There are two main types of eel – the Shortfin and the Longfin. There are fewer eels today because of the loss of wetlands and commercial fishing. Eels migrate up streams as elvers to find suitable adult habitat. After many years (15-30 years for shortfins, 25 years for longfins, and sometimes up to 80 years) they migrate to the Pacific Ocean to breed and die. Eels are secretive, mainly nocturnal and prefer habitats with plenty of cover.
For at least 65 million years, long-finned eels (Anquilla dieffenbachii) have been swimming up and down New Zealand’s waterways.The long-finned eel is one of the largest freshwater eels in the world and it is found only in the rivers and lakes of Aotearoa, New Zealand. Longfin eels are threatened fish.
About the Long-Finned Eel
Habitat
Long-finned eels can be found throughout New Zealand. They live mainly in rivers and inland lakes but can be found in almost all types of waters, usually well inland from the coast.
They are legendary climbers and have made their way well inland in most river systems, even those with natural barriers. Elvers (young eels) swimming up river will climb waterfalls and even dams by leaving the water and wriggling over damp areas. It is not unheard of for an eel to climb a waterfall of up to 20 metres.
Size
When eels begin life, they are a tiny one millimetre in length. During their life, they can grow up to two metres long.
Compared with many other fish, eels are slow growing - a long-fin may grow only between 15-25mm a year. They can also live for many years. Large long-fins have been estimated to be at least 60 years old.
The biggest eels are usually old females that have been slow to reach sexual maturity and, for reasons that are not yet understood, have not migrated to sea to breed.
The biggest long-finned eels reported have weighed as much as 40 kg. Pictures of fishers and huge eels used to appear regularly in local newspapers. But today, you’ll seldom find an eel heavier than 10 kg. Commercial fishing has meant that a big proportion of our very large eels have now disappeared.
Appearance
In shape, eels are elongate, slender-bodied fishes, almost tubular. When they are small, they have relatively smooth heads but as they grow the head becomes bulbous, with a prominent muscular dome behind the eyes.
They change shape again when they get ready to migrate to their breeding grounds. The head becomes much more slender and tapered, almost bullet-like and the eyes enlarge to up to twice their normal size.
Like all fish, eels have scales and fins. The long-finned eel is so named because its top (dorsal) fin is longer than its bottom fin.
While they have the appearance of being scaleless, tiny scales are embedded deeply within their thick, leathery skin. The eel’s skin is very sensitive to touch. This helps it to "see" in its watery environment.
In colour, long fins are usually dark brown to grey black. Very occasionally, long-finned eels found in the wild are partially or even wholly bright yellow in colour.
Food
Eels eat "live" food. Small long-finned eels living amongst the river gravels will feed on insect larvae, worms and water snails. When they get bigger, they begin to feed on fish. They will also eat fresh-water crayfish and even small birds like ducklings.
During the day, eels are secretive, hiding under logs and boulders or under riverbanks. Occasionally, they may be seen out hunting for food but most of their hunting takes place at night.
Eels hunt by smell rather than sight. Long-finned eels have a well-developed sense of smell. They have tube nostrils that protrude from the front of their head, above their upper lip.
They also have a very large mouth with rows of small, sharp, white teeth. The top teeth form an arrow shape on the roof of the eel’s mouth.
Breeding
Long-finned eels breed only once, at the end of their life. When they are ready to breed, they leave New Zealand and swim five thousand kilometres up into the tropical Pacific to spawn, probably in deep ocean trenches somewhere near Tonga.
When they reach their destination, the females lay millions of eggs that are fertilised by the male. The larvae are called leptocephalus and look nothing like an eel –they are transparent, flat, and leaf-shaped. The larvae reach New Zealand by drifting on ocean currents.
Before entering fresh water, the leptocephalus change into a more familiar eel shape, although they remain transparent for up to a week after leaving the sea. These tiny "glass" eels enter fresh water between July and November each year, often in very large numbers.
Eels take many years to grow and it could be decades before an individual is ready to undertake the long migration back to the tropics to breed. The average age at which a long-finned eel migrates is 23 years for a male and 34 for a female. The adults never return as they die after spawning.
Threats
While long-finned eels are still relatively common, fishing has had a significant impact on the species. Europeans showed little interest in eels as a fishery until the 1960s when commercial catches rose steadily.
In 1975, eels were the most valuable fish export after rock lobsters. Five years later, they were the fifth most valuable finfish export. This big increase in fishing effort led to significant stock reductions in some areas, with a marked decline in the average size of the eels caught.
Habitat loss also affects eels. Changes caused by hydro development, drainage and irrigation schemes and river diversions affect eels by reducing their habitat and the water available for aquatic life. Culverts and dams can also impact on eels by preventing their migration.
Eel habitat is also impacted by pollution. Sewage and effluent from meat works and pulp and paper plants discharged into rivers can remove large quantities of oxygen from the water. The result of this oxygen depletion is that the fish will either die or move away.
Eels on the Move
Hydro dams stop eels moving freely up and done some rivers. To get around this problem eel passes have been built across some dams to help the eels complete their journey. Young eels may also be moved across the dams by hand.