Ray Mullet's expert analysis

• European eel
• Salmon
• Mouth-brooding cichlid
• Sea horse

The European eel was for a long time regarded as a very mysterious animal. Until quite recently no one had even seen a baby eel. People had all sorts of weird ideas about where eels came from. The ancient Greeks thought that eels had some strange way of reproducing, like scraping off pieces of their skin that then turned into adult eels.

We now know that baby eels are born in or near the Sargasso Sea in the western Atlantic Ocean - far from where we might expect to find them. When they hatch, the young fry do not look like eels, but like tiny curled up leaves, so it is not surprising that people thought they had never seen a young eel. At this stage they are called leptocephalii (you can see a leptocephalus in the figure on the right).

They then drift across the ocean to Europe, 5600 km (3480 miles) away, swept by Gulf Stream and the North Atlantic Drift. During the journey across the Atlantic they develop into elvers or glass eels, named because they are almost transparent.

When they reach the European coast the tiny eels swim up rivers, feeding and growing for the next 6-20 years. Once they achieve maturity, the adult yellow eel as they are now called, make their way back downriver, helped by the current. Their instinct to reach the sea is very strong. On wet nights the eels will even travel overland in the wet undergrowth.

At this stage they look quite different to the young eels that entered the river several years ago - their eyes are bigger, their heads wider and their bellies have turned silver or bronze (and so are sometimes called silver eels).

When they reach the sea the eels begin the very long, and still rather mysterious, journey back to the breeding grounds of the Sargasso Sea.

Fish that spend most of their lives in freshwater but migrate down river to spawn at sea are called catadromous.

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European eel
(click image for a larger version)
© The Environment Agency


Leptocephalus

Salmon
(click image for a larger version)
© Erling Svenson


Salmon swimming upriver
© The Environment Agency

Like the eel the salmon travels great distances to spawn, up to 4830km (3000 miles). The adult salmon leave their home in the sea, and travel back to the very river where they were born - even if this river is thousands of miles away. We don't fully understand how the adult salmon find their way across such great distances - they may navigate like birds, using the sun or stars. We do know that, as they get closer to the coastline, they use scent to find the right river - smelling for the special cocktail of soil, vegetation and bedrock.

The journey upriver is also very difficult for the salmon. They swim against the current, often as much as 1,125km (700 miles), sometimes covering 48km (30 miles) a day. They may often leap waterfalls several metres high and fight their way up numerous shallow rapids.

Once they reach the place of their birth, in the upper reaches of the river, the salmon breed. The female salmon scoops out a shallow nest, called a redd, in the gravel on the riverbed, into which she lays her eggs. The male salmon fertilises the eggs at the same time. The eggs are sticky, and will attach themselves to the gravel at the bottom of the nest. The eggs are gently covered with gravel to keep them safe, then the parent salmon (now called spent kelts) leave to try to return to the sea. Many will die on the way, weakened by their efforts to spawn. Those that survive, called mended kelts, often spend up to 18 months feeding before fully recovering.

There are eleven different species of salmon. The Atlantic salmon will normally spawn two or three times in its life, returning to the same spot each time. However, the Pacific salmon only breeds once, dying in huge numbers shortly after the eggs are laid, and providing a seasonal feast for wildlife such as bears and birds.

The newly hatched baby salmon stay in the river, feeding on insect larvae, until they reach about 13cm (5") long, when they become known as parr. At this point they have special black, oval markings. If there is plenty of food available, they can lose those markings in their second year but sometimes this can take up to four years. They then become known as smolts and the dark oval markings are then replaced with a silvery, shiny skin. After a further year or two they are ready to migrate to the sea for the first time, where they feed and grow rapidly into adults, before the whole cycle is begun again.

Fish such as salmon are anadromous, that is, they spend most of their lives in the ocean but migrate into freshwater to spawn.

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Cichlids (pronounced sick-lids) are a large family of freshwater fish. In the wild they are found in South and Central America, north to Texas, Africa, Madagascar, Asia Minor and coastal regions of India and Sri Lanka. However, they are so easy to look after that they are often captive bred and used for food, and have accidentally populated rivers worldwide, often displacing the native species. They are also popular with aquarists (fish keepers). Some cichlids can grow to over 60cm (48 ins) long.

This particular fish - Oreochromis niloticus - was originally found in the shallows of most major Northern and Central African river systems. It is quite a hardy fish - surviving varying temperatures, salinity and water levels. They have even been found alive and well in sewers!

The male of this cichlid does not travel very far to breed. Instead he displays to attract a female to the centre of his territory - a shallow spawning pit, about twice his own size. This species is quite aggressive and will defend his space against other males of the same species, and even males that just look similar! Once a female has been attracted to the pit, and has laid her eggs, she gently collects them up in her mouth where the male then fertilises them. Once his job is done the male chases the female away and tries to mate with another willing female.

A large female can produce up to 2000 eggs that she keeps in her mouth, safe from predators for 5-7 days - she is described as a 'mouthbrooder'. She carefully 'chews' the eggs to keep them clean and well oxygenated, until they hatch. She cannot feed at this time so it is just as well the eggs only take a few days to hatch. The female protects the fry for another 5-7 days after hatching, during which time they leave her mouth to feed. If threatened by predators the young all rush back to her mouth, which becomes more and more crowded as they grow.

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Cichlid
(click image for a larger version)
© Below Water

Sea horse anatomy


Seahorse
(click image for a larger version)
© Below Water

Sea horses are not very strong swimmers, and only travel a few hundred metres from their home base. As a result they don't have to travel very far to breed. However, the sea horse is very unusual as it is one of the few animals where it is the male that carries and gives birth to the young.

Following a courtship display that lasts several hours, during which both partners change colour to reflect their mood, the female puts her eggs in a special pouch on the belly of the male. As many as 2000 eggs are fertilised and develop there. The eggs also hatch in the pouch, where the father cares for them, gently altering the salinity of the water to prepare the babies for life outside.

When it is time for the baby seahorses to be born the male gently forces them out of a special opening in his pouch. This can take a few hours and seems to be quite painful for the father. He gives his young no more attention once they are born, and many soon become prey for other fish. It is estimated that just 5 out of every 1000 seahorses make it to adulthood. This seems a small figure but is often better than most other fish, suggesting that keeping the young in a pouch increases their chance of survival.

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