The Working Waterfront

The changing climate of Atlantic salmon

Warmer winters with mixed precipitation a problem

Catherine Schmitt
Posted 2022-02-03
Last Modified 2022-02-03

Atlantic salmon face challenges, many of which—dams, pollution, predators—have been problematic for a long time. But as temperatures rise, precipitation and melt increase, and seasons shift, the pressure is increasing.

In their migrations from river to sea and back again, Maine’s wild Atlantic salmon, on the Endangered Species List since 2000, are experiencing climate-related threats throughout their range and life cycle. That was one takeaway from the Atlantic Salmon and Their Ecosystems Forum hosted by NOAA Fisheries and Project SHARE in January.

In Maine, warming has occurred most in winter. More frequent rain-on-snow and freeze-thaw events cause ice to break up and scour river beds that are home to salmon eggs and just-hatched fry.

Research in the Penobscot River found that delayed fish lose fat reserves…

Still, even in a warmer climate, there’s lots of good salmon habitat. In the heat of the summer, when temperatures climb and water levels drop in streams and rivers, young salmon seek out plumes of cold, aerated water. These “refugia” areas, which are plentiful in, for example, the Sandy, Piscataquis, and Narraguagus rivers, will become more critical for salmon as their world continues to warm.

Precipitation has become more frequent and intense, and resulting higher stream flows also might help compensate for warmer temperatures. In recent years, salmon smolts are starting their ocean-bound migration earlier.

Scientists with the U.S. Geological Survey are mapping river sections that tend to have more water during the summer and experimenting with time-lapse photographs from game cameras to monitor stream water levels.

Salmon smolts are more likely to survive the journey during years with high flows, according to a study of 1,536 tagged fish migrating down the Penobscot River by Alejandro Molina-Moctezuma, a Ph.D. graduate from the University of Maine now at Lake Superior State University.

Salmon smolts are also more likely to survive if they have spent more of their life in the wild versus the hatchery, especially when they get to the ocean, where more change awaits them.

Unlike on land, in the sea, most of the warming occurs in summer, when newly arrived salmon smolts are feeding in the Labrador Sea and older salmon are closer to Greenland. Declines in salmon populations have been linked to a changed ecology in the Northwest Atlantic Ocean triggered by changes in Arctic circulation in the early 1990s that changed salinities and populations of phytoplankton, zooplankton, and the small fish that salmon eat.

Some salmon return to spawn in the Penobscot River after only one winter at sea. Others stay for a second year, but these fish are not growing as well as their pre-1990s counterparts, according to Gulf of Maine Research Institute-led analysis of scales from captured fish. Only those salmon that have put on enough weight will be able to return to freshwater to spawn.

Those adult salmon that do survive—and scientists can only study the survivors—travel back to their home rivers in late spring and early summer. Marine growth is especially crucial for Penobscot River salmon, which have the longest migration of North American salmon.

Once in the river, they move upstream into spawning habitat in cool, flowing tributaries.

That is, if they can get there.


According to NOAA Fisheries, Gulf of Maine Atlantic salmon have unimpeded access to only 8 percent of their historic river habitat. Most salmon are delayed by dams which mean they spend more time in potentially lethal water temperatures below dams, said Sarah Rubenstein, a graduate student at the University of Maine.

Warm water is taxing for cold water fish. Rubenstein’s research in the Penobscot River found that delayed fish lose fat reserves and could potentially run out of energy before they were able to spawn. “Effectively, one out of three returning adults would be eliminated from spawning because of delays,” said Rubenstein. “Our model also indicates a 65 percent decrease in the number of fish that would have enough reserves to spawn successfully and survive to repeat their migration and spawn again.”

All these findings perhaps were overwhelming to scientists at the forum. But just as overwhelming, if not more, is that given all these findings, Atlantic salmon continue to flourish in the cool streams of the Maine woods, and that some continue to migrate to distant ocean waters and return home a year or two later.

“We have to trust in Atlantic salmon,” said Mary Colligan, who recently retired from the federal government after a career working on endangered species recovery. “They have shown incredible persistence, despite what we have done to them.”

The persistence of salmon continues to motivate efforts to reconnect streams and improve fish habitat by replacing aging culverts, removing defunct dams, and protecting refugia. More than 100 miles of river have been restored since 2018, said Ben Naumann of the U.S. Department of Agriculture Natural Resources Conservation Service. “A mile or two at a time, in 66 projects with 25 landowners, we have restored more than 100 miles of river since 2018.”

Scientists don’t often use words like “trust.” But what else is owed to a species that has lived through multiple ice ages and changing ocean conditions over millions of years, that has sustained people and inspired culture, and that continues to ascend the rapids of a warming world?

Catherine Schmitt is the author of The President’s Salmon: Restoring the King of Fish and Its Home Waters.