How do ocean-spanning salmon find their way back to the river in which they hatched years before when it’s time for them to spawn? US scientists believe they have come closer to solving the long-standing mystery.
A new study by researchers led by Oregon State University (OSU)’s College of Agricultural Sciences suggests that the fish use chains of microscopic magnetite crystals embedded in their tissue as both a map and compass, allowing them to navigate using the Earth’s magnetic field.
The team subjected juvenile chinook salmon (Oncorhynchus tshawytscha) to a brief but strong magnetic pulse.
Such pulses reverse the polarity of magnetic particles and are known to affect magnetic orientation behaviour in many species such as bats, birds, sea turtles and lobsters, but they had never before been linked to behavioural changes in fish.
The pulsed salmon’s behaviour was compared with that of ‘unpulsed' control fish in a magnetic coil system. The scientists found that if they shifted the “magnetic map” the control fish would be randomly oriented, but the pulsed fish would display a preferred heading.
The iron ore magnetite, or lodestone, is the most magnetic of the Earth’s naturally occurring minerals, but the researchers believe that the salmon use the crystals to navigate only when in the sea.
“In the river they seem to rely upon chemical signals,” said Prof David Noakes of OSU, adding that research was continuing into the fishes’ ability to return to the exact spot where they hatched.
“We’re trying to figure out the life-cycle of the salmon from the points of highest information – when they go from fresh water to salt water, and when they turn around and come back.”