Salmon shift migration timing to cope with a changing climate

Nature is nothing if not cyclical. From the steady spin of electrons around an atom’s nucleus, to the Earth’s seasonal passage around the sun, to the exchange of birth and decay that govern all organic life, natural processes often seem tuned to an unseen timepiece.

Scientists have spent millennia studying the cycles taking place around them, so when one as established as annual salmon migration apparently desynchronizes, those in-the-know take notice.

“When you see these really massive changes in [salmon] migration timing, it’s definitely something that made us all stop,” said Ryan Kovach, a fisheries research biologist for the U.S. Geological Survey. Kovach led a study to determine what might be causing these changes in Southeast Alaska, and what they could mean for the ecosystem on a larger scale. Their results were published online in Global Change Biology.

Representing the USGS and University of Alaska Southeast, the researchers analyzed long-term population data for four significant salmon species: chum, coho, pink and sockeye. The data showed that the former three species have been migrating earlier than normal, while sockeye are migrating later.

The shifts in timing alone were alarming, but it’s the details that raised the biggest questions. Certain populations had diverged from their historical migration patterns by several weeks. Initial analysis also revealed that these changes had occurred over only a few generations.

“When we started to dig a little bit deeper, we found out that at least one of those population changes in migration time was due to evolution by natural selection,” Kovach said.

As the study expanded to include most of eastern Alaska, the researchers considered what force could act on an entire region’s salmon populations, prompting a genetic predisposition toward earlier or later migration. With temperatures in Alaska on a steep rise, the climate seemed to be a model candidate.

“This kind of fits into the whole climate change picture,” Kovach said. “Changes in migration timing are probably the best documented biological response to climate change.”

Long-term salmon run data from weirs like this one helped show alarming shifts in migration timing. (Credit: Evan Barrientos)

Rising temperatures, Kovach explained, affect when and how rapidly winter snowpack melts. Earlier snowmelt has led to lower summer streamflow in Alaska, and low streamflow can make upstream passage even more difficult — or downright impossible — for spawning salmon. This could explain why most Alaskan salmon are migrating earlier: to hit the streams just as the snow melts, when streamflow is at its peak. Sockeye salmon usually migrate in July, when streamflows are already at their lowest. Their shift to later, rather than earlier migration could be an effort to utilize the rains of late summer.

As the salmon alter their migration habits to compensate for a changing climate, other species will have to adapt as well. Bears “know” to feed at certain spawning streams at particular times during the year. The entire life cycles of some parasitic aquatic invertebrates are dictated by the presence of salmon. And humans, of course, have built a huge industry on the commercial harvest of salmon in Alaska. But the reaction of the larger ecosystem to shifting salmon population dynamics remains to be seen.

“There’s a clear link between salmon migration timing and the behavior of other species,” Kovach said. “We just don’t know what that impact is, yet.”

Further research, Kovach says, will focus on the details behind the selective mechanisms that produce these changes in timing. While there’s little question that some sort of climatic phenomenon is at work, he says the fisheries science community needs a “finer scale understanding of what that may be.”

“We need to get a better handle on this if we really want to make sure that we can adequately sustain these fish into the future,” Kovach said. “It behooves us to dig deeper and figure out what it is that’s driving these strong changes.”