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Cod have been disappearing from the waters between Massachusetts and Maine, and shrimp populations are so depleted that the commercial shrimp season in the Gulf of Maine has been cancelled for the last three years.

At the same time, lobster are flourishing — alongside a host of species that have never before thrived in New England waters.

Why the wild ups and downs? One reason is that nearby waters are warming much faster than the rest of the ocean, making the environment newly unbearable for some longtime residents — and newly appealing for others. But overfishing has played a role as well, disrupting the balance of the watery ecosystem in unexpected ways.

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It’s hard to predict where all this is headed, whether for individual species or local fishing communities. But it needn’t be a story of doom and gloom. These undersea changes may well dampen the prospects of the cod industry, but they will also bring new opportunities.

What’s happening in nearby waters?

Rapid and unusual warming.

Temperatures in the Gulf of Maine have been increasing faster than just about every other blue spot on the planet. According to a recent report in Science, Maine’s waters are in the top 0.1 percent when it comes to rapid warming.

While that team focused on the waters to the east and north of Massachusetts, that superwarm area they reported on actually extends significantly south, surrounding the Bay State.

The effects of the warming have been dramatic, directly contributing to the decline of the cod population, according to the researchers.

Cod is a cold water fish, and the Gulf of Maine is already at the southern edge of its livable habitat. So even small changes in temperature can have a big impact.

Over the last decade, there’s been a 75 percent decline in the cod stock of the Gulf of Maine, which has translated into tighter fishing quotas and shrinking hauls.

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Has overfishing played a role?

“The Gulf of Maine cod stock has been chronically overfished,” is how the researchers put it in Science. NOAA, the federal agency that oversees commercial fishing, also said that “overfishing is occurring.”

Still, there’s little doubt that warming has made the situation worse.

For years now, there have been strict limits on the amount of cod commercial fisherman can catch in New England. It’s possible that — if the water temperature hadn’t risen — those quotas would have sufficed to keep the cod population stable. Once you account for warming, however, suddenly those fishing restrictions look far too lenient. The fishing rules put in place by regulators haven’t really taken these temperature changes into account.

Is warm water better for some fish?

Our warming waters aren’t just about cod. Look at lobster.

In 2014, commercial lobstermen in New England brought in 65 percent more lobster than they had in 2004. And the reason isn’t just that lobsters prefer warmer water — though the warmth does seem to help lobsters grow faster. It’s also about the disappearing cod. Cod eat young lobster, so when you take cod out of the environment, the lobster have more room to thrive.

Lobster aren’t the only animals doing well in the warming New England waters. Several species have moved up from the South, including black sea bass, squid, and even Maryland blue crab.

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Do newly-thriving species present opportunities for fishermen?

Those making their living off the sea can go after lobster, sea bass, or Maryland blue crab; ultimately they may have little choice. But a host of obstacles stand between today’s cod-trawler and tomorrow’s sea bass fisherman.

For one, each fish has its own habits and life-cycles, which means new fishing equipment and new kinds of expertise. Also, there are regulatory issues, since having a permit to catch groundfish like cod doesn’t necessarily give you the right to fish for other kinds of fish.

Even if you did return with a large haul of some once-exotic species, it would be harder to move your product to market, since current distribution systems in places like Gloucester are largely organized around cod.

Until new habits and practices are in place, there’s an increased possibility of conflict between fishermen. Think of it this way: When a species like summer flounder moves north from North Carolina to New York, who has the right to fish for it? The North Carolina fishermen who’ve been living off that stock for generations? Or the New York fishermen who have lost some of their own native species and are trying to adapt to climate change?

Both can make a strong claim, and as yet there isn’t a clear set of guidelines for how to adjudicate.

Meeting these kinds of regulatory and economic challenges will require some far-reaching changes. For now, fishing communities continue to struggle. The number of people who report being in the “fishing, hunting, and trapping” industry in Massachusetts has dropped by a third over the last decade, according to Census data. Several key centers of commercial fishing, like Gloucester and New Bedford, have found themselves on the front lines of the state’s heroin epidemic.

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Can you protect the ecosystem and also help fishing communities?

Fishing communities are very much part of the marine environment, thriving when oceans thrive, suffering when ecosystems destabilize. It should be possible to find solutions that benefit both.

For instance, fishermen and fish would both be helped by better scientific models, so that regulatory agencies could predict how the effects of a particular fishing quota might ripple through the environment.

It’s also possible that the whole species-by-species quota system no longer makes sense, and that it will have to be replaced by some more holistic approach. Telling fishermen how much cod to catch is just too blunt an instrument. Just as important, for instance, is the method used. Trawling the ocean floor for ground fish, for example, kills lots of other marine animals and tears up essential resources.

Ultimately, fishing communities are likely to need a good deal of economic support to adapt to the dramatic ocean changes happening all around them. That could mean money for new equipment, additional training, off-ramps for those who want to try new careers, or incentives to help the industry market and distribute their new products (cod shmod ... try redfish!).

Is the warming going to stop?

The “what goes up must come down” principle doesn’t seem to hold in this case. Just because the waters around Massachusetts have been warming at an unusually rapid pace doesn’t mean they’re bound to level off, especially given that the entire planet is expected to get hotter.

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Even if you add up all the climate pledges made by all the countries at December’s international global warming conference in Paris, it’s still likely the world will heat up by at least 3 degrees over the course of this century.

That’s going to continue transforming the nature of life in our oceans and the fortunes of fishermen.


Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the U.S. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz