Green Blog: The Sockeye's Secret Compass

Every summer, millions of sockeye salmon flood into the Fraser River in British Columbia, clogging its shivering waters with their brilliant blushing bodies.

Scientists and spectators alike have long been awed by the sockeye?s audacious struggle to swim upstream to spawn. And while it has been known for years that a salmon can smell its way up the river to find its natal stream, no one has been able to explain just how these beautiful and economically vital fish find their way back from the open ocean, 4,000 or 5,000 miles away, to the right river mouth.

Now, research from Oregon State University provides the first evidence that sockeye are guided home after two years at sea by a memory of the magnetic landscape of the river. The results of the study appear in the latest issue of Current Biology.

Many animals including seals, sea turtles and some migratory birds have all been shown to use magnetic fields to navigate. Just last summer, scientists discovered tiny iron crystals in the nose of rainbow trout, a close relative of the sockeye, that allow the fish to detect the changes in the earth?s magnetic field.

Nathan Putman, a postdoctoral researcher at Oregon State and the lead author on the study, took advantage of 56 years of fishery data and the unique geography of the coastline of British Columbia to show that sockeye recognize what home should look like, at least, magnetically speaking.

?When sockeye have gotten nice and fat out in the Pacific and start heading home to spawn, they have to decide which way to swim around Vancouver Island,? Dr. Putman said. ?There?s this 300-kilometer [185-mile] piece of land blocking their entry into the Fraser River from the ocean, and they either have to swim north by way of the Queen Charlotte Strait, or south through the Strait of Juan de Fuca.?

The fishery data Dr. Putman examined tracks whether salmon return by way of the southern waterway, which is shared by the United States and Canada, or the northern route, which is the exclusive economic property of Canada. Hidden in all the numbers collected by fishery officials for divvying up the catch, Dr. Putman discovered that when the magnetic field of the northern passageway was similar to what was experienced by the fish two years ago upon leaving the Fraser, more of the salmon chose the north route. When the magnetic field of the south was more similar, more went that way.

?When the sockeye? leave the Fraser as juveniles, that first encounter with saltwater is like a kick in the face,? Dr. Putman said. He said that it was as if to say: ?This is important You have to remember this magnetic map. And they do.?

?As the sockeye swim southward along the coast on their way back home, they are looking for a magnetic signal that is close in intensity to what they remember from years ago,? he said.

Other factors like food availability and especially water temperature can also affect the path taken by the sockeye, but Dr. Putman said he was surprised by the degree to which the magnetic fields seem to be controlling the route.

?I know it might seem fantastical to some people that fish have evolved compasses in their noses,? he said. ?But remember, a sockeye only gets to do this once. They have just one chance to spawn and pass on their genes so there is huge selective pressure for them to get it right.?

Source: http://green.blogs.nytimes.com/2013/02/07/the-sockeyes-secret-compass/?partner=rss&emc=rss

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