Last year, a team of scientists presented evidence of a correlation between ocean salmon migration and the Earth's magnetic field, helping to explain how fish navigate across thousands of miles of open water to find their river of origin. This week, a study detailing a series of experiments at the Oregon Hatchery Research Center in the Alsea River basin confirmed the connection.
"Researchers exposed hundreds of juvenile Chinook salmon to different magnetic fields that exist at the latitudinal extremes of their oceanic range. Fish responded to these 'simulated magnetic displacements' by swimming in the direction that would bring them toward the center of their marine feeding grounds," according to a press release announcing the results.
"What is particularly exciting about these experiments is that the fish we tested had never left the hatchery and thus we know that their responses were not learned or based on experience, but rather they were inherited," said Nathan Putman, a postdoctoral researcher at Oregon State University and lead author on the study. "These fish are programmed to know what to do before they ever reach the ocean."
The experiement involved the construction of a large platform with copper wires running horizontally and vertically around the perimeter. By running electrical current through the wires, the scientists could create a magnetic field and control both the intensity and inclination angle of the field. They then placed 2-inch juvenile salmon called "parr" in 5-gallon buckets and monitored and photographed the direction in which they swam.
A magnetic field characteristic of the northern limits of the oceanic range of Chinook salmon encouraged the salmon south, while a far southern field pushed the fish north. It seems the fish possess a "map sense" determining where they are and which way to swim based on the magnetic fields they encounter.
"The evidence is irrefutable," said co-author and senior scientist David Noakes of OSU. "I tell people: The fish can detect and respond to the Earth's magnetic field. There can be no doubt of that."
While Putnam said not all of the more than 1,000 fish swam in the same direction, there was a clear preference by the fish to swim away from a magnetic field that their "map sense" deemed "wrong."
"What is really surprising is that these fish were only exposed to the magnetic field we created for about eight minutes," Putman pointed out. "And the field was not even strong enough to deflect a compass needle."
Putnam said the salmon must be extremely sensitive to magnetic fields, since Earth's is relatively weak. Therefore, structures containing electrical wires or reinforcing iron could potentially affect the fish's orientation early in their life cycle, the researchers said.
"Fish are raised in hatcheries where there are electrical and magnetic influences," Noakes said, "and some will encounter electrical fields while passing through power dams. When they reach the ocean, they may swim by structures or cables that could interfere with navigation. Do these have an impact? We don't yet know."
"Juvenile salmon face their highest mortality during the period when the first enter the ocean," Putman said, "because they have to adapt to a saltwater environment, find food, avoid predation, and begin their journey. Anything that makes them navigate less efficiently is a concern because if they take a wrong turn and end up in a barren part of the ocean, they are going to starve."
However, Putnam said he doubts the magnetic field is the only tool used by the salmon to navigate.