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Eels

by  Paddy Ryan

Where do freshwater eels come from? Aristotle claimed they sprang straight from river mud, and some people thought they grew from bits of skin, or even horsehair. In 1896 an Italian discovered that a tiny ‘saltwater fish’ was in fact the larva of the freshwater eel. We are gradually learning more about their remarkable life journey, which begins and ends in the ocean.


Overview: features and distribution

Freshwater eels are fish, and belong to the family Anguillidae. Slow growing and long lived, they begin life in the sea, and then spend many years in fresh water as adults. Finally they return to sea to spawn, after which they die.

Do they bite?

Many people are scared of eels, because they are snakelike and slimy, and can slither over land. There are very few reports of eels attacking, but if they do, their teeth can grip. In one incident a longfin eel bit the wetsuit of a diver, who had to use a knife to release its hold.

Features

Unlike nearly all other freshwater fish, eels have a long cylindrical shape, and continuous dorsal (back), caudal (tail) and anal fins. They have pectoral (side) fins but no pelvic fins.

Covered in a layer of mucus, eels are extremely slippery. Although they seem to lack scales, under the microscope you can see a mosaic of tiny scales in the leathery, slimy skin.

Eels can travel over land, slithering through wet grass to get to a pond or lake. As long as their skin stays moist they can absorb oxygen through it, surviving long periods out of water.

Albino eels

Occasionally people find an albino (yellow or white) eel. These ones don’t have the usual dark colouring on their backs for camouflage, and the yellow or white pigment shows up clearly in the water. They often don’t last long in the wild, as birds overhead spot them easily.

Distribution

Freshwater eels are found worldwide. Of 15 recognised species, most occur in the waterways that flow into western Pacific and Indian oceans. There are none that spawn in the eastern Pacific or the South Atlantic.


New Zealand eels

Eels are New Zealand’s top native freshwater predators – no other species prey on them when they are adult. Māori identified many different varieties, mainly by colour. But when European settlers first arrived in New Zealand there was debate about the number of species.

There are three species, all from the Anguillidae family of fish:

  • longfin eel (Anguilla dieffenbachii), found further inland
  • shortfin eel (A. australis), found on the coast
  • spotted eel (A. reinhardtii), which may have recently arrived from Australia and is found only in northern rivers.

The longfin and shortfin eels are found all around New Zealand. They have very varied coloration, so this is not the best way to identify them. The most reliable distinguishing feature is the length of the back fin – hence their names.

Slippery catch

Eels were important food for Māori, and it took effort and ingenuity to catch them. Traps, nets and other devices were often complex and sophisticated. In the 19th and 20th centuries, many New Zealand children went eeling, and catching one was almost a rite of passage.

Longfin eel

The longfin eel is found only in New Zealand. Travelling far inland, it may be the country’s most widely distributed freshwater fish. The pectoral (side) fins have 16–20 bony rays. This species is usually dark brown to grey-black on the back.

One study found that males swim out to sea to breed and die at an average age of 23 years and females at an average age of 34.

They are probably the world’s biggest eels. They may grow up to 1.75 metres, and the biggest caught and measured so far weighed 24 kilograms. Any New Zealand eel over 1 metre long and caught inland is probably a longfin specimen.

Shortfin eel

The shortfin eel is quite widely distributed in the South Pacific, and occurs in Australia, New Caledonia, Lord Howe Island, Norfolk Island, Fiji and possibly Tahiti.

Its dorsal fin starts close to the anal fin, and the pectoral fin has 14–16 rays. This eel is olive green and frequents lowland waterways. It can grow to 1 metre and 3.5 kilograms.

Males mature, breed and die at 14 years, and females at 22.

Spotted eel

The spotted eel also occurs in eastern Australia, New Caledonia and Lord Howe Island. It was first confirmed in New Zealand waters in 1997, and is currently only found in rivers from Taranaki to Northland.

It has a distinctive colour pattern – mottled or blotched on the back, with yellowish pectoral fins. This eel grows to 1.5 metres and 14 kilograms. The maximum reported age is 41 years.

Diet and feeding

Eels were once considered a threat because they ate introduced trout. But they are now economically important in New Zealand, and there has been considerable research into their diet.

Big longfin eels may eat species such as juvenile trout – larger prey than that of the shortfin or spotted eel. One 9-kilogram eel was reported to have eaten an entire shoveller duck.

Small (less than 50-centimetre) longfin and shortfin eels generally feed on snails, insects, worms, grubs, crayfish and small fish.

Eels feed mainly at night, using their powerful sense of smell to track prey. Once an eel is close, taste buds on its head and sensors along its sides help locate the victim.

A big one

In 1882 Canterbury naturalist T. H. Potts listed some unsubstantiated reports of impressive catches, including a 43-kilogram eel caught by a shepherd at Stoneyhurst, and eels over 59 kilos from Lake Wakatipu. The largest recorded longfin eel is a 24-kilo specimen taken from Lake Waihola, south of Dunedin, in 1974.

At Lake Ellesmere (Te Waihora) in Canterbury, shortfin eels longer than 50 centimetres increasingly become fish-eaters, preying on cockabullies and smelt. Feeding drops off during winter in both species, especially in southern New Zealand.

Growth

Growth rates are slow. Longfin eels grow at around 2.5 centimetres a year at a length of 30 centimetres, and they slow down to only 1.5 centimetres a year at a length of 1 metre. Shortfin eels grow a little faster.

These slow rates are a result of temperature, food supply and competition. Faster growth rates have been achieved in trials to fatten eels in farm ponds.


Life cycle and breeding grounds

Freshwater eels have a remarkable life cycle, which begins and ends in the ocean. Spawning has never been observed.

Life cycle

Adult eels: spawning at sea

Adult eels probably spawn at some depth in warm seas. New Zealand’s shortfin eels produce 1.5–3 million eggs, and the longfins 1–20 million eggs. Males fertilise the eggs. After spawning, the adults die.

Larvae: drifting to land

Fertilised eggs hatch at the surface and become leaf-shaped larvae, floating on ocean currents towards the coast. They have teeth, but it is not clear for what purpose – they may store calcium for bone development. Their skin may absorb nutrients, as researchers have not found food in the larvae.

Glass eels: migrating into estuaries

Once the larvae reach land, an extraordinary transformation takes place: they become slender, transparent eels, known as glass eels. They arrive at New Zealand’s coast from July to December, with numbers peaking in spring (August–October) – the time of whitebait migration. Glass eels migrate into river mouths or estuaries in astounding numbers.

Up and over

Hydroelectric dams are an obstacle to elvers (young eels) swimming upriver. Some dams have special passes, allowing them to get round the massive concrete walls. But they don’t always need this help. Elvers can climb the 43-metre Arapuni Dam on the Waikato River, and the 75-metre Patea River dam in Taranaki.

Elvers: swimming upriver

Glass eels soon turn grey-brown, and in this form they are known as elvers. They migrate upriver, often in swarms and usually at night. Young elvers can climb waterfalls, but lose this skill as they grow.

Adult eels

Elvers become adults, with bigger heads and fatter bodies. After many years in fresh water, eels migrate back down the waterways to the sea. It is thought that males fertilise the eggs once the females spawn out at sea.

Migration to the sea

When they reach breeding size, eels change from ‘yellow-bellies’ to ‘silver-bellies’. The yellow-grey underside becomes grey-white, the head shape changes and the head, back and pectoral fins darken.

Shortfin males migrate in February–March, and longfin males in April. The females soon follow, and both males and females die after spawning. Studies show the species also migrate at different ages:

  • Shortfin males at an average of 14 years (38–58 centimetres), females at 22 years (50–100 centimetres).
  • Longfin males an average of 23 years (48–74 centimetres), females at 34 years (75–180 centimetres).

It is not known how long the journey takes. One female longfin eel that was tagged took 161 days to swim from Canterbury’s Lake Ellesmere (Te Waihora) to a point 160 kilometres north-east of New Caledonia.

Barriers across waterways have hampered their route. One estimate suggests that hydroelectric dams have blocked the longfin eel’s access to the sea in 35% of its habitat.

Fish out of water

As a young man, author David Graham worked on a Dannevirke dairy farm where eels slithered through wet grass from one waterway to another. ‘It was quite a common occurrence to leave home in the dark to bring the cows home to be milked and arrive back with several of these Eels, which were easily stunned with a blow on the head, and used for a meal.’ 1

Life cycle: a long-standing mystery

For centuries, larval eels were thought to be a separate species: they occur in the ocean and look different from adult, freshwater eels. Then in 1896 the Italian zoologist Giovanni Grassi reported that Leptocephalus brevirostris, known as a saltwater fish, was in fact the larva of the European freshwater eel. But just where at sea they bred was a mystery.

Searching for breeding grounds

In a 1923 paper, Danish biologist Johannes Schmidt stated that American and European eels spawned in the Sargasso Sea, in the Atlantic. In 1926, after sailing his research vessel Dana II to Australia and New Zealand, he concluded that New Zealand eels probably bred somewhere east of New Caledonia. But the exact locations are still not fully known.

Longfin eels

At migration, longfin eels are more ready to reproduce than shortfin eels. Scientists thought this meant their spawning grounds were closer to shore. However, a study showed that the longfin eel had the longest larval stage of any Pacific freshwater eel reported. This suggests that the larvae actually hatch further away from New Zealand, possibly near Tonga. They were also the biggest specimens when they reached coastal waters – so they may have been at sea longer, and travelled further.

Footnotes
  1. David. H. Graham, A treasury of New Zealand fishes. Wellington: A. H. & A. W. Reed, 1953, p. 129. › Back

Export and fishery management

Eels for export

Freshwater eels have become an important export, often live or as processed products. They go mainly to South-East Asia, Europe and Canada. Annual sales ranged from $800,000 to $3.5 million between 1990 and 2004.

Live eels and products such as smoked fillets, whole smoked eels or skinned segments are available from local processors and restaurants.

Overfishing

Worldwide, eel production is declining because of overfishing of glass eels (juveniles) and adults, and damage to the environment. Many countries farm eels, usually sourced from wild glass eels.

International demand for wild glass eels for use in eel farms is strong. The market is subject to fluctuations, and high prices can be paid.

Fishery management

Through the Quota Management System the New Zealand government controls and monitors commercial fishing, limiting the total catch to ensure sustainability.

In 2000, the South Island’s shortfin and longfin eels came under the quota system. Both species were treated as a single stock. In 2004 the North Island eel fisheries were included, but the two species were managed separately. Treating South Island shortfin and longfin eel fisheries as a single stock is questionable. Longfin eels breed later in their lives than shortfin eels, which makes them more vulnerable to overfishing.

Long-lived longfins

Scientist Don Jellyman figured out the great age of some longfin eels in Lake Rotoiti – the oldest eels yet documented. He found that one female eel was 106 years old by reading the growth rings on its ear bones (otoliths). An average generation time of 93 years suggests that conservative harvest levels must be set in managing these fish.

Breeding in captivity

A New Zealand research team at the Mahurangi Technical Institute in Warkworth has hatched baby eels from eggs of the shortfin eel – the first time that commercial quantities of freshwater eel have been bred in captivity. The commercial and ecological implications of this breakthrough could be considerable.

Tame eels have become a tourist attraction in places such as Tasman in Nelson and the Anatoki River in Golden Bay.


External links and sources

More suggestions and sources

More links and websites


How to cite this page: Paddy Ryan, 'Eels', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/eels/print (accessed 23 May 2019)

Story by Paddy Ryan, published 24 Sep 2007