Joe Orlando, 60, who fishes from a Gloucester, Mass., base, said the effect of the ban was terrifying.“It’s completely, completely over,” he said. “I got a house, kids, payments.”

But many other fishermen do not blame climate change. They blame the regulators, calling the moratorium cruel and needless, because they say their latest cod catches are actually better than in recent years. More than a few talk of a conspiracy between scientists and environmentalists to manufacture a fishing crisis that will justify their jobs.

Scientists say the truth is more prosaic: Although the gulf is generally warming — 2012 was the hottest year on record — the last year was cooler, and kinder to cod. Moreover, the gulf’s remaining cod have congregated in deeper, colder waters in southern Maine and Massachusetts, where their abundance masks their scarcity elsewhere.

“A fisherman’s job isn’t to get an unbiased estimate of abundance. It’s to catch fish,” said Michael Fogarty, the chief of the ecosystem assessment program at the Northeast Fisheries Science Center of the National Oceanic and Atmospheric Administration, the federal agency that monitors sea life. “The world they see is a different world than we see in the surveys.”

That said, much about warming’s effect on the gulf remains unclear. Years of overfishing have winnowed some fish populations, muddling efforts to measure climate change’s impact. Fishermen, scientists and regulators often disagree over whether the current changes are temporary or the new normal.

And in fact, the latest warming is not unprecedented. Weather records document a steady, if slow warming of the region’s waters since the 1850s, and a 50- to-70-year climatic cycle set off unusual ocean warming in the 1950s. A similar cycle is believed to be heating up the northwest Atlantic today.

But scientists say those cyclical effects are now being turbocharged by human-caused climate change. The gulf has been at least two degrees warmer than its historical 50-degree average in each of the last five years. In 2012, it measured four degrees higher, according to the National Oceanic and Atmospheric Administration. If that is a clear win for sea bass, and a loss for cod, the consequences for some species are not so easily tallied.