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Fish Crossing Genetic Borders as Oceans Warm

Scientists have identified a ‘biogeographic boundary’ for commercially valuable marine animals in the northwest Atlantic that divides genetically distinct populations of the same species. As temperatures rise, that dividing line is moving north.

Written by Matthew O. Berger Published on Read time Approx. 5 minutes
Climate species11
An American lobster hides among the kelp in the Eastern Shore archipelago of the Atlantic Ocean. Nick Hawkins

There isn’t anything obviously different about the waters north or south of 44.61 degrees north latitude off Nova Scotia’s coast. No new land mass emerges as you cross the parallel. No new ocean currents sweep in.

Yet within 100km (62 miles) of this line is the boundary between north and south for at least five very different types of marine animals, marking a dividing line between the genetically distinct populations of the species.

This multispecies boundary, however, is likely to move farther north as waters warm due to climate change, bringing the populations of commercially valuable species with it, according to a new study published Wednesday in the journal Science Advances.

The paper reveals this new “biogeographic boundary” separating the northern and southern populations of American lobster, Atlantic cod, European green crab, northern shrimp and sea scallops. Despite their differences, all those species share that breakpoint: Below it are genetically distinct southern populations of the various species; above it, genetically distinct northern populations.

“What’s more remarkable is how tightly aligned this breakpoint is to climate,” said Ryan Stanley, lead author of the study and a researcher based in Nova Scotia at Fisheries and Oceans Canada, a government agency.

Typically, when analyzing responses to climate change, he said, researchers will look at a single species, but the similarities they discovered across such different creatures allowed them to examine them together.

The researchers took the discovery of this new multispecies boundary and tried to see what it could tell them about the future of these species, all hugely commercially important, with the exception of green crab, which is invasive.

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Average winter bottom temperature (2002–2012) in the northwest Atlantic. The overlays show average binary clustering to the north (blue) and south (red). (Ryan Stanley, Fisheries and Oceans Canada)

They predict that by 2075 the line between northern and southern populations will move north: around 300km for northern shrimp, around 275km for lobster, around 225km for green crab, just under 200km for cod and just under 100km for scallops, by far the least mobile of the five species.

Both the northern and southern populations of all five species would also shift north as ocean temperatures rise. The northern population of lobster would see the biggest shift, with the center of its range moving north by 400km – about the distance by sea from Boston to the Canadian border.

For lobster, that shift would largely be the result of expanded habitat, meaning it wouldn’t be a shift so much as an increase in territory. But other species would move north while losing habitat to the south. Cod, in particular, would see both its northern and southern populations shift north by 150–200km, but it would lose about 10–15 percent of both its northern and southern habitat. Northern shrimp would gain a fraction of habitat for its southern population but lose around 30 percent of its northern habitat. Warmer waters have already contributed to declining catches and fishery closures for some segments of the northern shrimp industry in recent years.

These changes echo shifts already being seen on the other side of the Atlantic. In the North Sea, for example, all but one species of fish had shifted its distribution northward by 2005.

And the predictions lend further support to similar ones about shifts off the U.S. East Coast. A study published last year, for instance, predicted a “northward shift of the thermal habitat for the majority of species” there. In the Gulf of Maine, which it noted is warming faster than 99 percent of the rest of the global ocean, habitat loss could mean, “that species currently inhabiting this region may not remain in these waters under continued warming.”

It’s a phenomenon Tom Nies is already seeing.

“We’re definitely seeing changes in distribution here,” said Nies, executive director of the New England Fishery Management Council in Newburyport, Massachusetts, which manages cod and sea scallop fisheries. He notes, though, that those shifts aren’t all northward.

Atlantic cod. (Auscape/UIG via Getty Images)

Nies cites cod in the Gulf of Maine as an example. “At present, we’re seeing cod locating into colder, deeper waters there, not necessarily to the north,” he said.

But while cod see their range shrink, other species, such as fluke, are seeing theirs grow – though that’s only part of the story of how climate change is impacting fisheries, he said, noting it also affects how fast fish grow and how successfully they reproduce. That North Sea study, for example, found that the species that had shifted to the north had shorter lifespans and were smaller in size.

The impacts in the northwestern Atlantic are still a little unclear, Nies said, and are complicated by the fact that there are many other possible factors for struggling stocks, such as northern shrimp. “These stocks have been heavily exploited for years, so sometimes trying to differentiate between the impacts of fishing and climate change is difficult,” he said, “but climate change is definitely having an impact.”

The new study raises another complicated question, but one that might give hope to adapting fisheries. If five very different species share the same boundary, as the study suggests, perhaps other species do as well – maybe they’re similar enough to be able to thrive in new environments as they move northward into the waters vacated by cod or shrimp.

Stanley said it “could stand to reason” that, potentially, the similarities extend beyond these five species. He said he plans to test whether other species also diverge along the biogeographic boundary they found.

Jake Kritzer, director of diagnostics and design at the Environmental Defense Fund’s Fishery Solutions Center in Boston, sees this as a big unanswered question. “Would the same pattern hold for species that are currently not found in large numbers in the New England region but might become more abundant there?” he wondered.

To him, the new study suggests for the first time that populations moving north from the U.S.’s mid-Atlantic region might have similar underlying genetic makeups as those already to the north.

“The fact that things like black sea bass, squid, blue crab are showing up in the Gulf of Maine doesn’t mean that they’re actually going to form economically stable, viable populations there, but it’s giving some hope that as some species move out of the region others might be able to take their place in fishermen’s portfolios,” Kritzer said. “But the jury is still out.”

The biggest remaining questions for fishery experts are the policy implications.

“We need to know how much fish we are going to be able to catch,” Nies said. “I don’t know that we really have the ability to quantify what these changes mean, to the point where you can say, ‘OK, you’re going to be able to catch X amount of fish long term.’”

The paper suggests the northern lobster population could gain more than 25 percent additional habitat by 2075. But what that will mean for managers is still impossible to say right now. “Will yields go up 25 percent? That’s the type of information that’s hard to develop and hard to come by because there’s so many variables involved,” Nies said. “It’s difficult to take these things that say change is coming and say, ‘This is what it means.’”

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