Too Hot to Handle? Salmon and Climate

Karen Dunmall opened her mail one day in 2012 to find a package awaiting her - inside was the head of a salmon.

Receiving fish heads via the post is, to be clear, routine for Dunmall. A PhD student at the University of Manitoba, Dunmall directs the Arctic Salmon Project, a research initiative documenting the increased prevalence of Pacific salmon in the western Canadian Arctic, where salmon are historically scarce. Subsistence fishers receive $50 gift cards in exchange for their salmon - the whole body, or only the head - which Dunmall scours for clues that might reveal the fish’s origin. She analyzes the chemical composition of ear bones to pinpoint home waters and sifts through stomach contents to determine diets. She’s also developed a Pacific salmon guide to help fishermen identify their unusual catches.

“I wander around the Arctic and people don’t even know my name,” Dunmall says with a laugh. “They just call me the salmon lady.”

So what made the 2012 delivery unusual was not that it was a salmon head - it was an Atlantic salmon head. The fish had come from Clyde River, an Inuit community on the glacier-carved coast of Baffin Island in Canada’s Nunavut Territory, more than 400 kilometers above the Arctic Circle. Although Atlantic salmon aren’t entirely unfamiliar to residents of the eastern Arctic, the fisherman who caught this specimen found it noteworthy enough to hand it over to the Department of Fisheries and Oceans office in Iqaluit, which shipped it to Dunmall. She’s received several more Atlantic salmon in the years since, and, while she lacks the data to call it a trend, says the phenomenon deserves exploration.

“What we’re seeing in the western Arctic is that more Pacific salmon are showing up, and it seems to be related to environmental conditions,” she says. “We’re working with communities to better understand if Atlantic salmon are following similar patterns, and, if so, what it might mean for the marine environment and subsistence harvest.”

If Atlantic salmon are indeed infiltrating the Canadian Arctic in greater numbers, there’s an obvious potential culprit: climate change. Our planet’s oceans have absorbed more than 90 percent of the excess heat generated by global warming, driving many species poleward toward cooler climes. The most dramatic changes have occurred in the Northwest Atlantic, where creatures from black sea bass to butterfish have followed North America’s coastline toward higher latitudes.

For some creatures, like American lobster, warming oceans have already boosted productivity. Salmon may also benefit from balmier conditions by gaining access to once uninhabitable Arctic waters. They are turning up in growing numbers off Norway’s remote Svalbard archipelago, leading researchers to speculate that northern European stocks are already finding sustenance in the Arctic.

On the whole, however, it’s a virtual certainty that Salmo salar will ultimately suffer. In southern New England, at the thermal limits of Atlantic salmon’s range, fisheries managers have implicated climate change in the failure of runs to recover on rivers like the Farmington and the Connecticut. In Europe, models suggest that salmon could disappear from river basins throughout Portugal, Spain, and much of France by 2100 as triggerfish, sea bream, and other new arrivals turn up along the Irish coast.

“If you’re a cold water species, you’re probably not going to be too happy in a situation where all these warm water species are having a ball,” says Ken Whelan, the Ireland-based research director of the Atlantic Salmon Trust. In 2018, the Trust launched the Missing Salmon Project, a tagging and tracking effort aimed at identifying why fewer than four percent of smolt that leave United Kingdom rivers find their way home—a fraction of historic returns. Although Whelan is years from pinpointing the problem, he suspects that climate is an “overarching issue” exacerbating existing problems, like aquaculture and predation. “To me, that’s the most concerning thing: that these sorts of effects could be cumulative,” he says.

Atlantic salmon, of course, are fish of two worlds, and climate change stands to alter every phase of their complex life cycle, from the survival of parr to the reproductive success of spawners. The accumulation of marine and freshwater threats is why, in 2016, scientists at the U.S. National Oceanic and Atmospheric Adminis­tration declared diadromous fish the Northwest Atlantic’s most vulnerable group of species and placed Atlantic salmon atop the list.

The clamorous alarm bells notwithstanding, the salmon’s demise is not a "fait accompli" nor is it entirely beyond our control. Conservationists are already taking measures to make our continent’s rivers more hospitable in the face of a warming world. In the end, however, the species’ survival may hinge on whether society can reduce its greenhouse gas emissions and whether a legendarily resourceful fish, which has endured climate change before, can adjust to the rapid man-made version now underway.

“Whether Atlantic salmon have enough capacity to adapt is a natural experiment that’s in progress right now,” says longtime researcher, and former ASF vice-president of research, Fred Whoriskey. “We’ll just have to see.”

Even in the best of times, a salmon’s life is a gauntlet of threats. Over millions of years, the fish have evolved to navigate the obstacle course of anadromy, their genes calibrated to endure migration, evade predators, and find food. As their world grows unfamiliarly warm, Atlantic salmon may discover that their finely tuned physiology and behavioral ecology no longer serve them quite so well.

