The development of hydroelectricity has changed the genes of salmon

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Researchers have documented the artificial evolution of a natural salmon population.

Just a century ago, the Eira River, which flows into the innermost part of Norway’s Romsdalsfjorden, was famous for its big salmon. The average Eira salmon at that time weighed 12 kilograms and catching a 20 kg salmon was not uncommon.

Today, the average salmon in the Eira River is four kilograms. In other words, the size of the salmon has shrunk by a third in a few decades.

What happened to the big salmon in the Eira River?

Ingerid Julie Hagen is one of the scientists who have looked into this question. Using genetic analyses, long time series and statistical models, Hagen and his colleagues found the explanation – and documented human-induced evolution in a natural population.

“We found that mass reduction is an evolutionary response to reduced water flow. Like many other salmon rivers, the Eira is affected by hydroelectric power generation. Within just a few generations, salmon have adapted to the low water flow and don’t grow as big anymore,” says Hagen.

Less water, smaller salmon

The water flow in Eira has gradually decreased due to three different hydroelectric developments. When comparing water flow and salmon size over the same time period, a clear trend emerges; the size of the salmon decreased as the amount of water increased.

Atlantic salmon populations are adapted to the environment of their native river.

In big rivers with a lot of water, it’s an advantage to be big. In small rivers with little water, the reverse is true.

“A likely scenario in the Eira River is that the big fish simply haven’t entered their spawning grounds and therefore haven’t been able to spawn to the same extent as before,” Hagen says.

What caused the change?

Scientists investigated whether the reduction in body mass was a genetic response to altered water flow in the Eira River, environmental changes in the ocean, in addition to a host of other underlying causes. potential underlyings.

Recently, scientists discovered two genes of great importance for sexual maturation and body size in salmon: vgll3 and six6.

Did the evolution of these genes cause the Eira salmon to decrease in size?

The answer lies in the genes

“We found that genes for puberty were subject to strong selection. While the large salmon gene variant was the dominant variant before the development of hydropower, the small salmon variant is dominant today. Since the small salmon variant is more common, salmon in the Eira River are more likely to mature at a younger age and therefore also at a smaller size. Changes in both genes could explain 84% of the body mass reduction seen in Eira salmon,” says Hagen.

This study is a clear example of the importance of genetic variation for the resilience of populations to environmental change; if Eira salmon did not have the genetic variation for early sexual maturation, they would not have been able to adapt to the altered flow regime.

Reference:

Jensen et al. Large-effect loci mediate the rapid adaptation of salmon body size after river regulation, PNAS2022. DOI: 10.1073/pnas.2207634119

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Read the Norwegian version of this article on forskning.no