''We are causing magnetic noise''

Several human-made fields, which are stronger than the Earth's magnetic field, are hampering navigation abilities of animals, says NATHAN F PUTMAN , a post-doctoral researcher at the department of fisheries and wildlife, Oregon State University, US. Edited excerpts from an interview Why is the ability to navigate important for animals? Different parts of an animal's range are best suited for different activities. Some locations might serve as good shelter, others as good feeding grounds, others as places where it is safest to reproduce or rear the young. Animals need to efficiently transit between these different locations, whether a few metres apart or across continents. How do animals ensure they navigate correctly? A butterfly cannot look from Maine and see that it is making progress toward Mexico, nor can a salmon see from the Aleutian Islands that it is getting closer toward Vancouver. What these animals need is an orientation system that informs them whether they are heading in the correct direction. The earth's magnetic field provides such information. They can determine where they are on the globe, based on what the magnetic field feels like. This is possible because there are predictable gradients in the field's intensity (stronger toward the poles and weaker towards the equator) and the angle that field lines intersect the surface of the earth (that is, field lines are perpendicular to the surface at the magnetic poles and gradually become parallel as one moves to the magnetic equator). These two gradients form a bicoordinate grid over many parts of the earth that allows animals to assess their location.

Given that the earth's magnetic field is quite weak (relative to some human-made fields), for animals to use such subtle magnetic fields for navigation requires an exquisite sensitivity. Unfortunately, this may put the animals in conflict with some of the things that humans do. How is this happening? Humans are doing all sorts of things that increase the magnetic "noise" in our environment. Much of this is being done while we still have a very rudimentary idea of how animals use magnetic cues. Whether offshore wind farms or wave energy devices will disrupt the migration of marine animals is difficult to know, but these projects are moving forward regardless. What do we do to avoid the problem? It would be wise for us to learn more about how animals use the earth's magnetic field to navigate as we go forward modifying what appears to be an important environmental feature for many species.