Pink salmon populations in Alaska have evolved to migrate earlier in the season, reacting to rising stream temperatures.

Fluctuations in water temperature can adversely affect fish populations, leading to an increased risk of disease and mortality. When streamwater began to warm faster than normal, pink salmon populations took the hint, leaving their freshwater homes earlier than usual.

Now, migration occurs nearly two weeks sooner than it did 40 years ago, according to a study sponsored by the University of Alaska, Fairbanks, and University of Alaska Southeast. The rapid adaptation is a sign that larger organisms can cope with climate change, said David Tallmon, an associate professor of biology and marine biology at UAS and author of the paper.

"It demonstrates evolutionary change in response to warming temperatures," he said. "It provides some hope for species to adapt to changing climate."

The study, published in the Proceedings of the Royal Society B, looked at pink salmon populations in Auke Creek, analyzing 32 years of genetic data beginning in the 1970s. Scientists examined 17 generations of the salmon, and they sampled five to 30 fish every day during the odd years from 1983 to 1993, plus 2001 and 2011. Their findings show a 20 percent reduction in late-migrating fish, which made up only about 10 percent of the sampled population as of last year.

The switch toward an earlier migration pattern was exceptionally apparent between 1989 and 1993. Stream temperatures during peak migration in 1989 were the second highest on record. From then on, scientists observed substantial genetic changes in the offspring from that spawning generation, the study says.

But evolution isn't always a good thing, because genetic shifts are a give-and-take process. When an organism adopts one positive attribute, another is weakened, potentially reducing the overall fitness of the organism.

An evolutionary impact
Changes in genetic makeup also come at a cost in terms of biocomplexity. The Auke Creek late-migrating salmon populations have all but disappeared in favor of early-migrating salmon. This significantly reduces the genetic variability in the population and would make it difficult for salmon to cope should temperatures reverse.

Pink salmon have adapted so far, Tallmon said, but no one can predict the extent to which they can stretch. It is possible that they will not continue to evolve in the coming decades.

"We may exhaust the ability of these wild populations to adapt -- and stuff hits the fan," Tallmon said.

It's a possibility that could negatively affect local and national fishing industries.

Pink salmon "represents a substantial portion of some fisheries. In southeast Alaska, Prince William Sound and in the Kodiak area, it's a significant part of the commercial fishery catch," said Eric Volk, a fisheries scientist for the Alaska Department of Fish and Game. "We're talking about hundreds of millions of pink salmon fry released from hatcheries each year."

While all signs point to climate as a catalyst for evolution, Tallmon cautions against applying his team's findings across the board.

It's unlikely, he said, but this incidence of evolution could be an isolated event. More research is required before scientists can ascertain whether this applies to all pink salmon and not just the ones in Auke Creek. However, that doesn't change the overall message of the study.

"People should be aware that climate change can have evolutionary impacts, and that's something we need to be aware of and measuring," Tallmon said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500