Strait Science: Warming waters may be good news for pink salmon

Last summer saw a huge surge of pink salmon far and above normal numbers, both around Nome and throughout the region. According to Ed Farley, Program Manager for the Ecosystem Monitoring and Assessment Program at the Alaska Fisheries Science Center, this summer’s pink salmon boom may be part of a larger trend with climate change at its root.

Annual winter sea ice in the Bering and Chukchi Seas has been decreasing at a faster and faster rate in recent years.

On average, between 2004 and 2013, there were about three months of open water in the Chukchi and Northern Bering Seas, Farley reported. Climate models at the time predicted four months of open water by 2020 and five months by 2040.

“However, those of you who live in the region and those of us that are studying up there, we know that the model’s not actually getting us where we’re at now,” he said in a recent Strait Science talk. “In fact, we’re exceeding everything that these models were predicting.”

Recent years have seen some parts of the Chukchi and Norton Bering Seas completely devoid of sea ice all year because of warming ocean temperatures, and freshwater in rivers and lakes has seen warming to an even greater extent.

Many species are struggling to deal with these rapid changes, but some species like pink salmon, Farley said, can take advantage of opportunities and thrive.

Farley explained that the pink salmon life cycle takes two years, which is relatively fast compared to other salmon species. A pink salmon spends one year in freshwater as it grows from an egg to a smolt, followed by another year in the ocean as it grows from a juvenile to an adult.

Pink salmon from the Norton Sound region spend their first summer in the Northern Bering Sea before migrating south of the Aleutians for the winter and intermingling with pink salmon from southern Alaska, Canada and Asia.

When they return to the Bering Sea the following summer, they swim upriver to spawn in the massive runs that regional residents are familiar with. But not all pink salmon breed exactly where they were born.

More so than other salmon species, pink salmon are great at finding places to colonize, Farley said. “And so if there’s new habitat that’s forming up in the Beaufort Sea and the Chukchi in terms of freshwater warming, they’re going to find it.”

Indeed, just the last ten years have seen a massive increase in pink salmon found in rivers both along the North Slope of Alaska and the northern coast of Siberia.

Last year, there were even enough salmon around Wrangell Island to feed polar bears waiting for the fall ice to form, a phenomenon that just decades ago would have been unthinkable.

“There’s a number of papers that have documented that pink salmon do better when they’re in warmer freshwater environments,” Farley explained. “So, in this case, we believe that the warmer freshwater environment will favor higher survival.”

He added that warmer ocean temperatures may help them grow faster during their year at sea, which may help protect them from marine predators and cause the population to boom even more.

Another interesting quirk of pink salmon is that there are actually two populations: the odd-year spawners and the even-year spawners. While one population is growing up in freshwater, the other one is out in the ocean, and because of their offset timing the two populations don’t intermingle.

While historically the even-year population has been larger, the recent trend has seen bigger growth of the odd-year population.

Farley mentioned one paper suggesting that the odd-year population does better in warmer waters because 10,000 years ago, its ancestors waited out the ice age in warmer, southern areas while the ancestors of the even-year population adapted to colder water.

He also said that while the pinks may be doing well, other salmon species are having more trouble adapting to the changing temperatures. Studies have shown that king salmon numbers have been decreasing, and more research needs to be done to evaluate how silvers salmon are responding.

As for pinks, though, numbers will likely continue to grow as waters continue to warm, and the surge of pinks seen this summer may be the beginning of a new normal.