Baby salmon with no prior migratory experience are able to orient themselves according to the Earth's magnetic field in the direction of the marine feeding grounds frequented by their ancestors, showed a new study published Thursday that suggests the fish may inherit a kind of built-in GPS that always points them home.

Nathan Putman of Oregon State University and his colleagues tested the offspring of adult Chinook salmon from the Willamette River Basin in Oregon for evidence of orientation preferences that would indicate use of an inherited magnetic map.

The fish, which hatch in freshwater but migrate to sea and spend several years foraging in the ocean like other species of semelparous Pacific salmon, had not yet embarked on their seaward migration and were thus navigationally naive.

The researchers used a magnetic coil system to expose young Chinook salmon to magnetic fields that exist near the latitudinal extremes of their typical oceanic range.

They found a magnetic field characteristic of the northern limits of the oceanic range led Chinook to orient southward while a far southern field encouraged the salmon to orient northward.

"Our findings are certainly suggestive that before the fish even hit the ocean, they have information about how they should orient to reach, or remain in, favorable locations," Putman said.

"In essence, the fish act as though they have a map based on the magnetic field," Putman added.

Further study showed that Chinook use the combination of magnetic intensity and inclination angle to assess their geographic location and guide themselves accordingly.

Given that such navigational systems have also been reported in sea turtles, the researchers speculated that similar navigational systems also exist in other marine species with similar life- history patterns, such as tunas, eels, sharks, seals and whales.

The findings were published in the US journal Current Biology.