The European eel (Anguilla anguilla) is common in fresh water lochs, rivers and streams all over Harris and in the slightly salty waters of river mouths and coastal lochs where seawater and freshwater mix.
This amazing species leads a long and adventurous life. The longest eel ever caught was a whopping 133 cm (that’s as high as P3 children). The oldest was 84 years old.
Adult European Eel (Photo: Wikipedia)
Life cycle and habitat
Eggs are released at sea
Like salmon and sea trout eels move between fresh waters and the sea at different times in their life. But, unlike salmon and trout, eels produce their eggs in the sea not rivers. Eels can swim over 4000 miles from Harris to the Sargasso Sea near Bermuda to spawn. Eels from all over Europe meet here, deep in the ocean, to spawn. A female eel can lay ten million eggs in one go! Female fish release eggs into the water and nearby males release sperm or 'milt'. The milt fertilizes the eggs and eels start to grow inside. After they have spawned the adult eels are tired out and die.
Lots about the life of adult eels at sea is a mystery. Because the eels may swim down to 300 meters in huge oceans finding them is very, very hard. We know more about young eels, (called larvae), as there are many millions of these and they drift nearer to the top of the sea where they can be caught in nets by scientists on boats. Eels hatch from the eggs after just a few days......
Floating Back to Harris
When they first hatch eel larvae are leaf shaped and known as 'leptocephalus'. Their shape helps them to drift along with the ocean currents which they ride all the way back to Harris.
The larvae’s long journey back to Harris takes almost a year. By the time they get near to the shores of Harris our eel larvae have changed (metamorphosed) to look like mini adults, except that they are see through (transparent).
Glass Eels (Photo: Wikipedia)
They soon become pigmented like the adults and then they are called elvers.
Young eels are masters of disguise and change colour twice again before becoming adults.
When they reach Harris eels surf up rivers on high tides and then live in places where water becomes less salty. As they grow eels often move higher up rivers, this is why the biggest eels in Harris are usually found in high lochs and burns while small eels are found in rivers closer to the sea. If you look carefully you can spot eels moving around small waterfalls and even over wet grass or heather. Some eels never move totally into freshwater, instead they keep swimming between the sea and river mouths but amazingly, others have been found living over 600 m above sea level, (this is about the height of Ullaval in North Harris).
Life in Freshwater
In rivers eels swim at night (this is called being nocturnal) and feed on animals like freshwater snails and flat worms and small fish. There are not many little bugs in lochs and burns high in the Harris hills and eels there eat more fish than those in rivers closer to the sea.
Because they live in cold water and have small amounts of food our eels grow much slower than those in hot places such as France. This means that they have to stay in fresh water for many years before they have enough fat to go to sea. In the colder, higher burns of Harris some eels even take a long sleep through the winter.
Return to Sea
Eels are between 50 and 100 cm long when they go back to sea and between 7 and 87 years old. Female eels stay in freshwaters for longer than males and grow bigger. When they are getting ready to go back to sea eels turn silver, their eyes get bigger to help them see better in the deep, dark ocean and their front fins get much bigger to help them swim a long way.
On dark summer nights when there is no moon silver eels migrate down Harris rivers. They choose dark nights so that they do not get eaten by otters and big birds. They want to reach the sea so much that they even slither over short stretches of moorland just like snakes. We don’t know for sure how eels find their way back to the Sargasso Sea (or if all eels go to the Sargasso, perhaps there are other spawning grounds) but they swim deep in the water and probably detect magnetic currents which help them go in the right direction, like a compass.
By measuring how fast eels in tanks can swim biologists have calculated that it would take eels about six months to complete their long journey to the Sargasso Sea.
The importance of Eels in Harris
Eels are an important species in the freshwaters of Harris and their healthy numbers here may help to save eels in other countries.
The freshwaters of Harris have low numbers and less variety of fish in them compared to rivers on the mainland. This means that eels are a more important food for cormorants, herons, sea gulls, ducks and otters here in Harris than they would be in places with lots of different fish.
Some anglers do not like eels because eels eat small trout and salmon. However, eels are a favourite food for otters and herons which would otherwise eat more big trout and salmon, so eels can be good for fishing. In some parts of the world people like to eat eels but this is not common on Harris.
In many countries of Europe eels have almost disappeared in the last 30 years and now eels might disappear for ever. One reason for this could be that thousands of millions of little glass-eels used to be caught in France, Spain and Italy for people to eat. Other reasons why eels may have disappeared are pollution of streams with nasty chemicals and sewage, damming of rivers so that eels cannot move up them, and the spread of new eel illnesses.
Happily eels have not disappeared from Harris or Scotland. Eels from all over Europe come together to spawn so our eels could produce the elvers, (young), that are urgently needed in rivers as far away as Morocco in Africa.