The team's study focused on the changes recorded over a 20-year period to Atlantic salmon populations in Asturias, Spain - considered to be the "vulnerable" southern limit of the species' natural range. "Salmon develop quite distinct population structures because of their ability to home to their natal rivers," explained co-author Jamie Stevens, from the University of Exeter's School of Biosciences. "If you have such a defined system, they will quite quickly develop genetic profiles that become definitive to a particular river system." He said the unique characteristics meant that the fish adapted to the conditions found within a particular river. "There is a whole bunch of things: river chemistry, ability of the fish to withstand things like temperature, behavioural factors like run time to the sea and return time to spawning grounds," Dr Stevens told BBC News. "The reason why we do not want those structures broken down is because we know that those local populations have a range of adaptations that can give the fish an advantage within that river."