists with the Maine Department of Marine Resources, which has kept tabs on reports of shell disease off the Maine coast, were unsuccessful.

Bayer said he does not have statistics for how prevalent the disease might be in Maine, but he guessed 1-2 percent of the lobster population in the gulf may be affected. He said he knows it has increased because the institute has been receiving noticeably more photographs, emails and phone calls this summer from Maine fishermen who have found affected lobsters in their traps.

“It’s not massive numbers,” Bayer said, compared to what has been reported in the past decade off the coasts of Rhode Island and southern Massachusetts.

David Cousens, president of Maine Lobstermen’s Association, said Thursday he’s not overly concerned about shell disease becoming more prevalent off the Maine coast, as long as the water temperature in the gulf stays cool. Water in the gulf of Maine was abnormally warm in 2012, which led lobsters to molt unusually early, but this year so far the water at depths where lobsters live, several meters down, have been in the typical 50-degree Fahrenheit range.

According to Cousens, the frequency of shell disease among lobsters in the gulf has increased in the past couple of years from two or three in 1,000 to about six in 1,000.

“I’m concerned, obviously, but not panicked,” Cousens said. “I don’t think (this increase) is anything to worry about.”

Cousens said he believes water temperature is a factor, because it is the kind of environmental factor that could put added stress on lobsters, which prefer relatively cold water. The bacteria is always in the water column, he said, and last year’s temperature increase in the gulf could have been a trigger for the comparatively higher rate of shell disease.

“(Lobsters) only get it when they are stressed,” Cousens said.

Joseph Kunkel, a research professor at University of New England, said Tuesday that he is hesitant to read too much significance into the reports he has heard of lobster shell disease creeping into the gulf. The sample size for the numbers of affected lobsters found in the Gulf of Maine is so small, he said, that a small pocket of diseased lobsters could skew the estimates for how prevalent the disease is.

“There supposedly is some creep in the southern range of the Maine coast and the New Hampshire coast,” Kunkel said Tuesday. “I’m not convinced by the numbers yet.”

Still, if the numbers have increased from, say, 0.1 percent to 0.3 or 0.6 percent, he added, it would be a cause for concern. What has happened in southern New England is quite serious, he said.

“But we’re far away from that now (in the gulf),” he said. “That’s something we want to avoid.”

Kunkel said relative acidity levels or pollution in the ocean could act as a trigger for activating shell disease among some lobsters. He said he thinks the vulnerability of lobsters after they shed their old shells but before their new ones harden may play a role.

“There’s no smoking gun yet, and there are a lot of opinions,” Kunkel said.

Bayer and Kunkel each said that there should be more funding devoted to researching shell disease, so that a cause can be determined before it might become a major concern in the Gulf of Maine. Bayer added that the Lobster Institute hopes to conduct a survey of lobstermen this fall to get more data on the presence of the disease along the Maine coast.

“We may see less as the season progresses,” Bayer said. “It’s hard to know what is going to happen.”