Climate change drives salmon evolution

For salmon trying to make it upriver to spawn before a hot summer hits, slow and steady loses the evolutionary race.

Salmon DNA records stretching back over 30 years show that nature has increasingly selected for fish that migrate from the ocean earlier in the year. It is among the first pieces of genetic evidence that climate change is driving the evolution of a species.

Many species have changed their migration patterns over the past few decades in response to warmer temperatures. What is difficult to tell is whether the species are changing their behaviour or evolving genetically ? or both.

Thanks to an old experiment, researchers at the University of Alaska at Fairbanks were able to confirm that genes play a role in at least one animal's response to warmer temperatures ? the pink salmon (Oncorhynchus gorbuscha). Its migration from the ocean to the river is controlled largely by its genes.

In the 1970s, Anthony Gharrett was studying why some salmon migrate up Alaska's Auke Creek a month later than the rest of the population. Gharrett selectively bred some late-migrating fish so that they would share a rare genetic mutation that had no impact on the salmons' survival prospects. This allowed him to identify the late migrators even before the migration began.

By the time Gharrett ended these salmon-breeding investigations in 1985, 26 per cent of the late-migrating salmon population had the genetic marker, compared to 3 per cent of the rest of the population. He continued to collect DNA from the migrating salmon population every few years.

Fast forward to 2011, when Ryan Kovach and David Tallmon analysed Gharrett's 32-year-long fish DNA record. They found that the number of salmon with the genetic marker ? more likely to be late-migrating fish ? was relatively stable throughout the 1980s.

Disappearing marker

But between 1989 and 1993 there was a steep decline in the marker's frequency. It is now found in about 3 per cent of all fish, and it is no longer possible to distinguish early and late-migrating salmon by studying the frequency of the genetic marker in the population.

The speed with which the marker has disappeared from the population suggests that something in the salmon's environment changed between 1989 and 1993, making late-migrating fish less fit and increasing the number of fish in the general population with an early-migrating heritage. Stream temperatures in 1989, during the peak migration time, were the second highest on record, the researchers note.

It is an exciting result, says Lisa Crozier of the US National Oceanic and Atmospheric Administration in Seattle. The findings fit with many observations of salmon changing their migration patterns, although each population is different: some are migrating later in the autumn rather than earlier in the spring, for example. If these shifts have a genetic component, she says, the selection of certain salmon could have long-term effects on their genetic diversity.

While it's good to know that the salmon are adapting, Kovach says, an altered salmon migration pattern is "worrisome" for other animals, such as those that depend on eating the fatty fish in order to hibernate or produce milk for their young on schedule.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2012.1158

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