Wild salmon genetic diversity in decline due to ‘stocking’
Acollection of salmon scales kept by generations of Swedish fishermen and fisheries biologists over a century has given a unique insight into the human impact on fish populations.
The Swedish study focused on the Baltic region, which is heavily dominated by hydropower. The resulting dams either severely restrict or completely prevent salmon accessing their natal rivers to spawn.
As a mitigation measure, since the 1960s five million ‘stocked’ (hatchery-reared) salmon have been released annually to boost numbers, but these stocked fish have had an effect on the genetic make-up of wild populations.
Researchers focused on 13 Swedish rivers and compared DNA retrieved from 893 archived Baltic salmon scales from the 1920s with 787 contemporary samples. They found a homogenization of the populations, with local genetic adaptations being lost.
The study supports a growing body of scientific evidence that shows stocking salmon reduces the overall genetic fitness of wild populations. Long term it may reduce the ability of the salmon to adapt to environmental change.
“We suggest developing a science-based common strategy across the entire Baltic Sea area and countries around it to handle future rearing and stocking of Baltic salmon,” says lead author, Dr Johan Östergren of the Swedish University of Agricultural Sciences. “This includes reducing stocking amounts and potentially phasing out some or all stocking – or at least discussing such a phase out.”