A study suggests that Pacific salmon are born with an in-built "magnetic map" that helps them to migrate over thousands of kilometres.US researchers believe the fish are sensing changes in the intensity and angle of the Earth's magnetic field to establish their position in the ocean.The study is published in the journal Current Biology.The epic journey of the Pacific salmon is one of nature's greatest migrations.The fish hatch inland in rivers and streams, before swimming for hundreds or even thousands of kilometres to reach the open ocean.After several years of foraging at sea, they make their way back to the same freshwater sites where they spawn and then die.Lead author Dr Nathan Putman, from Oregon State University, said: "The migration is a lot of effort and it is definitely challenging, and looking at it from the outside, it doesn't seem necessarily intuitive how they could manage that."Turn northPrevious research has suggested that the fish use the Earth's magnetic field to find their way, with an earlier study led by Dr Putman revealing that Sockeye salmon may possess a memory of the magnetic field where they first entered the sea to find their way back home to their spawning ground.But now the team says that the fish may also have an innate sense of the world's magnetic field.To investigate, they looked at Chinook salmon hatchlings, which had not yet made a migration out to sea.Because the intensity and inclination of the Earth's magnetic field change depending on where you are on the globe, the researchers exposed the fish to the sorts of magnetic fields they might experience on their journey through the ocean."We put the fish in buckets, we change the magnetic field around them, and the fish change direction in response to the field," explained Dr Putman.For example, if they altered the magnetic field so it mimicked the northern extreme of the salmon's range, the fish oriented south. If they changed the field so it was the same as that experienced by salmon at the very southern end of their range, the fish turned around and pointed north.Dr Putman explained: "To try to observe meaningful behaviour in the lab, we needed to have a good prediction of what the fish should do. Since none of these fish are found north of a certain magnetic field, we assumed that they are happiest to the south of that."So if they are using the magnetic field to find out where they are, they should think, 'Oh I am a bit north of where I should be', and go south. And likewise with the southern magnetic field."He added: "It's like they have a map. They know something about where they are based on what field they are in."Because the fish that were studied had never before made a migration, the scientists think the fish are born with this magnetic sense rather than it being a skill that is learned.The team believes other sea creatures such as turtles, sharks and whales may also use the same tactics to roam the oceans.(BBC)ANN.Az