How do salmon find their way home?

THE summer months see the gradual return of the majestic Atlantic salmon to rivers around Ireland after their arduous homeward migrations from far-flung oceanic feeding grounds.

The incredible life cycle of the salmon begins in shallow freshwater streams in November or December when the female digs a spawning nest (a ‘redd’) in the streambed with her tail, into which she deposits several thousand eggs to be fertilised by one or several males. She then covers the fertilised eggs with gravel and the young are on their own from there on.

The tiny fry emerge the following spring and quickly establish feeding territories within the stream. By late summer they have developed into par, with vertical stripes for camouflage. The parr feed on aquatic insects and after one to three years (typically two in Ireland) theyey undergo a process known as ‘smoltification’: they lose their parr marks, become silvery, begin swimming with the current, instead of against it, and acquire the ability to survive in seawater. The ocean now calls.

Smolts leaving Irish rivers migrate with the North Atlantic drift towards rich feeding grounds in the Norwegian Sea, with some eventually moving further west towards Greenland. Along the way they are joined by salmon originating from rivers in Spain, France, the UK and Scandinavia and may even mix with salmon from Canadian and US rivers.

Some salmon, known as grilse, return to freshwater in summer after only a single winter of feeding at sea, while others may spend two, occasionally even three, winters at sea. In Ireland, these older, larger fish (sometimes weighing more than 10kg) typically return to the rivers in the spring and are highly prized byy anglers.

Once they re-- enter freshwater theyy cease feeding, so why leave the bountiful ocean so early if you don’t need to spawn till much later in the year? Recent research by our group shows that rivers with large lakes tend to support more spring-migrating salmon, most likely because lakes provide a haven from predators where salmon can hide in the cooler layers.

Salmon are renowned for ‘ homing’ to their natal rivers, often to within metres of where they were born. Of the initial few thousand eggs produced by a female, only a few fish may survive to spawning age, and these then still have to run the gauntlet of fishermen and poachers, leap waterfalls and scale dams before reaching their final destination.

How do these lucky few find their way back home? The exact mechanisms remain something of a mystery, but decades of research suggest they can use the Earth’s magnetic field and possibly even the Sun and stars to navigate in the ocean.

As they get closer to home, they recognise and orientate towards the unique smell of their home river. Crafty experiments show that young salmon learn the chemical signatures associated with their home stream and lower parts of the river systems through which they migrate to the ocean. They later use these retained ‘odour memories’ to guide their return migration to natal areas, a process known as sequential imprinting. The migratory instincts and homing behaviours of salmon certainly also have a strong genetic basis.

Some scientists suggest that returning salmon may be able to recognise the smells of other fish originating from the same population, forming shoals together in the ocean and then co-migrating to the shared home stream.

We may never fully understand the exact mechanisms by which these ‘King of Fish’ find their way home after their remarkable oceanic migrations, but if we knew all the answers then science would cease to be any fun!