It can grow up to 2 meters and live up to 100 years. It is not a python but a fish... Its bite is not very painful, but it is very scary.

Do you know what this fish called "river monster" is?

Image source: NZTODAY RV Lifestyle

This is the New Zealand longfin eel , a freshwater eel that is unique to New Zealand. Its English name is New Zealand longfin eel, and its scientific name is Anguilla dieffenbachia.

Some people may think they look a bit scary because they can grow up to 2 meters, their bodies are black and slimy, they can crawl on the grass, crawl deep inland to the streams behind people's houses, eat not only plants but also small fish or ducklings, and can live to be 100 years old...

Researchers hold a female longfin eel. Image credit: Lindsay Hawke

Long-lived giant "river monster"

As one of the largest eels in the world, the New Zealand longfin eel is only 1 mm long at birth, but can grow up to 2 meters in length and weigh up to 40 kg when it reaches adulthood .

Their average lifespan is 35 to 52 years, and some even live to be "centenarians" . Compared to many other fish, these large, long-lived fish grow slowly, with New Zealand longfin eels growing only 15 to 25 mm per year. However, factors such as habitat type, water temperature, population density, age, and food supply can affect their size and growth rate.

Females are generally larger, mature later, and live longer than males of the same species . The largest individuals are usually females, which are reported to weigh up to 50 kg and live to be over 100 years old.

They generally don't bite unless they feel threatened. People who have been bitten say it's not very painful, but it's quite scary. Image source: fs.fish.govt.nz

In other parts of the world, only two kinds of freshwater eels can reach the body size of the New Zealand longfin eel: one is the Australian longfin eel, and the other is the longfin eel (commonly known as flower eel and eel king) which is also found in my country.

Unlike these two freshwater eels, which have stripes or spots on their bodies, the New Zealand longfin eel is very dark in color, mostly dark brown or black, and in very rare cases, golden longfin eels appear. The golden color appears because the dark pigment in the longfin eel is lost, allowing the yellow pigment to appear.

Golden longfin eel, Photo credit: Mike Joy

Can climb, can climb waterfalls

Longfin eels can not only swim, but also climb . The larvae (young eels) can swim upstream, climb out of the water, and wriggle on wet and rough surfaces, clinging to the rock walls through surface tension.

They can even climb dams or waterfalls. Some longfin eels have been reported to climb waterfalls as high as 20 meters . People have also seen eel larvae climbing vertical surfaces up to 43 meters high . However, when they grow to about 12 centimeters, they "lose" their climbing ability, probably because they are too heavy and surface tension no longer works.

In addition to this, the eels will walk across wet grasslands on land , and can migrate over 130 kilometres inland in just one summer. Breathing through their skin, they can "walk" on land for up to 2 days! As a result, they can be found in rivers and lakes at all altitudes across New Zealand.

Swipe left to see the climbing eel. Image credit: Jacques Boubée

In freshwater, the longfin eel is a top predator and an omnivore that relies on a very keen sense of smell to hunt . Depending on the food available, the longfin eel will eat plants and also prey on other aquatic organisms such as insects, worms, freshwater crayfish, and occasionally ducklings. Hunting activities are mainly carried out at night, and during the day they often hide in crevices, logs, or boulders.

However, during the day, they are easily attracted by food (especially meat). Some hunters were almost scared to death by the sudden appearance of long-fin eels when they were gutting their prey in the stream.

Many New Zealanders also crack eggs into the water to "lure the eels out of the hole", and some people even use potato chips to succeed!

When faced with prey larger than themselves, they will use a "death roll" like a crocodile to bite their prey. Image source: "River Monsters"

They reproduce only once, then die.

In the autumn (April to June in the southern hemisphere), adult eels leave New Zealand's rivers and streams and travel a long distance to the South Pacific to spawn . Scientists speculate that the spawning site is near Tonga, which may be located in a very deep trench. Until now, people have never observed longfin eels spawning.

When they reach their destination, the females lay millions or tens of millions of eggs, which are fertilized by the males. How the males fertilize the eggs is still a mystery. The adults then die.