To understand the challenges that climate change poses to Atlantic salmon, imagine that you are a fish yourself—perhaps a fry hatched in the Miramichi River. The Miramichi is not merely the mother of all eastern Canadian salmon rivers, it is also, says Nathan Wilbur, director of New Brunswick programs at the Atlantic Salmon Federation, “ground zero for the impacts of climate change on Atlantic salmon.” North of the Miramichi, Wilbur explains, Canada’s rivers remain, at least for now, consistently cold; further south, watercourses like Maine’s Penobscot River regularly exceed salmon’s heat tolerance. “The Miramichi is the dividing line,” Wilbur says.

Lately, however, the Miramichi has tipped toward the hot end of the temperature spectrum. Thus, the first threat that a fry, scarcely longer than a paper clip may face in the Miramichi is heat. Although a bit of warming can help you grow faster, you start feeling stressed as soon as the thermometer exceeds 20 degrees celsius. At 23 degrees, you start seeking cold-water refuges as your metabolism hastens and lactate builds in your tiny muscles. If water temperatures reach 28 degrees, you have but a week to live; if it reaches 33 degrees, you’re dead in 10 minutes. In 2018, says Wilbur, the Miramichi exceeded 23 degrees on 57 days, most of them consecutive. “The fish were under incredible stress,” he says.

Survive hot water, not to mention herons, mergansers, and bass, all of which are harder to dodge when you’re lethargic during your fry and parr stages to become a smolt, and you’ll confront your next hurdle: migration to the sea. At this point, timing is everything. Hit the ocean too early or too late and you might miss your prime feeding window, collide with predators, or even struggle to regulate your physiology. Although your departure for the ocean is partly governed by seasonal changes in daylight, temperature also plays a major role.

In 2014, researchers found that, as rivers around the North Atlantic have warmed over the past 50 years, smolt migrations have shifted forward by 2.5 days per decade. That may sound subtle, but when you’re a hungry, vulnerable smolt, even the slightest mismatch between river and ocean could prove fatal.

If, that is, you make it to the ocean at all. In the Miramichi’s estuary, you will be pursued by schools of voracious striped bass—recently a million strong, up from just 5,000 in the 1990s. At that time, DFO shut down all fisheries for the southern Gulf of Saint Lawrence striped bass. Their bounce back is a conservation success story. But the predator’s comeback has proved problematic. On the Northwest Miramichi River, Wilbur says, smolt survival through the estuary has plummeted in recent years from 70 percent to less than 10 percent. Although it’s impossible to pin the decline entirely on bass, hungry stripers are, as biologist John Waldman has written, “a very smoky gun.” And while striped bass are native to the Miramichi, the hardy generalists range as far south as Florida, suggesting they may weather our hot future better than their salmon prey. “We can’t say that it’s climate change, but we know striped bass do well in warmer waters,” Wilbur says.

Should you escape the striper armada, congratulations, your life is about to get even more perilous. Climate change’s oceanic disruptions may not be as noticeable as parr floating belly-up in overheated streams, but they’re no less insidious. As temperatures have warmed, lipid-rich krill, the zooplankton equivalent of bacon cheeseburgers have been supplanted by smaller, less nutritious copepods; forced to fill your tank with lower-quality fuel, your growth slows and you remain more susceptible to seals, sharks, and other predators. Sea lice breed faster in warmer waters, heightening your risk of infection, particularly when you pass near salmon farms that are breeding grounds for contagions. Oceanographic patterns can go haywire, the Gulf Stream strengthening and the Labrador current slackening, disrupting the migration route hardwired in your brain. Your cold-blooded body, governed entirely by ambient temperature, ramps up its metabolism with the heat, devolving into the piscine equivalent of a gas-guzzler.

“You may actually get to the point where you’re churning along at such a fast rate that your food intake is being consumed,” says Whoriskey, “and you’re not storing up the fat that you need to survive over the wintertime.”

If, by some miracle, you avoid predators and endure sea lice and navigate shifting currents, your reproductive success is far from guaranteed. Two years after you left, your natal Miramichi is running even hotter than it did in your parr days, yet another test of your hyperactive metabolism. Heat-stressed and exhausted, you’re susceptible to even seemingly minor threats. like, say, a ten-minute struggle on the end of a tapered mono leader. Should you manage, at last, to dig your redd and deposit your eggs, your genes are hardly assured of persisting. Rain­storms, intensified by climate change, may scour away your future offspring before the cycle can begin again.

“It is extremely unlikely that Atlantic salmon as a species will become globally extinct within the next 100 years” as a result of climate change, a 2017 report by the Inter­na­tional Council on the Exploration of the Sea concluded. “However, it is very likely that some populations will suffer significant reductions in abundance.” Extirpation, or localized disappearance, is neither as permanent nor quite as devastating as utter extinc­tion. But for communities at the southern bounds of Atlantic salmon’s range, the losses will feel no less final.

