Fish farming may have been devised as a remedy to reinvigorate dwindling fish stocks but this human solution has spawned another problem: lower genetic diversity. Now, a study shows that the genetic makeup of Atlantic salmon populations from a century ago compared with the current stock across 13 Swedish rivers is more genetically similar than distinct, which researchers say could compromise the ability of the fish to adapt to climate change. In the study, researchers compared DNA retrieved from 893 archived Baltic salmon scales collected by fishermen and fishery biologists in Sweden from the 1920s with 787 contemporary samples. Of the 13 rivers in focus, five solely harbour salmon populations that have been reared by humans, the researchers wrote in the journal Proceedings of the Royal Society B. In the 1920s and 30s, there was very little industry in the area – but by the 50s and 60s the hydropower sector was booming. This is when large-scale fish rearing – formally called stocking – took off as a mitigatory measure. It is apparent the genetic changes started taking place in tandem with the stocking, said the lead author, Dr Johan Östergren of the Swedish University of Agricultural Sciences. The problems with stocking begin at the very outset: salmon chosen by so-called hatcheries are generally the same – they are selected for fast growth but are largely devoid of the special survival skills embodied by wild stock. If reared salmon escape their pens or stray away from their designated water body, and end up mixing with wild species, they all but guarantee their offspring an inferior draw in the genetic lottery. For example, if reared salmon from river A strays into river B, which exclusively has wild salmon, the genetic diversity as a result of reproduction between the two in river B will technically be enhanced. But overall, the species in two rivers will become genetically more similar, which could diminish the ability of the salmon to adapt to environmental changes. In fact, given that mixed offspring are likely carry inferior genetics and are therefore less likely to survive, stocking may not even reverse the decline in salmon populations in the long run. “All these decisions [to rear fish] were taken in the 1950s and 60s … but since then, they’ve never been reviewed scientifically. Maybe it’s time to actually have a more scientific base,” said Östergren. Carlos Garcia de Leaniz, the director of the centre for sustainable aquatic research at Swansea University, who was not involved in the study, said this latest research added merit to arguments that stocking was not the solution for salmon conservation, nor could it compensate for lost or degraded habitats. “It is another nail in the coffin of stocking, the … techno-arrogance approach to salmon conservation, one that simply addresses the symptoms (fewer fish) and not the causes (less water, less habitat, more fragmented rivers) by simply releasing fish,” he added. “This is a very solid study that adds to the growing body of evidence that shows that stocking is at best a waste of time, at worst an additional problem for the same populations it is trying to restore.”