A team of researchers from Sweden, the U.K and the U.S., has found that fish stocking in Sweden has led to a reduction in diversity in Baltic salmon. In their paper published in Proceedings of the Royal Society B, the group describes their genetic study of Atlantic salmon that live in the Baltic Sea and spawn in Swedish rivers.
Back in the 1960s, the researchers note, dams were built on many rivers in Sweden, preventing Atlantic salmon from spawning. In response, stocking operations (known as fish farming in the U.S.) were built to help boost salmon numbers.
Prior research has shown that fish farming operators tend to choose a certain kind of fish—they prefer those that grow fast, which tends to limit diversity. Prior research has also shown that fish that escape from stocking operations, which is quite common, have few survival skills. If the escaped fish mate with wild fish, the offspring tend to be less able to fend for themselves as they grow, and the population becomes less diverse. The researchers with this new effort noted that the stocking operations have been running for over a half-century, which made them wonder if the stocking operations may have been impacting diversity of the salmon population in Sweden. In searching the literature, they found that nobody had bothered to find out what sort of impact stocking might be having on the salmon. So their study involved obtaining salmon scale samples captured by a host of other entities (mainly fishermen) going back approximately 100 years. They then conducted a genetic study of the scales by screening 82 SNPs from 1,680 samples that were known to come from 13 rivers in Sweden.
The researchers found that the salmon were much less genetically diverse in modern times than they had been before stocking was introduced. This finding, they note, suggests that the stocking operations may have been conducted in vain—by adding inferior fish into the mix, stockers may have inadvertently been reducing fish populations.
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