Despite more than 20 years of declining lobster populations in southern New England and extensive studies of the shell disease that is a major factor in their decline, scientists are still struggling to provide definitive answers to help restore hope to those working in the local lobster fishery.

A new study of lobsters along the eastern Connecticut coast has found that the disease is linked to warming water temperatures, while progress is slow in efforts to identify probiotics to counteract the disease and to better understand why so many lobsters are blind.

“Epizootic shell disease first appeared around 1996 and became quite prevalent around 1999, and it continues to be prevalent,” said Maya Groner, who conducted the Connecticut study as a post-doctoral researcher at the Virginia Institute of Marine Science. “It’s been a challenge to figure out what the pathogen associated with the disease is. The best evidence suggests it may be a suite of bacteria that chews away at the carapace, but that suite of bacteria changes over the course of the disease.”

Her study found that the increased prevalence of the disease stems from warmer water temperatures that induce the lobsters to molt their shells earlier than usual.

Using data on 200,000 lobsters collected over 37 years in Waterford, Conn., as part of biological monitoring near the Millstone Nuclear Power Station, Groner found that about 80 percent of male lobsters have the disease during warm years, with females contracting the disease at a slightly lower rate.

“Molting their shell resets their health,” she said. “If they don’t molt, there’s no way they can recover. But now that they’re molting earlier in the spring, the molt happens before they’re even challenged with the disease.”

The earlier molt allows the disease to progress longer than if the lobsters molted in summer, as they typically do.

Groner found that for every 1.8-degree increase in the average temperature of the bottom water in May, lobsters molted about six days earlier. In early-molting years, disease prevalence doubled by September.

“It’s very consistent with trends we’ve seen with other marine diseases,” Groner said. “Organisms at the southern part of their range — like lobsters in Long Island Sound — are limited by temperature. They’re at their thermal tolerance limit. So as temperatures increase, they’re becoming stressed and less able to cope with diseases.”

University of Rhode Island fisheries researcher Kathy Castro has been studying lobsters for more than two decades, and she decided to look for a solution to help lobsters recover from the disease even though the precise cause of the disease is still uncertain. She is collaborating with URI colleagues who are studying probiotics on oysters.

“Why can’t we identify good bacteria that normally occur on lobsters, take the bad bacteria off, and repopulate their shells with good bacteria?” she wondered. “In essence, the idea works, but we don’t know what’s the right bacteria, how do we treat the lobsters, how often, and how to do it in a reasonable time frame.”

In a laboratory setting, Castro’s URI colleagues David Nelson and David Rowley isolated probiotics from healthy lobsters and tested them against what they believe may be the “bad bacteria.” The strategy looked promising. Initial trials on adult lobsters were positive as well. But it may not be practical.

“Our initial idea was that lobstermen could treat the lobsters on their boat,” Castro said. “But it’s hard to do; you have to do it in a lab. Maybe we still haven’t identified the right probiotic. And are we even working with the right pathogens?”

While that work is continuing, Castro is investigating why about half of the lobsters she has tested are functionally blind.

“That’s a more concerning issue to me than shell disease,” she said. “My question is, is it related to shell disease. The lobster’s endocrine control system is located in their eye stalk, so if a lobster is blind, is it molting incorrectly, and is that contributing to the disease.”

Castro said a colleague in Virginia thinks the cause of the blindness may be manganese, a neurotoxin that harms optic nerves and is released from sediments under low-oxygen conditions. But studies are just now under way.

“In my mind, it has to be related to shell disease. That’s my gut feeling,” Castro said.

One of the challenges to finding the answers has been inadequate research funding, Castro said, so much of the research is being done piecemeal.

“I really wish there was something fundamentally easy that we could do to solve all these problems,” she said. “That would be my greatest dream. But I know it takes time. And as much as we know about lobsters, there’s a lot more we don’t know.”

Rhode Island resident and author Todd McLeish runs a wildlife blog.