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GPS doesn’t work so well underwater and other types of tracking device are too heavy to attach to a salmon. However, thanks to recent advances in technology, Prof Audun Rikardsen of Norway's Tromsø University has been able to spy on the lives of salmon. He tells Marnie Chesterton about his trackers' amazing discoveries. (Photo: Atlantic salmon leaping on the falls of Shin on their way to their spawning grounds. Credit: Getty Images)