I have memories of the tiny Leith River (just a few metres wide) that runs through Otago University in Dunedin (New Zealand) being ‘chokka’ with spawning salmon. This run by the salmon to their place of birth has always interested me…
How do they do it? How do they head home from vast oceanic expanses often thousands of kilometres away from the small shallow mountain stream eddy from where they hatched, and drew their first gulp of oxygenated water?
Well according to a study published this week in Current Biology, it turns out that they [the salmon] do it not too dissimilarly from us. I mean if you were in Sri Lanka and wanted to return home to the western hemisphere for Christmas how would you get home?
Exactly – well almost. Perhaps if the image above was a tall ship, we’d be bang on. Anyway we utilise the predictability of the Earth’s magnetic field to align our compass. Well it turns out that our friends the salmon use the very same homing strategy.
The aforementioned Oregon State University study examined 56 years of fisheries data documenting the return of sockeye salmon to the Fraser River in British Columbia (BC) – and the route they chose around Vancouver Island showed a correlation with changes in the intensity of the geomagnetic field.
According to Nathan Putman, a post-doctoral researcher and lead author on the study:
“What we think happens is that when salmon leave the river system as juveniles and enter the ocean, they imprint the magnetic field – logging it in as a waypoint. It serves as a proxy for geographic location when they return as adults. It gets them close to their river system and then other, finer cues may take over.”
Given that the Earth has a predictable, consistent geomagnetic field that weakens as you move from the poles toward the equator (i.e. the North Pole has a magnetic intensity gradient of roughly 58 microtesla, while the equator has one of about 24 microtesla). So salmon originating from Oregon that have spent two to four years in the northern Pacific Ocean off Canada or Alaska would return as adults, the scientists speculate, journeying southward off the coast until they reached a magnetic field intensity similar to that of their youth. Putman reckons:
“That should get them to within 50 to 100 km of their own river system and then olfactory cues or some other sense kicks in”
The study also showed that the “drift” of the geomagnetic field correlated with which route the salmon chose. When the normal intensity level of a river shifts north, the salmon are more likely to choose a northern route for their return home. Likewise when the field shifted slightly south, they chose a southern route.
However Putman also pointed out that variations in sea surface temperature was also a major factor for the choice of route:
This “field drift” accounted for about 16 per cent of the variation in the migration route, Putman said, while variations in sea surface temperatures accounted for 22 per cent. The interactive effect between these two variables accounted for an additional 28 per cent of the variation in the migration route.
Salmon are a cold-water fish, and all things being equal, they prefer cold water. But the fact that they also demonstrate geomagnetic fidelity in choosing a route shows that this could be a major instrument in their biological toolbox to guide their way home.
- Salmon May Use Magnetic Field As Navigational Aid (albanytribune.com)
- How salmon make long journey home (bbc.co.uk)