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Wednesday, May 12, 2021

Where do salmons go to spawn

Their inbuilt magnetic compass and remarkable olfactory memory help the fish to navigate freshwater upstream till they reach the place of their birth

Written by Ranjit Lal |
April 11, 2021 6:30:06 am
Salmon are born in freshwater streams where they will remain safe from predators as they grow.

I LOVE pan-seared salmon with lemon-butter-wine sauce and, on innumerable occasions, have watched, on television, hulking great grizzly bears stuff their faces with the delicious fish till they’re bursting — and then fussily pick only at the brains and fat — leaving most of the rest. Of course, it doesn’t go to waste — gulls and other scavengers clean up and what’s left of the carcass returns valuable nutrients to the soil, enabling the trees nearby to grow tall and strong. Invariably, these salmon have been caught in “salmon runs” — when they return “home” to spawn and die towards the end of their lives, leaping clear of the rapids they encounter on their way upstream.

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What zaps you is the fact that these fish (which, one has never considered to be overburdened with brains) are returning to the very same streams where they were born — after spending two to eight years, living it up in the big wild ocean, either the Pacific or Atlantic. Talk about “ghar wapsi”! But why?

Salmon are born in freshwater streams where they will remain safe from predators as they grow. At this time of their lives, they’re called “smolts” and their education involves imprinting themselves on the smells of their streams: the rotting vegetation in the water, the fish and insects, the dust and mud. Once they grow, the desire to “leave home” (as is among teenagers) increases, their bodies change, making them suitable for life in the salty seas. Because their parents died before they were born, they continue to imbibe and memorise, on their own, the smells they encounter en route to the sea.

At the ocean’s mouth, they make use of the most important inheritance from their parents: a clear, built-in sense of the Earth’s magnetic field at that place. In their brains, they have magnetite — iron particles sensitive to the Earth’s magnetic field — and they use this sense to navigate the vast featureless oceans where they will swim and swirl in schools (and, hopefully, not get caught or snapped up by predators), eating and getting fatter, for the next seven or eight years.

But then, like most of us, they’ll feel the urge to “settle down” and start a family, leaving hundreds of fertilised eggs strewn in the very same nursery beds where they were born and grew up. The magnetic compass now guides them back to where their natal stream joined the ocean and, now, their remarkable olfactory sense kicks in: they can pick out a single familiar (or not) molecule in the water from a million (even a billion) parts of water. The old olfactory memories come flooding, correcting their course if it fades, playing something like that children’s game, “Hot and Cold”. And when they know they are home, the females will spawn, the males will fertilise the scattered eggs, and shortly after, both will die.

So, why do they return to their old nurseries? Why not spawn in any freshwater stream? Because they know there’s no place like home, where they survived and thrived, and so will their progeny. If, however, there are new strange smells in the water, say, from pesticide run-offs, or fertilisers, or pollution, or acidification, it may cause them to pause and even turn back in confusion. Salmon fishermen will never wash their hands upstream in the water, when the salmon are swimming towards them, knowing it could spook them. Therein lies the need to keep freshwater streams pollution-free.

Other famous oceanic creatures renowned for their sense of direction are sea turtles. It’s the lady turtle that returns to the same beach where she was hatched, perhaps decades earlier, to lay and bury her eggs in the sand. It is believed, they know that on these beaches they are immune to the parasites present (because they survived!), and so want their young ones to have the same immunity. Baby turtles break out of their eggs at night, struggle to the surface and then instinctively make hell for leather towards the sea — drawn by the reflected light of the moon and stars.

Many get confused by the lights onshore — from hotels, houses, street lamps — and turn around. But those who do make it swim straight out, and now, their genetic inheritance, a built-in compass, helps guide them around the ocean currents. We’re still not sure exactly how they plot their routes but the angle and intensity of the earth’s magnetic field appears to be used.

Most of us would love to revisit our childhood homes (especially if we’ve had a normal, happy childhood) but most will return bitterly disappointed because the place no longer looks like what we remember. It’s much the same with salmon and sea turtles.

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