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.
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.
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.
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.
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 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.