The chemistry of animal tissues reflects the composition of food and water in the area where they live and feed, and can act as a natural tag. Using this idea, the Southampton team, working with scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas), the Game and Wildlife Conservation Trust (GWCT) and the National Oceanography Centre (NOC), looked at the isotopes of carbon contained in historical records of scales of Atlantic salmon. The scales grew while the salmon were feeding at sea, so the carbon isotope values of the scales reflect the values of their diet in the feeding grounds. The team compared the scale values through time with satellite records of sea surface temperature across the North Atlantic. The locations of sea where the time series match best are most likely to be the areas where the fish have been feeding.

"As every single salmon contains the natural chemical tag, we can now see where fish from individual rivers go to feed in the Atlantic," lead author Dr Kirsteen MacKenzie said.