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Salmon May use Magnetic Maps to find their Way Home
2/10/2009
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Long-standing mystery yields to new research; Salmon and sea turtles share epic journeys and unerring homing instincts 
by Craig Weatherby 


Sea turtles and salmon rank near the top of nature's most impressive navigators.
 
No matter how far they journey, both possess the uncanny ability to find a natal needletheir birthplace, that isin a humongous oceanic haystack.
 
Salmon hatch in rivers, then migrate hundreds of miles out into the ocean before returning to their home river several years later to spawn.
 
Salmon return to the freshwater river of their origin to spawn, although some may stray to other nearby streams.
 
And some populations of sea turtles cross entire oceans and roam for more than a decade before returning to reproduce on their home beach.
 
The question of how marine animals migrate such vast distances to return to their own birthplacesometimes bypassing other suitable locations along the wayhas mystified scientists for more than a century.
 
Scientists speculate that natal homing evolved because individuals that returned to their home areas to reproduce left more offspring than those that tried to reproduce elsewhere.
 
Researchers have proven that once salmon get close to their birth river, a keen sense of smell guides the fish the rest of the way.
 
Sense of smell may explain how both creatures home in on their birth rivers and beaches.
 
But it has been unclear how they find their way close enough for smells or other aquatic cuessuch as to guide them on the final, short legs of their epic journeys.
 
Marine biologists at the University of North Carolina summarized the conundrum in a recent article, in which they propose a magnetic solution to the mystery:

“Salmon are known to use chemical cues to identify their home rivers at the end of spawning migrations. Such cues, however, do not extend far enough into the ocean to guide migratory movements that begin in open-sea locations hundreds or thousands of kilometers away.

“Similarly, how sea turtles reach their nesting areas from distant sites is unknown. However, both salmon and sea turtles detect the magnetic field of the Earth and use it as a directional cue” (Lohmann KJ et al. 2008).
 
Now, marine biologists at the University of North Carolina at Chapel Hill think they might have unraveled the secret of how salmon and sea turtles can get close enough to their ultimate goal for their “noses” to lead the rest of the way.
 
Magnetic explanation for homing accuracy
At the beginning of their lives, salmon and sea turtles may read the magnetic field of their home area and “imprint” on it, according to a new theory in the latest issue of the journal Proceedings of the National Academy of Sciences.
 
The Earth's magnetic field varies predictably across the globe, with every oceanic region having a slightly different magnetic signature.
 
By noting the unique “magnetic address” of their birthplace and remembering it, animals may be able to distinguish this location from all others when they are fully grown and ready to return years later, researchers propose.
 
As the authors wrote, “…we propose that salmon and sea turtles imprint on the magnetic field of their natal areas and later use this information to direct natal homing. This novel hypothesis provides the first plausible explanation for how marine animals can navigate to natal areas from distant oceanic locations” (Lohmann KJ et al. 2008).
 
Previous studies have shown that young salmon and sea turtles can detect the Earth’s magnetic field and use it to sense direction during their first migration away from their birthplace to the far-flung regions where they spend the initial years of their lives.
 
The new study seeks to explain the more difficult navigational task accomplished by adult animals that return to reproduce in the same area where they themselves began life, a process scientists refer to as natal homing.
 
According to lead author Kenneth Lohmann, Ph.D., “What we are proposing is that natal homing can be explained in terms of animals learning the unique magnetic signature of their home area early in life and then retaining that information. We hope that the paper will inspire discussion among scientists and eventually lead to a way of testing the idea.”
 
The hypothesis builds on previous studies with sea turtles by Lohmann and his team. In 2001, they showed that baby turtles use magnetic information to help guide them during their first migration across the Atlantic Ocean. And in 2004, they discovered that sea turtles several years older possess a more sophisticated magnetic map sense that helps them navigate to specific areas rich in food.
 
“For animals that require highly specific environmental conditions to reproduce, assessing the suitability of an unfamiliar area can be difficult and risky,” Lohmann said. “In effect, these animals seem to have hit on a strategy that if a natal site was good enough for them, then it will be good enough for their offspring” (UNC 2008).
 
The study notes that the Earth's magnetic field changes slightly over time and thus probably only helps animals arrive in the general region of their birthplace. Once an animal is close to the target, other senses, such as vision or smell, may be used to pinpoint specific reproductive sites. Salmon, for example, are known to use smell to locate spawning grounds once they have drawn near.
 
Lohmann said one problem making it difficult to test the new theory is the low survival rate of sea turtles. Only one out of about 4,000 baby sea turtles survives to adulthood and returns to its natal site to breed. A similarly small percentage of baby fish survive.
 
Lohmann also notes that if the theory is correct, it could lead to new ways of helping save sea turtles and salmon:
“Ideally, it might be possible to steer turtles to protected areas where we would like them to nest,” Lohmann said, noting the animals’ endangered status. “It might also be possible to use magnetic imprinting to help re-establish salmon populations in rivers where the original population has been wiped out” (UNC 2008).
 
 
Sources
  • Collin SP, Marshall JN. Sensory Processing in Aquatic Environments. SpringerNew York, 2003. DOI 10.1007/b97656
  • Lohmann KJ, Putman NF, Lohmann CM. Geomagnetic imprinting: A unifying hypothesis of long-distance natal homing in salmon and sea turtles. Proc Natl Acad Sci U S A. 2008 Dec 9;105(49):19096-101. Epub 2008 Dec 5. Review.
  • University of North Carolina at Chapel Hill (UNC). No place like home: New theory for how salmon, sea turtles find their birthplace. December 10, 2008. Accessed online at http://uncnews.unc.edu/news/science-and-technology/no-place-like-home-new-theory-for-how-salmon-sea-turtles-find-their-birthplace.html

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