Through the oceans
A look at underwater migration stories.
Migration under the sea
Some deep ocean fish migrate daily. They swim up to the surface to feed at night and then return to the depths by day. Dolphins migrate north and south each year. They navigate by using echolocation to recognise the shape of the seabed and the coastline – just like bats do in the air.
Even some lobsters migrate. The spiny lobster spends the summer in the warm coastal shallows of California, where it lays its eggs. In October, as the shallows get colder, it migrates towards the warmer waters of the deeper ocean. Hundreds walk in single file, clasping pincers to form a living chain of lobsters.
The journey of a whale
Baleen whales mostly feed on tiny marine creatures called plankton. They swallow thousands of them by sieving great gulps of water through the special fleshy filters called baleen that they have inside their mouths.
During summer, plankton concentrates in the waters of the Arctic and Antarctic. The whales gather here and feed non-stop, putting on mountains of fat, called blubber. Then, in autumn, they migrate to warmer tropical waters to breed. Here they have no need to feed, but live off their blubber.
Each species of whale has its own migration route. Grey whales spend the summer in the Arctic Ocean, feeding from June to October. In winter, when the ocean freezes, they migrate south down the coast of America towards the warm waters of the equator.
Here, during February, the females give birth. A young whale (or calf) suckles its mother’s milk to grow strong enough for the long journey north again in summer. Each year it migrates with its mother, and by the time it is mature – at 12 years – it may already have travelled more than 100,000 kilometres. That’s two-and-a-half times around the world.
About freshwater eels
Young eels (called elvers) hatch from eggs in a deep part of the western Atlantic Ocean called the Sargasso sea.
The young eels migrate back across the Atlantic towards the coast of the UK, drifting in the warm waters of the Gulf Stream. This journey takes about two years. By the time they arrive they are about 5cm long.
They gather in large river mouths like the Severn Estuary, and then head upriver in search of a new freshwater habitat. The urge to migrate is so strong that eels sometimes leave the water to wriggle their way overland.
The eels spend about 12 years inland, getting bigger all the time. Once the males are 36cm and the females 46cm long, they are mature enough to breed. In July, they return to the sea and begin the long journey back to their breeding grounds. When they reach the warm waters of the Sargasso sea, they lay their eggs and die.
Sea turtles travel thousands of kilometres through the oceans, following the currents and navigating using the earth’s magnetic field, but they nearly always return to breed on the beaches where they were born.
In spring, male and female turtles meet in the shallows for courtship and mating. Then, on a dark, moonless night, the females come ashore to lay their eggs. They use their flippers to dig a pit in the dry sand on the beach and lay their eggs in it, hidden from predators and safe from the waves.
They then shovel sand over the eggs until nothing can be seen and crawl back to the sea and swim away. The whole process takes about two hours. Each female lays three to four clutches, at 12–14 day intervals, each containing more than 100 eggs. She then doesn’t return to lay eggs again for at least another three years.
When the eggs hatch, 56 days later, the babies dig their way out of the sand at night and hurry down the beach to the sea. By day, predators such as crabs, dogs and vultures are waiting to snap them up. Many babies never even reach the ocean, and fewer than 3 per cent survive to become mature adults. But those that do can live for more than 50 years.
The fight of migrating salmons
Female salmon lay their eggs upstream. When the eggs hatch, the young salmon (called fry) drift slowly downstream. Most die, but the survivors gradually get bigger. Once they reach the sea, after about two years, they set off on a journey of thousands of kilometres to reach their ocean feeding grounds. Here they continue to grow, doubling their weight each year as they feed on smaller fish.
After about four years, when the salmon have grown into strong adult fish, weighing 10 kg or more. They swim back across the ocean to the mouth of the river where they were born. Here they wait for a strong enough water flow, before heading upriver.
By now, the males are sporting their bright breeding colours. Nothing but death can stop the salmon migrating upstream; they will fight the fiercest currents and even jump up waterfalls.
By October, the adult salmon have reached the place where they were born. Here they mate and the females lay their eggs in a shallow scrape in the riverbed. This is called spawning. After spawning, 95 per cent of all adults die. But those that survive head back out to sea to start their journey all over again. Meanwhile, in spring, their eggs hatch and a new generation of salmon prepares to start the great adventure.
The epic journey of the salmon also causes other animals to migrate. In Alaska, grizzly bears and bald eagles gather each year at salmon spawning sites upriver. The huge numbers of exhausted and dying fish provide a welcome feast for these big predators. The bears eat as much as they can in preparation for their winter hibernation.