Over the past century, the salmon populations in the Baltic Sea have become more and more genetically similar. The reason for this is probably the common practice of putting farmed salmon outside in order to replenish the natural stocks threatened by environmental degradation and the construction of hydropower plants, reports an international team of researchers in "Proceedings B" of the British Royal Society. The genetic standardization of the salmon may reduce their ability to adapt to changing environmental conditions. The study points to the long-term consequences of a practice that is widespread worldwide.
The Atlantic salmon (Salmo salar) spends most of its adult life in the sea, in the Atlantic Ocean.
A subspecies lives in the Baltic Sea.
In autumn, the animals migrate to the upper reaches of the rivers in Europe and North America to spawn.
The young salmon then migrate back into the sea when they are around two years old, until after a few years they return to their place of birth to spawn there themselves.
In the past century, salmon stocks declined in many places, including in the Baltic Sea.
Overfishing and environmental pollution threaten the animals, but above all the construction of hydropower plants, which hinder the natural migration of salmon in the rivers.
Atlantic salmon jump over a rapids at Fotlandsfossen in Norway
Source: picture alliance / blickwinkel / A
Originally, young salmon migrated from at least 80 rivers into the Baltic Sea, write the scientists working with Johan Östergren from the Swedish University of Agricultural Sciences. Today there are only 28 left. 16 of these rivers belonged to Sweden; around 90 percent of the young salmon in the Baltic Sea came from them. In order to compensate for the losses, around five million young salmon - known as smolts - are released annually, making up around 60 percent of the young salmon there. This practice is the "most extensive and longest replenishment experiment in the world," the researchers write.
“The salmon rearing facilities are maintained by the power plant operators, they are legally obliged to do so,” explains Harry Vincent Strehlow from the Thünen Institute for Baltic Sea Fisheries in Rostock, who was not involved in the study. “The salmon that rise to spawn are caught, the eggs are stripped and then incubated and reared in fish hatcheries.” After about two years, the smolts are usually returned to the estuaries of the rivers.
The scientists around Östergren now wanted to find out whether the decades of practice had changed the genetic make-up of the salmon.
To do this, they analyzed characteristic gene areas of 1680 salmon from 13 Swedish rivers.
In eight of these waters salmon spawn naturally, in five there is no natural reproduction at all.
The researchers obtained the genetic material on the one hand from dried scales that had been collected in museums since 1920, and on the other from animals now caught in rivers.
The evaluation showed that a large part of the populations examined had converged genetically more and more closely over the past few decades. The researchers attribute this to the fact that, as a result of the release of young salmon by humans, the gene flow between the different populations increased, and as a result, they became more and more similar. Genetic diversity increased within individual populations, again because new genes were introduced into the populations by the farmed salmon. The particularly large stocks showed fewer changes, presumably because more of their own young salmon grew in them.
At the beginning of the rearing of salmon, native fish were still used as breeding stock, explain the researchers.
However, non-native animals were increasingly used in all rearing stations in order to be able to guarantee reliable offspring.
This contributed to the standardization of the gene pool.
If the farmed salmon were released directly into the sea - and not into rivers - there would be another problem: These salmon lack the character for a fixed place of birth, to which they migrate to spawn themselves.
That is why they “roam” around and distribute their genetic characteristics widely.
According to the researchers, this was mainly practiced in the 1980s and 1990s.
The genetic standardization affects the fitness of the animals at the individual level and at the population level, write the researchers.
Local genetic adaptations would, at least in the short to medium term, be replaced by non-adapted genetic variants, which should reduce the resistance to future environmental changes.
It is conceivable that, for example, local genetic adaptations to higher water temperatures, which could be useful in times of climate change, could be lost.
For the salmon populations in the Baltic Sea, the genetic homogenization has probably led to negative biological consequences, the researchers summarize.
In solving the problem, a joint, scientifically based hatching and release strategy across the Baltic Sea would help, said Östergren.
“If this were also adapted to the possibilities of the fishery to actually catch and harvest the majority of the released fry, then the problem would be less significant.” Because large rivers or large populations have a certain resistance to the transfer of genes It seems that it is also important to keep wild salmon populations as large as possible.
Salmon farming on land - not everyone likes it
The sales of salmon are increasing from year to year.
In Germany it is the most popular fish and demand is also increasing in other countries.
One reason why animal breeding has to be outsourced - to land.
“Basically, every river has its own subpopulation, which has its own characteristics that are adapted to the respective habitat,” says Strehlow.
Genetic impoverishment could lead to the loss of this diversity and thus also of the ability to adapt.
It is questionable whether the practice of releasing juvenile fish will continue in the long term.
"Sooner or later the EU Commission will demand the preservation of wild populations," says Strehlow.
"That is not compatible with the operation of hydropower plants." If you want to preserve the stocks, it makes sense to improve the habitats of the salmon in the rivers, for example by removing weirs or creating gravel beds for spawning.
"You can achieve a lot with it."