The Great Salmon Migration

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?

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.

Salmon leaping.

The crux of these alterations is that though the Pacific salmon are marine animals, they return to the very river system they were born in to spawn, and are able to tolerate the freshwater conditions. The overall journey may span thousands of miles. Out of the five migratory species of Pacific salmon, it is the sockeye which travels the farthest, migrating from the west coast of North America as far as northern Russia and southern Japan!

Leaping salmon.

After spending up to five years feasting in the bountiful water of the Pacific, the adult salmon return to the coast. The method by which they discern their own birthing-places is still something of a mystery, though their accuracy in re-locating these sites is impressive.

It is thought that they rely on their vision for much of the trip, though experiments with (temporarily!) chemically-blinded salmon have shown that they utilise their sense of smell to pinpoint the exact river system. Many salmon will die on this journey, and those that make it often arrive battered and bruised. They must travel against the flow of water for the entire journey, and this includes travelling up waterfalls – something which has been the subject of many a stirring picture.

Salmon leaping. All rights reserved, used with permission.

After spawning, their purpose fulfilled, they die where they were born. The ability to live in two differing conditions is energetically very expensive, as is the ability to locate a specific spot across many thousands of miles. So what's the advantage? How could such a complex and dangerous system have come about?

From what we know about evolution, it may be supposed that because a salmon is born at a particular spot, that spot will also be an ideal one for it to spawn its own young and be assured that they will survive (not that I reckon a salmon can be 'assured'). Though the trip is risky, it might perhaps be more risky at a population level if individuals began selecting other, potentially less stable areas in which to spawn.

Alaska, the final destination for many Pacific salmon.

There are a few grey areas with evolution: either a system works and the young survive to pass on the trait, or it doesn't and the trait hits the wall. Clearly the great salmon migration, though perhaps not the most efficient system, does the trick admirably.

Sources: 1, 2, 3

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