Summer Series 3: Slippery devils – longfin eels
Lurking in the depths of freshwater waterways, all around New Zealand, longfin eels are the most common fish in our rivers. The native longfin eel, at up to 1.6 metres in length, is something to be in awe of, especially when there's a crowd of them – and they aren't the most attractive thing you've ever seen.
There are three native species: the longfin eel, shortfin eel, and the Australian longfin. "Both shortfins and longfins are widespread throughout New Zealand with shortfins preferring slow flowing rivers and lowland lakes, while longfins prefer faster water and are found further inland than shortfins. The Australian longfin eel is found only in the upper half of the North Island," says NIWA Principal Scientist Dr Don Jellyman.
Wriggly, long, slippery fellows, characterised by a thick layer of slime on their skin, longfin eels aren't the best to touch. Dark brown or grey coloured, they have tiny scales like most fish, yet feel smooth.
The longfin eels are great climbers. When they are young (up to 12 cm in length), they can climb vertical surfaces upwards of 30 metres!
The longfin eel can also be occasionally seen travelling cross country through the grass when it's raining, looking for lakes or rivers to hop into.
They like blood, and tend to come out at night, preferring to hide in burrows in the river bed during the day. Eels hunt at night by smell rather than sight. They are very secretive and are rarely seen through the day.
They have a horny upper lip, that encases a nasal cavity with a heightened sense of smell, they can detect food in the water more than a hundred metres a way! Eels are widely distributed over most of the river bed at night, which is when they look for food in shallow waters.
Their large mouths have rows of small, sharp, white teeth with the top teeth forming an arrow shape on the roof of the eel's mouth.
Longfin eels can eat small fish whole. For items too big to eat whole they tear pieces off their food by taking hold of it with their small sharp teeth and spinning, up to 12 revolutions per second, which enables them to tear pieces of food.
Eels are very stretchy, so they can stuff themselves with food, and, like the boa constrictor devour a lot. They love live food: crayfish and fish are particular favourites.
Throughout New Zealand, eels sustain important commercial, recreational, and Māori customary fisheries. Shortfin and longfin eels are smoked and exported.
They are an important food source to Māori. "Tuna (eels) are taonga (treasured) species, their importance to Māori is reflected in their whakapapa (ancestry), tikanga, and kawa (customs and culture). Plentiful and healthy tuna in the waterways are an indicator of wellness for people and their environs," says NIWA Chief Scientist Charlotte Severne.
The biggest longfin eels reported have weighed as much as 40 kg. Longfin eels can live up to one hundred years old.
In cold waters eels grow slowly, only between 15-25 mm a year. And the biggest eels are old females who are slower to reach sexual maturity.
Longfin eels travel thousands of kilometres at the end stages of their life, journeying to the Pacific Ocean to spawn, and then die. When they make this migratory journey their head flattens and they become more streamlined for the trip. Their eyes become bigger to improve their vision as they set off downstream, towards the sea, for this massive migratory journey. Interestingly they don't feed for the 5-6 months it takes to make this migratory trip. Their tiny offspring travel back to New Zealand, mainly swept along on ocean currents.
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.
About Dr Don Jellyman:
NIWA Principal Scientist Dr Don Jellyman has been studying eels for upwards of forty years. Currently he is looking at improving methods for assessing shortfin eel populations in lowland lakes in New Zealand. Dr Jellyman is also involved in studying the behaviour of migrating eels in Lake Manapouri, with a view to maximising their opportunity to escape to sea.
To illustrate this story: Download photos from: http://ftpmedia.niwa.co.nz/summer_series/Longfin_eel/
Other information about this item:
Species Fact File: Longfin eel
Common names: Longfin eel
Māori name: Tuna
Scientific name: Anguilla dieffenbachia
Type: longfin eel
Size: 160cm –2 metres
Lifespan: Up to 100 years for females. Males only live to half the age of females.
Diet: Small longfin 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.
Reproduction: Longfin eels breed only once, at the end of their life. When they are ready to breed, they leave New Zealand and swim 5000 kilometres to the south Fiji basin to breed.
When they reach their destination, the females lay millions of eggs that are fertilised by the male.
The larvae look very different to an eel – they are transparent, flat, and leaf-shaped. The larvae reach New Zealand by drifting on ocean currents.
Before entering fresh water, the larvae 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.
Things you need to know: They occasionally bite fishermen.
They can swim backwards!
They are our largest freshwater fish
Something strange: Their heads change shape when they migrate, and their eyes become bigger.
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