Forty years of farmed salmon ... and one genetic mystery

Scientists at the Norwegian University of Science and Technology are trying to determine the genetic differences between farmed and wild salmon -- and the effects of those differences -- as a way to help protect the country's unique wild salmon stocks.

Beginning in 1971, aquaculture researchers combed 40 of Norway's best wild salmon rivers to find the soundest genetic stock they could. These fish, selected for their ability to grow rapidly and use food efficiently, formed the breeding lines for Norway's wildly successful salmon aquaculture industry. Nearly 40 years and 10 salmon generations later, the industry has grown by a factor of more than 600, and had a turnover of roughly $3 billion US in 2007.

But producing more than 170 million farmed salmon results in at least some escapees -- according to Statistics Norway, the official government statistics office, roughly 450,000 farmed salmon and trout escaped from Norwegian fish farms in 2007. In comparison, an estimated 470,000 wild Atlantic salmon approached the Norwegian coast in 2007 to spawn in one of Norway's salmon rivers. It's known that escaped farmed fish can interbreed with wild salmon, and thus must have changed the genetic and physical makeup of today's wild salmon stocks. But how much? Biologists at the Norwegian University of Science and Technology (NTNU) are trying to answer this question by breeding special fish families to determine the exact genetic differences between farmed and wild salmon stocks.

130 different salmon "families"

Led by Ole Kristian Berg and Sigurd Einum, professors at NTNU's Department of Biology, researchers including have established 130 different salmon "families," where the father's contribution comes from sperm taken from Norway's first generation of farmed salmon (stored in a sperm bank), and the mother comes from a selection of Norwegian salmon rivers as well as from farmed stock.

The result is a combination of specially bred fish that can be compared to today's stocks of wild fish. When the specially bred fish are five centimeters long, they will have grown enough so that their physical characteristics, as well as their genetic makeup, can be compared to wild salmon of today.

Wild salmon under siege

Norway is home to the world's most genetically varied wild salmon stocks on the planet, with genetically distinct groups found in the country's 452 different wild salmon rivers. But since 1970, wild salmon stocks have been reduced by roughly 80 per cent. Fully 10 percent of the country's salmon rivers have lost their populations, with another 32 rivers severely threatened because of the effects of hydropower development, acid rain, sea lice and the invasion of the parasite Gyrodactylus salaris.

In 2008, scientists from the Norwegian Institute for Nature Research, a government research institute, determined that fully 35 per cent of all salmon in the Surna River, one of Norway's most important wild salmon rivers, were in fact farmed fish "That is very high for such a big salmon river," says Kjetil Hindar, a senior researcher at NINA.

"In rivers that have been affected by diseases or by parasites like Gyrodactylus, wild salmon stocks are weakened and are particularly vulnerable," says NTNU's Berg. "It is easy for these stocks to be affected by wild salmon whose genes have been diluted by farmed fish."