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The largest salmon have disappeared, but people continue to go fishing in Eira’s beautiful scenery.

Hydropower development altered the genes of salmon

Researchers have documented man-made evolution in a natural salmon population.

Barely a century ago, the river Eira, which flows into the innermost part of Romsdalsfjorden in Norway, was famous for its large salmon. The average Eira salmon at that time was 12 kilograms, and catching a 20 kg salmon was not uncommon.

Today, the average salmon in the river Eira is four kilograms. In other words, the size of salmon has reduced to a third over a few decades.

What happened to the large salmon in the Eira river?

Ingerid Julie Hagen is one of the scientists who pondered this very question. With the help of genetic analyses, long time series and statistical models, Hagen and her colleagues found the explanation - and documented human-induced evolution in a natural population.

“We found that the reduction in mass is an evolutionary response to reduced waterflow. Like many other salmon rivers, the Eira is affected by hydroelectric power production. In just a few generations, the salmon have adapted to the low waterflow and no longer grow as large,” Hagen says.

Less water, smaller salmon

The waterflow in the Eira gradually reduced due to three different hydropower developments. By comparing waterflow and salmon size in the same period, a pattern clearly emerges; the size of the salmon has decreased in step with the amount of water.

Populations of Atlantic salmon are adapted to the environment in their natal river.

In large rivers with much water, it is an advantage to be large. In small rivers with little water, the opposite is true.

“A likely scenario in the river Eira is that large fish simply did not enter their spawning grounds, and were therefore not able to reproduce to the same extent as before,” Hagen says.

Three hydropower developments have reduced the River Eira’s waterflow (dotted lines). In 1953, 1962 and finally in 1975. Body size (blue points) follows the reduction in waterflow (red line).

What caused the change?

The scientists investigated whether the reduction in body mass was a genetic response to the altered waterflow in river Eira, environmental changes in the ocean, in addition to a set of other potential underlying causes.

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

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

Morning catch, 17th of June 1939: This tweed-clad gentleman landed three monsters during the morning hours of a summer’s day in 1939, each weighing over 20 kilograms.

The answer lies in the genes

“We found that the puberty genes have been subjected to strong selection. While the large salmon variant of the gene was the dominant variant prior to the hydropower development, the small salmon variant is dominant today. With the small salmon variant being more frequent, the salmon in river Eira are more likely to mature at a younger age and therefore also at a smaller size. Changes in the two genes could explain 84 per cent of the observed body mass reduction in the Eira salmon,” Hagen says.

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


Jensen et al. Large-effect loci mediate rapid adaptation of salmon body size after river regulation, PNAS, 2022. DOI: 10.1073/pnas.2207634119


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