VINALHAVEN, Me. — At 3:30 in the morning on a Friday in late May, the lobstermen ate breakfast. Outside, their boats bobbed in the labradorite 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 grow Maine’s fishery fivefold to a half-billion-dollar industry, among the most valuable in the United States . But last year the state’s lobster landings dropped by 22 million pounds, to 111 million.

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