The circumstances for Atlantic salmon may be dire, but they’re not hopeless. The Miramichi River is honeycombed with frigid pockets, groundwater-fed seeps and brooks that bubble into the mainstem and create pools that remain several degrees colder than the surrounding river. To an angler, these cold spots offer enticing fishing holes. To a salmon, they’re life-savers.

In 2010, Emily Corey, a PhD student at the University of New Brunswick, discovered just how vital these cold harbors can be. That spring, Corey inserted PIT tags - microchips the size of a pencil tip - into 600 fish to monitor their movements around the watershed. In July, river temperatures climbed above 26 degrees for days on end and even briefly spiked to a lethal 31 degrees. As Corey scanned the river with a mobile tag-reader, she found it nearly devoid of fish until she reached the precious cold spots. “Wherever was coolest and deepest, that’s where the adults were,” Corey recalls with amazement, “and then you’ve got a cloud of parr on top of that.”

Often, Atlantic salmon parr are aggressive, fiercely defending their feeding areas from rivals. That summer, though, they lived in harmony, adhering to what Corey calls “almost a schooling behavior.” Guarding territories, in other words, was less important than staying cool. Remar­kably, some fish had traveled as far as eight kilometers to take shelter. “Any area that was cooler, they found it,” Corey says.

Protecting these cold-water refuges is today a focus of conservation work on the Miramichi, a crucial adaptation strategy in the face of warming rivers. The Depart­ment of Fisheries and Oceans manages the river under a “warm water protocol,” which protects adult fish that use cold pools on their way to spawn. If river temperatures climb above 20 degrees for 48 consecutive hours, DFO closes 26 cold-water refuges to fishing to protect salmon from additional stress. If it gets hotter than 23 degrees, fishermen are only permitted to ply the river in the morning.

In 2018, to the chagrin of some outfitters, the cold pools were shut down for nearly a month and a half, from July 5 to August 21.

Regardless, conservationists and anglers are not only safeguarding chilly refuges, they’re enhancing them. Since 2014, the Miramichi Salmon Association has completed nine cold-water restoration projects, using heavy machinery to deepen holes, deflect flows from creek mouths into the main current, and install boulders to make cold pools even more enticing. Smarter forestry management can help, too. Protecting forests in the headwaters of the Miramichi watershed, and leaving broad riparian buffers bracketing streams, slows down runoff after rainstorms, reducing erosion and keeping chilly water in the river longer. “We don’t think forestry regulations are stringent enough to protect cold water and flows,” Wilbur says.

For all their value, however, such local land use decisions can only accomplish so much. In one 2016 study that analyzed four decades of data, researchers found that land use changes, like sheep overgrazing, had little impact on the freshwater survival of Atlantic salmon and brown trout in Ireland’s Burrishole catchment. What did matter: climate. In years with wetter, warmer winters, conditions that will become more common as the climate changes—salmonids suffered lower survival rates.

And land use changes, of course, do little to protect salmon at sea, when the fish are subject to a mind-boggling array of climate-related stressors that we still scarcely understand. “We have to keep telling ourselves that salmon are not freshwater fish,” says Whelan. “We have to see these creatures as marine fish, and sit down with the scientists studying herring, mackerel, and other pelagic stocks to see what story they’re telling us.”

That story: Unchecked climate change may well, in the end, overwhelm our conservation efforts. According to the Fourth National Climate Assessment, a blockbuster 2018 report produced by the U.S. government, the world will experience two to three degrees of warming by century’s end even if we manage to dramatically reduce our greenhouse gas emissions. And although nearly all of the world’s nations signed the Paris Accords, a 2016 deal aimed at curtailing global emissions, industrialized countries have made little progress toward meeting the agreement’s goals—which many researchers believe wouldn’t be adequate to hold warming below two degrees anyway. And that was before the second-largest carbon emitter, the United States, withdrew from the accords in 2017.

If there is long-term hope for Salmo salar, it lies in the salmon’s own proven tenacity in the face of environmental changes. The species has already overcome the fluctuation between an ice age and a successive era of global warming. Could the Atlantic salmon sent to Karen Dunmall be a sign Salmo salar will find a way to endure once again?

Only time will tell, but in the meantime it is essential to understand the extent of the salmon’s northern range extension and identify and protect areas with cold springs and shaded streams. As the Arctic becomes more habitable, human interventions to preserve and enhance coldwater refuges may yet allow Atlantic salmon to survive and, perhaps, thrive again one day. Now that would be cool.


Ben Goldfarb is an independent environmental journalist and the author of Eager: The Surprising, Secret Life of Beavers and Why They Matter (Chelsea Green Publishing, 2018).