You see the eels that we know that are in our rivers, they’re known as yellow eels, and they're a sort of browny yellow in colour. And they are the stage that’s maturing from being a very young eel right up to the completely mature eel. And they just spend their time eating, and they’ll eat anything animal, fish, frogs, crustaceans, and as they grow bigger they take larger and larger fish and larger and larger crustaceans. They’ll lose their yellow brown colour and become silver eels, and it’s these ones that have the desperate urge to migrate to the sea. And they’ll even travel overland in order to get to the sea. But most of them end up migrating down the major rivers into the sea. And then they undertake the massive 5,500 kilometre swim all the way to the Sargasso Sea.
Because they leave Europe, in the autumn, they’ve sort of arrived at the Sargasso Sea by early spring, and that’s when they spawn. They lay their eggs, and the eggs hatch into these tiny little larvae called leptocephalis larvae. Imagine a tiny little fish seven millimetres long, it’s feeding on the various small animals in the zooplankton. Now the reason the Sargasso Sea is so rich in zooplankton is because it’s a huge deep pool, about 500 metres deep, right in the centre of what's effectively a slow whirlpool, because there's a continuous clockwise current flowing around the edge of the Sargasso Sea, and the huge upwelling of nutrients that happen there mean that there's large amounts of seaweeds growing and lots of animals feeding on them, a lot of plankton, so it’s really the ideal spot for an eel to be able to spawn and leave its offspring.
And then as the leptocephalis larvae grow they get caught up in the Gulf Stream, and they're carried along in that, and then they get into the North Atlantic Drift. So they're moving, they're moving eastwards right from the Sargasso Sea across the Atlantic Ocean, and it takes them, in total, about a year to reach the European coastlines. But over that time they’ve actually become transformed. They're gradually changing, so they lose their sort of flattened shape, so that by the time they reach the European coastline they’ve become more rounded in cross-section and they’ve become glass eels, and these are very tiny. And they reach the river estuaries, and there they spend quite a long time because they need to change their physiology completely from being a seawater fish into a freshwater fish, and that is a massive change and it takes time.
Because it means that, whereas in the sea, the fight, the physiological fight, is that salts are always going into the body of the fish, and the fish has these special cells that pump the salts back out into the seawater so maintaining the normal composition of the blood and the body fluids. But in freshwater it’s the other way around, the fish are now fighting the loss of salts because their body contains a lot more salts than freshwater. So now, in freshwater, the cells are now pumping salts back into the body. So we mustn't underestimate what a big change that is and that’s why it takes a while. As that’s happening the glass eels become transformed into elvers, and these are the small fish that actually undergo this huge migration up the rivers. So they get out of the estuary, into the river and they migrate upstream.
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