The fertilized eggs float to the surface and hatch into very flat, leaf-shaped, transparent larvae that are only a few millimeters in diameter. These transparent little eels drift with the ocean currents for more than a dozen months before eventually reaching the coast of New Zealand.

When they reach the coast of New Zealand, the larvae develop into transparent "glass eels" . Then, they enter the estuary from the ocean and migrate upstream. Once in fresh water, their bodies gradually darken in color and they become slender, wriggling eels. Schools of eels swim upstream, spending months or even years searching for their home.

Glass eel. Photo credit: Gregory Watkins-Colwell

In their freshwater home, longfin eels live for decades or even a hundred years until they hear the call to reproduce. When they are ready to migrate to their spawning grounds, they begin to "transform," such as their heads becoming more elongated, like a bullet, their eyes becoming larger for better vision in low light, and their bellies becoming silver to make them more invisible to predators.

When they migrate to their ocean spawning grounds, males are about 25 years old, and females are 35 to 40. The largest longfin eels seen inland are usually older females, who mature more slowly and, for unknown reasons, do not migrate to the sea to reproduce.

The life cycle of a longfin eel. Image source: Te Ara The Encyclopedia of New Zealand

Numbers are declining

In the past, longfin eels were an important food source for the Maori , often caught and dried or smoked to preserve for winter consumption, and they are also an important symbol of Maori culture and identity.

But today, most freshwater eels in New Zealand are short-finned eels, and long-finned eels have become relatively rare .

There are two species of freshwater eels native to New Zealand: the longfin (top) and the shortfin (bottom). The longfin gets its name because its upper fin (dorsal fin) is longer than its lower fin. The shortfin, on the other hand, has upper and lower fins of similar length. Shortfins don't live inland like longfins do. Image credit: Department of Conservation, Te Papa Atawhai

The vulnerability of the New Zealand longfin eel is partly due to its long life cycle. In addition, although the young eels can climb, the dams built by people also prevent many eels from moving freely in the river; sewage and wastewater discharged into the river by factories also take away a lot of oxygen in the water, causing the eels to die or forcing them to migrate; and factors such as overfishing, habitat loss and climate change are also threatening their survival.

Image source: Downes, Thomas William, 1868~1938

Today, the eel runs observed are getting smaller and smaller, and it’s becoming increasingly difficult to spot the terrifyingly large longfin eels.

References

[1] Eels. Retrieved from https://www.doc.govt.nz/nature/native-animals/freshwater-fish/eels

[2] (https://www. nvinteractive. com/), NI On a pathway to extinction? An investigation into the status and management of the longfin eel. Retrieved from https://pce.parliament.nz/publications/on-a-pathway-to-extinction-an-investigation-into-the-status-and-management-of-the-longfin-eel

[3] https://pce.parliament.nz/media/sh4f5niq/pce-eels-final2.pdf

[4] New Zealand Longfin Eel | Freshwater Fish | Auckland Zoo. Retrieved from https://www.aucklandzoo.co.nz/animals/new-zealand-longfin-eel

[5] New Zealand Longfin Eel | Freshwater Fish | Auckland Zoo. Retrieved from https://www.aucklandzoo.co.nz/animals/new-zealand-longfin-eel

[6] Tuna – working with glass eels. Retrieved from https://www.sciencelearn.org.nz/resources/423-tuna-working-with-glass-eels

[7] Hunt, E. (2021). A creature of mystery: New Zealand's love-hate relationship with eels. the Guardian. Retrieved from https://www.theguardian.com/environment/2021/apr/24/a-creature-of-mystery-new-zealands-love-hate-relationship-with-eels

[8] MacLean, D. (2020). Teeth like sandpaper: Northland boy recounts eel bite. NZ Herald. Retrieved from https://www.nzherald.co.nz/northern-advocate/news/teeth-like-sandpaper-northland-boy-recounts-eel-bite/HYPWKTE3HEA5THZPG4QCDVWCUM

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