HOOK AND FUR
By Bob Brown
How migrating salmon find their way back to rivers where they were born has been one of those mysteries that have never been fully explained. However, a new study published in the February issue of Current Biology, a scientific journal, suggests salmon find their way back to their home river by sensing the river’s unique magnetic signature.
The Columbia River Bulletin reported in its Feb. 8, issue that the research team used data from more than 56 years of salmon fisheries catches to identify the routes salmon had taken from their most northerly destinations (probably near Alaska or the Aleutian Islands) to the mouth of the Fraser River, their home river. The data was then compared to the intensity of the Earth’s magnetic field at pivotal locations in the salmon’s migratory route.
Nathan Putman, a post–doctoral researcher at Oregon State University and the lead author of the study, said Earth has a magnetic field that weakens with proximity to the equator and distance from the poles that gradually changes on a yearly basis. Therefore, the intensity of the magnetosphere in any particular location is unique and differs slightly from year to year.
Located directly in front of the Fraser River’s mouth is Vancouver Island, which blocks direct access to the river from the ocean. However, salmon can access the river from the north via the Queen Charlotte Strait or from the south via the Juan De Fuca Strait. The study showed the intensity of the magnetic field largely predicted which route the salmon used to detour around Vancouver Island. In any given year, the salmon were most likely to take whichever route had a magnetic signature that most closely matched that of the Fraser River when the salmon initially swam from the river into the Pacific Ocean.
Putman said, “Those results are consistent with the idea juvenile salmon imprint on (i.e. learn and remember) the magnetic signature of their home river, and then seek the same magnetic signature during their spawning migration. It has been long known some animals use the Earth’s magnetic field to orient themselves and to follow a straight course. However, scientists have never before documented an animal’s ability to learn the magnetic field rather than simply inherit information about it or to use the magnetic field to find a specific location.”
Putman added the empirical evidence of magnetic imprinting in animal represents the discovery of a major phenomenon in behavioral biology. In addition, this study suggests it would be possible to forecast salmon movements using geomagnetic models- a development that has important implications for fisheries management. Once salmon reach their home river, they probably use their senses of smell to find the tributary in which they were born. However, over long distances magnetism would be a more useful clue to salmon than odors because magnetism, unlike odors can be detected across thousand of miles of ocean.
Scientists don’t know exactly how early and how often salmon check Earth’s magnetic field in order to identify their geographic locations during their trip back home, but for salmon to be able to go from some location out in the middle of the Pacific 4,000 miles away, they need to make a correct migratory choice early and they need to know which direction to start going in. For that, they would presumably use the magnetic field. Salmon have to get it right because they only have one chance to make it back to their home river, so it makes sense they may have more than one way to get there. The magnetic field is amazingly consistent, so that is a strategy that can withstand the test of time. But, they may also use the sun as a compass, track waves breaking on the beach through infrasound, and the of use smell.
Putman plans to follow through with experiments on varying the magnetic field for salmon in a laboratory setting, using the Oregon Hatchery Research Center in Oregon’s Alsea River basin.