Update: December 10 – I won a travel award! I’m going to ScienceOnline! A hearty thanks to NESCent, and you should all go read the other awesome winning posts – on bed bug ground zero, bee housekeeping, and evolutionary escapes from environmental toxins.
Pink salmon spend two salty years in the ocean before they return to their birthplace to spawn and to die. If that birthplace was Auke Lake, near Juneau, Alaska, then a returning salmon can only reach its final destination by passing through a narrow opening in the weir at Auke Creek, which drains the lake into Auke Bay. Every year, thousands of pink salmon pass through the weir’s trap, both adults fighting upstream and juveniles coasting the other way. Each one of those fish is counted by researchers who stand thigh deep in the cold water, monitoring the trap every day between March and October. This marathon fish count stretches back to the 1970s, and has provided one of the most detailed records of a salmon population anywhere in the world. Combined with a fortuitous little genetic experiment performed at the weir in 1979, the Auke Creek data have also given us some long-sought evidence that the annual rhythms of the natural world are evolving in response to climate change.
Many things have changed in the decades since the fish counting started. Average stream temperatures are higher by more than one degree celcius, the salmon are returning to the lake nearly two weeks earlier, and the entire migration season falls within a narrower window of time. Although we can’t say for sure that the migration shifts are caused by the temperature change, it falls into a pattern that has been observed for many other organisms all over the world. Birds, butterflies, frogs, flowers, plankton – to name just a few – are slightly shifting the timing of their big, seasonal life events, all consistent with a response to a warming climate.
But do these timing shifts count as evolution? Without evidence of genetic change in a population, such shifts might be just the result of individuals adjusting within their normal range of behaviors. Genetic evidence to the contrary is extremely hard to come by – so even though biologists have long believed that the many examples of shifting seasonal traits must include some examples of rapid evolution, they haven’t had the hard genetic data to show it.
Luckily, some three decades ago, fisheries biologist Anthony J Garrett started an obscure little experiment at Auke Creek. Recently, that experiment was extended and repurposed by Ryan P Kovach, a graduate student from University of Alaska, Fairbanks, and David A Tallmon of University of Alaska, Southeast, to confirm that the Auke Creek salmon have indeed evolved.
In the 1979 experiment, Garrett tinkered with the genetics of late-migrating salmon just enough to let him trace their fortunes. Historically, the fish counters could distinguish between two relatively distinct populations that migrated about twenty days apart – the “early run” and the “late run.” Interested in these sub-populations, Garrett looked for a gene variant to use as a genetic “marker” for the late-run fish. The genetic marker he chose was naturally present at low levels in the population, but seemed likely to be selectively neutral – neither harming nor helping the fish that bore it. He captured all of the very last migrating pink salmon of the season and only spawned those that carried the genetic marker. The offspring of those fish rejoined the naturally spawned population, and by the time the next generation returned, late-run salmon had a five-fold increased frequency of the genetic marker compared to the early run.
Because all those diligent Auke Creek fish counters in waders were also taking DNA samples throughout the spawning seasons, we know that the frequency of the late-run marker stayed constant for about a decade, confirming that the marker was indeed selectively neutral. The “marked” fish and their descendants kept turning up reliably late until 1989, when stream temperatures during the spawning season reached the second highest on record.
By 1991, the late-run marker had faded back to the low natural levels found in the early-run fish, and in parallel, the fish counters saw a dramatic decrease in the number of salmon turning up late. In a single generation, the distinct late-migrating subpopulation had practically disappeared, making the average migration time of the entire population significantly earlier. In 2011, twenty years later, the data looked much the same as in 1991 – which means the Auke Creek salmon population is probably still dominated by descendants of the 1989 early run.
So this is interesting news for biologists looking for evidence of climate change-driven evolution. But what does it mean for salmon? Today, Auke Creek pink salmon are as abundant as ever, and thanks to that hot 1989 summer, the population is now adapted to a slightly warmer climate. But because of that adaptation process, they are also less genetically diverse and less behaviourally diverse, which means they might not be so lucky when up against other natural selection events in the future. There is also a limit to how early a salmon can spawn. If temperatures continue to rise, at some point Auke Lake could cease to be a viable salmon spawning ground, with effects that would ripple through the region, both ecologically and economically. It would also bring an end to the salmon counting.
Ryan P. Kovach, Anthony J. Gharrett and David A. Tallmon
Proc Biol Sci. 2012 Sep 22; 279 (1743):3870-8