Animal globetrotters IV

Fish, reptiles and amphibians that migrate

After a break of one week, due to unavoidable circumstances, we bring you today the final article in the series

 on the fantastic long-distance journeys made by our animal friends. Having dealt with birds, mammals and insects, we will now examine how and why fish, reptiles and amphibians migrate...

Many types of fish and amphibians migrate for feeding and breeding purposes. They do so on time scales ranging from daily to annual, and distances ranging from a few metres to thousands of kilometres.

Both freshwater and salt water fish migrate. Fish that travel between salt water and fresh water are grouped as diadromous. Those that live in the sea, but breed in fresh water are called anadromous.

And fish that live in fresh water and breed in sea water are catadromous. Amphidromous fish move between fresh water and salt water during some part of their life cycle. However, they do not migrate for breeding purposes. Fish that migrate within fresh water only are potamodrous and those that migrate within salt water only are oceanodromous.

The most famous anadromous fish is the salmon. These fish undertake lengthy and arduous (difficult) journeys from the ocean where they spend their adult lives, back to their birth place, which may be fresh water streams or rivers.

Some of the potamodrous fish, migrating from lake to river and back again, include species such as barbels, catfishes, carp, minnows and North America's garpike.

Atlantic salmon

The Atlantic salmon begins its life in a river in the wild mountain streams of Scotland or Norway. After hatching from an egg in the spring, it usually spends about four years in the river. Then, as an adult, it commences its outward marathon migration.

At this stage, it is referred to as a Smolt; a young salmon, ready to migrate to the sea. It travels down river to

 the ocean, and during this time, the smolt's physiology adapts (changes) to enable it to live in the great ocean. Even in the ocean, it spends four years travelling and maturing before it undertakes its return journey as an adult salmon.

What's amazing is that it retraces its path upstream to the very river in which it hatched about eight years ago! How does it manage to do this, you may wonder. The salmon is able to get back to its birthplace due to its highly developed olfactory sense. It enables the salmon to distinguish (recognise) between different rivers purely by scent.

Now, isn't that amazing?

The journey the salmon undertakes is not very easy. It's really a case of 'survival of the fittest,' because in order to get to its birth place, it not only has to swim against the tide up stream, but also leap over waterfalls as well as all other natural and man-made obstacles.

It is a long, exhausting journey and unless the salmon is a strong specimen, it cannot make this journey. In fact, this migration process helps to weed out the weakest fish and ensure that only the healthy ones survive to pass on their genes to the next generation.

Two other species of fish that migrate are eels and tuna. But many other species of fish too are masters of migration.

European and North American eels

Most species of eels always live in the sea, but certain species such as the European eel, mature in fresh water and then migrate to the sea to spawn.

The European eel begins its life in the Sargrasso Sea of the North Atlantic. It hatches from its egg as a 10 mm long larval form known as leptocephalus. As the larva grows larger, it drifts farther out from its birth place,

 across the Atlantic by the Gulf Stream.

It reaches the European coast three years later.Entering estuaries, it then metamorphoses(changes) into the familiar 'glass eel' stage. It is after this stage that the eel begins its upstream journey, not on its own, but in vast hordes.

In its outward-bound journey seeking rivers in which to live, the eel covers a distance of up to 5,000 km (3,100 miles). Once the eel finds the river to settle in, it lives there for a number of years, until it transforms into a fully mature, adult eel.

Then it is ready to make the journey downstream into the sea again, although not much sightings of this has been recorded. However, there are many reports of downStream journeys made by its North American counterpart.

Once the North American eel reaches the eastern coast of North America, it starts the final phase of its migration; it heads for the Sargrasso Sea, where it spawns and thereby completes its life cycle. The European eels move across the Atlantic on the Gulf Stream.

Like the salmon, the eel too is highly sensitive to olfactory stimuli. However, it's not the only thing that helps it in its long-distance journey, to complete its life cycle.

It uses many environmental cues to navigate, such as fluctuations in water movements, seismic activity and also the tiniest of electrical fields generated by water currents. During larvae stage, eels are also helped to drift passively by ocean currents.

Bluefin tunas

As the tuna does not leave the ocean during migration, it is a oceandramous fish. But, you may be surprised to learn that the tuna makes a longer journey than the two masters of migration we mentioned before; the salmon and eel. The bluefin tuna swims north from its breeding grounds in search of prey, covering up to 10,000 km (6200 miles), sometimes.

Reptiles and amphibians

Among the reptiles and amphibians that migrate are sea turtles, frogs and crabs. But not all their journeys are long-distance ones. For instance, frogs may migrate from one pond to another, rarely travelling more than a couple of miles.

Some aquatic animals however are more adventurous and undertake incredible journeys across entire oceans.

The sea turtles, especially the Loggerheads and Green sea turtles are most notable among them. Loggerheads hatch in Japan and spend the next two years swimming about 10,000 km across the Pacific Ocean to their feeding ground in Mexico's Baja California.

After about five years, the turtles return to Japan. The Green sea turtles that live off the coast of Brazil migrate to Ascension Island (located off the coast of Brazil, 2,250 km away), to mate and lay eggs. The adults later return to Brazil, leaving the young to incubate and find their way home, alone!

One of the most abundant reptiles on Earth, are sea snakes and the yellow-bellied sea snake is believed to migrate. Even the land crabs of Christmas Island undertake amazing mass migration. Check out their fantastic journeys.