March 22, 2019
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Fishery watches the ‘creep into Maine’ of lobster shell disease

WALPOLE, Maine — Lobstermen plying the waters of the Gulf of Maine from the southern end of Maine’s coast to the Canadian border have seen historically high landings in recent years, offering a stark contrast to the lobster fishery in southern New England.

According to data from the Maine Department of Marine Resources, 2015 marked the fourth year in a row and the fourth year in the state’s history in which Maine lobster harvesters landed more than 120 million pounds of the crustaceans, with the 2015 figure totaling 121,083,418 pounds.

Alternatively, the crustacean fishery south of Cape Cod, a once lucrative industry, has been hit hard in recent years with steep declines in the region’s lobster population.

Though not believed to be the cause of the collapse in southern New England, epizootic shell disease has been seen in rising numbers in the warmer waters of these more southerly states and could pose a future threat to the currently booming population in the Gulf of Maine.

Maine’s iconic local industry does not appear to be in any immediate danger and, to date, the disease is rare in the state’s waters, occurring in a low percentage of the lobsters harvested in the Gulf of Maine.

According to Dr. Richard Wahle, of the University of Maine’s Darling Marine Center in Walpole, the disease first took off in the coastal waters of Rhode Island around 1997, spreading north to Cape Ann in northeastern Massachusetts before stalling out for a number of years.

The disease progressed to the Great Bay of New Hampshire before making it to Maine.

“We have gradually seen it creep into Maine,” Wahle said.

The research professor said up until 2013 the disease was found in just a few lobsters per thousand, but over the course of a year, its prevalence rose to a few per hundred, though these higher numbers were restricted to the southern end of the Maine coast.

The rate of a few diseased lobsters per thousand remained the norm for the northern end of the state’s seafront.

Wahle said the disease is characterized by lesions on the lobster’s shell.

“It starts off as a bacterial infection. The bacteria digest the surface of the shell, creating nasty-looking pits,” Wahle said.

“It tends to be most strongly associated with larger lobsters. It stands to reason these lobsters are holding onto their shells longer and the bacteria has a greater opportunity to establish a colony,” Wahle said.

He said younger lobsters molt more frequently than older members of the species and appear to be less impacted by the bacteria in question, Aquimarina homaria, a microbe that is naturally present on lobsters’ shells.

Wahle said an exact cause of the disease is not yet apparent.

“We don’t have a smoking gun,” Wahle said.

He said water temperatures vary greatly between northern and southern New England, with summer temperatures in the Bay of Fundy ranging from 50-51 degrees and those in Rhode Island hovering around 71-73 degrees.

He said lobsters experiencing environmental stressors can be more susceptible to the disease, with different sources of stress including warmer temperatures, ocean acidification, and the presence of chemicals, pesticides, and herbicides.

Wahle said a combination of these factors could be affecting the crustaceans’ susceptibility, but water temperatures alone are likely not the only factor to blame.

“I don’t see the temperatures themselves being as much of a threat,” he said.

Wahle said temperatures in the waters off the southern end of Maine’s coast average 59-64 degrees and lobsters don’t experience stressful conditions until 68 degrees.

He said the region’s ocean waters have been warming at an average rate of 0.035 degrees per year over the past 30 years, a rate of about a third of a degree per decade or 1 degree per 30 years.

However, the biologist said this rate has increased of late.

“In the past 10 years, warming has accelerated,” Wahle said.

Despite the ambiguous impact of warmth on shell disease, temperatures do seem to have an impact on the habitats of lobsters, with the range of the species shifting north.

“The whole distribution of the population is shifting northward. Lobsters seem to be receding from shallow waters in the south with a dramatic increase in population in eastern Maine, the Bay of Fundy, southern Nova Scotia, Prince Edward Island, and south (Gulf of) St. Lawrence,” Wahle said.

Monitoring

Katherine Thompson, of the Maine Department of Marine Resources, works as a lobster biologist out of Boothbay Harbor. Thompson said that though the department is keeping an eye on shell disease, it is not yet a concern.

“We still have very low levels in comparison to southern New England,” she said.

Maine’s lobster fishery is broken into several zones (A-G) stretching from Maine’s border with the Maritimes to the New Hampshire state line.

Thompson said the southern zones (F and G), have experienced slightly higher levels of shell disease than the zones to the northeast, but numbers of infected lobsters were still low and often limited to large or egg-bearing lobsters, which hold onto their shells longer than most lobsters.

Thompson said the disease only impacts the shells of the creatures and has no impact on edibility.

Thompson was quick to point out the low occurrence of shell disease in Maine waters to date.

“We are monitoring it in the context of what is happening in the greater range. It’s on the radar of the industry, not a concern as of yet,” Thompson said.

She said the disease is subject to ongoing research in hopes of determining causes of the disease.

One early warning system for the industry could be the American Lobster Settlement Index.

Wahle was instrumental in establishing the index, which studies baby lobsters roughly five to nine years before the crustaceans become large enough to harvest.

Wahle expressed optimism that the index can be used to project trends in the fishery.

More than 100 sites from Rhode Island to Newfoundland are sampled annually to monitor larval settlement, which indicates the strength of a year class.

By tracking lobster year classes from settlement to the time adult lobsters join the fishery, the index aims to reveal trends and give the industry time to react in the event of a downturn or upswing in population.

Started in 1989, the American Lobster Settlement Index is an interstate and international monitoring program consisting of a partnership between several organizations in New England and Canada.

 



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