VINALHAVEN, Maine — At 3:30 a.m. one day in May, the lobstermen ate breakfast. Outside, their boats bobbed in the water, lit only by the dull yellow of streetlamps across the bay. It was windy, too windy for fishing, but one by one the island’s fishermen showed up at the Surfside cafe anyway. Over pancakes and eggs, they grumbled about the season’s catch to date.

Some of the lobstermen said it was just too early in the season. Others feared that it was a sign of things to come. Since the early 1980s, climate change had warmed the Gulf of Maine’s cool waters to the ideal temperature for lobsters, which has helped increase Maine’s fishery fivefold to a half-billion-dollar industry. But last year the state’s lobster landings dropped by 10 million kilos, to 50 million.

Now, scientists and fishermen are worried that the waters will warm too much for the lobsters, and are asking how much longer the boom can last.

«Climate change really helped us for the last 20 years,» said Dave Cousens, who stepped down as president of the Maine Lobstermen’s Association in March. But, he added, «Climate change is going to kill us, in probably the next 30.»

Scientists say many factors have contributed to the boom, including overfishing of predators like cod and the lobstermen’s own conservation efforts. But without climate change, Maine’s lobster fishery would not be anywhere near as successful as it is today, said Richard A. Wahle, a professor at the University of Maine.

The Gulf of Maine has warmed faster than 99 percent of the world’s oceans for much of this century, driven by climate change in combination with natural variation. By 2050, that warming could cut lobster populations in the gulf by up to 62 percent, the Gulf of Maine Research Institute says.

One crisp overcast morning, Mr. Cousens, 60, hauled up trap after disappointing trap. It was early in the season, so few lobsters were expected. Even so, Mr. Cousens was disheartened.

In the 1990s, Mr. Cousens said, he could haul up to 36,000 kilos of lobster per year. But last year, his earnings fell 30 percent. «You can’t do that too many years in a row,» he said.

As temperatures in the gulf have increased, the favorable conditions for lobster reproduction have shifted northeast, in the direction of Canada.

But Mr. Cousens’s 24-year-old son Samuel isn’t worried, even though his boat, Adrenaline, has sent him more than $200,000 into debt. «I just put my head down and work,» he said.

Often, the younger Mr. Cousens will fish 14-hour days, 55 kilometers from the mainland. This has become the norm for younger fishermen, who are venturing farther offshore in bigger, faster, more expensive boats. Lobster populations are not only expanding northeast but are thriving in deeper waters as waters warm.

Offshore, the fishing is high-risk and high-reward, Mr. Cousens said. When you haul a trap up into a boat, he said, the feeling is exhilarating: You either see «dollar signs or dirt.»

Lobstering has always been a boom-and-bust business, but the conservation measures long enforced by Maine’s lobstermen may help stave off complete collapse, scientists say.

The lobstermen clip the tails of egg-bearing female lobsters and release them, a practice called V-notching that began voluntarily in the late 19th century and was later mandated by law. They throw back lobsters that have V-notches, alongside lobsters that are smaller than nine centimeters or larger than 12, measured from the eye socket to the base of the tail. These measures help conserve the brood stock, ensuring that the lobsters continue to repopulate.

A study last year found that these conservation measures could save the industry from sharp decline in the future.

«It’s going to give them some resiliency to the changes that we think are coming,» said Andrew Pershing, the chief scientist at the Gulf of Maine Research Institute.

Given the ominous signs, some are branching out. This summer, Krista Tripp, 33, is buying an oyster farm to complement her lobster fishing.

Diversifying is hard, Ms. Tripp said. She had always wanted to be a lobsterwoman, ever since she watched her father and grandfather hauling, measuring and banding the claws of the lobsters, in what she said resembled a dance. «They were so good, they were so fast,» she said. «I knew that that’s what I wanted to do.»

This summer, she plans to spend her mornings lobstering and her afternoons on the farm, wading through the shallow mud flats.

In the meantime, the lobstermen on Vinalhaven will continue to rise in the dark for breakfast at Surfside. Eventually, the sun will come up, and they will go out onto the water. Whether they will always find their traps full, however, is another question.

Climate change puts a half-billion-dollar industry at risk.

© 2018 The New York Times