Atlantic Salmon Are Evolving Not To Grow As Big And It's Not Clear Why But It's Probably Our Fault
Michael Marshall, Contributor-Science
19 Nov 2018
Atlantic salmon are evolving rapidly, and the result is they are not growing as large as they used to. The effect has been traced to a single gene.
Atlantic salmon (Salmo salar) hatch in rivers in Europe, where they spend their first few years before heading out to sea. Once they reach maturity, they return to their home river to breed.
One such breeding ground is the Teno River, which forms the boundary between Norway and Finland. Since 1972, citizen scientists have collected samples of the salmon's scales, which contain DNA.
"We knew from our earlier research that the age at maturity had been decreasing over this period," said Craig Primmer at the University of Helsinki in Finland.
When to spawn is a trade-off: a salmon that waits longer will grow larger and probably have more offspring, but it also risks dying before it can spawn at all. Primmer's team has found that the salmon are spending less time at sea growing, and instead heading back to their rivers to spawn sooner.
"Now we wanted to see if there were signs of this also at the genetic level, that is, whether it was an evolutionary change," said Primmer.
This is normally tricky, because most aspects of an organism's anatomy and behaviour are controlled by multiple genes. But in this case one gene, vgll3, was known to be the major player. The team has found that the version of the gene that causes later maturing has become 18 per cent less common over 36 years.
The results have been published in Nature Ecology & Evolution.
It is not clear why the salmon are evolving to be smaller at maturity. It could be that they are being affected by climate change in some way, or that fishing fleets have targeted the larger individuals, or something else entirely.
It would be misleading to say that the salmon's evolution is a good or bad thing. On the one hand, it suggests that they are adapting to the pressure they are experiencing, whatever it is, which seems encouraging. But it also makes clear that they are under some sort of pressure, and since we do not know what it is, we do not know how much they can endure.
According to the International Union for Conservation of Nature, Atlantic salmon are "least concern", meaning they are not at imminent risk of extinction. However, the species has not been assessed since 1996, so a second look might well revise that finding. Certainly, some of the local populations have declined steeply in recent decades.
The study is also a reminder that evolution can act rapidly when it wants to, even on species that only reproduce every few years. “This is another example de-bunking the myth that evolution takes millions of years,” said lead author Yann Czorlich of the University of Turku in Finland.
Similarly, in the Galapagos Islands an entirely new species of bird arose in just three generations. The island of Daphne Major was home to a single species of finch called the medium ground finch, until in 1981 a single Española cactus finch arrived on the island. It mated with a local finch and the offspring were fertile. They began reproducing among themselves, shunning matings with the resident medium ground finches. They are larger and their beaks are a subtly different shape.
Clearly, evolution can be quick. But for many species, facing a host of pressures from over-hunting, habitat loss and climate change, the question is whether it will work quickly enough.