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Image: Anita Scharf

Diadromous is not a word one hears every day, but this word is in fact the key to one of nature’s most startling migrations – that of the Pacific salmon. From the warm climes of the northern Sunshine State (California, that is) and the cool coniferous forests of British Columbia, to the chilling waters of the Bering Strait, these fish yearly perform migrations that cover thousands of miles.


Image: Jeff Hsu

Sockeye salmon.

The key to these wanderings is the ability of salmon species to function both in freshwater and seawater: diadromy. Ordinarily, animals are quite heavily specialised towards living in either one of these habitats. The physiological requirements are quite different for both, with vastly different demands made on the animal’s body. Freshwater animals are constantly trying to keep salt in their body and water out, while for seawater animals, the opposite is the case. So how do the salmon do it?

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Image: Pacific Northwest National Lab

Chinook salmon.

Usually as they travel through estuaries (which act as a kind of half-way house between the two habitats), Pacific salmon begin to make changes to the way in which their bodies function – especially in how much water they intake and release as urine. But it’s not all behavioural: special gill chloride cells help to correctly regulate the balance between salt and water in their bodies. Many species (such as the sockeye salmon) lose their distinctive red colour as they head out to sea.

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