Source: NZ Department of Conservation
Sleek, strong and slithery yet secretive about their sex lives. Eels/tuna are the top predators of the freshwater world with exquisitely evolved teeth, jaws, muscles and fins – and an epic sense of smell.
Many more things make eels unique and special – how much do you know?
We asked two of DOC’s biggest eel fans – Brittany Earl, a freshwater ranger and Marine Richarson, a senior science advisor, for their top 10 eel facts. Read on!
1) Eels are fish
Brittany: One of the questions I get asked most often is, are eels fish? And yes, they are!
To be a fish you have to live in water, have gills and a spine, but not have arms and legs, fingers or toes. Eels get a tick for all these.
Most fish also have fins and scales – and eels have these too – but their scales are microscopic and embedded in their skin. The skin itself is covered in a healthy layer of protective mucus. This slime is an adaptation that helps them move through water and even over land, as long as they stay wet.
Speaking of fins, there are two main species of eels in Aotearoa, longfin and shortfin. Longfins generally grow bigger, live longer (longer than some humans) and inhabit streams further inland than shortfins. Longfins are only found here and are the largest and longest-lived freshwater eels in the world. Iconic!
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2) Breeding is the last thing eels ever do
Marine: When the mood feels right – moonlight, rain and a good river flow – eels get the urge to head downstream and out to sea. They gather at an unknown location in the Pacific Ocean, breed and then… well, they die.
In a publication from 1944, eels are described as breeding in “a great deep off the coast of the Great Barrier Reef”. Truth is we still don’t exactly know where that ‘great deep’ is or even if it’s that deep. The latest searches point to somewhere between New Caledonia and Fiji.
In a recent study, NIWA scientists used satellite trackers to try and pinpoint the final destination of migrant eels. But the ocean is vast and the tagged eels evaded scrutiny.
One thing every eel scientist agrees on: it’s a one-way trip to the spawning grounds, where eels release eggs and sperm. The next generation must make the long journey to the shores of Aotearoa on their own. Talk about a rough childhood!
3) Silver for the sea, brown for freshwater – eels can change colour
Brittany: Eels have really cool transforming powers. They totally change their shape and colour over the course of their lives.
Heading back from the Pacific, the hatchling eels are flat, colourless, leaf-shaped larvae. Being completely transparent makes them (hopefully) invisible to predators while they drift and wriggle back to our shores. When they get here, the larvae have become eel-shaped but are still transparent. And they have an evocative new name – glass eels.
This oceanic disguise doesn’t work in rivers and streams, so they hang out at river mouths for a couple of weeks until they’ve switched into camo colours. It’s pretty crazy! They stay brown until the end of their lives, changing back into silver attire in readiness for their final ocean journey.
4) The ancients thought eels reproduced without sex
Marine: Freshwater eels live all around the world, and for millennia, fascinated people have been coming up with intriguing ideas about their love lives.
The ancient Egyptians thought eels sprang to life when the sun warmed the Nile. Aristotle believed they just popped into existence and their food came from rainwater.
Pliny the Elder theorised that eels, “rub themselves against rocks and the particles which they thus scrape from their bodies come to life”.
The first person to argue that eels must have male and female forms was a philosopher, savant, composer, medical writer and practitioner and … drumroll please… a woman, St Hildegard of Bingen.
It took another 600-odd years for (male) scientists to pick up and validate Hildegard’s hypothesis. The next challenge lay in finding eel testes.
In 1876 Sigmund Freud was a young marine biology researcher. He set out to find the ‘male apparatus’ that had recently been described by a contemporary. Dissecting 400 eels, he eventually found just two testes.
It was such a debacle that Freud abandoned this career and shifted his attention to the human mind. (Perhaps eels have something to do with the weirder aspects of psychoanalytical theories after all?)
What we now know is that sexual organs don’t develop until eels are on their way to their breeding grounds. That’s one eel mystery solved – but a few remain!
5) Glass eels have attitude
Brittany: One of the best parts of my job is going out and monitoring (basically counting and checking) the glass eels at the mouth of the Ashley/Rakahuri River in spring each year. I’m in the middle of it right now. It’s a DOC-led study that’s been going for 30 years.
Glass eels are pretty cute – and feisty! If you’ve ever handled an adult eel, you’ll know that they’re strong, slippery and have a lot of energy for their size. Glass eels are exactly the same but they’re tiny. I love watching them wriggle around, hiding in corners of the net or climbing out of my bucket.
The study involves looking at them under the microscope, so I get to see their hearts beating and what they’ve just eaten, like mayfly larvae and algae from the rocks. Of course, we put them all back carefully in the river afterwards .
6) Māori have more than 150 names for tuna
Marine: Māori have always been keen observers of the natural world, but the mātauranga (knowledge) on eels is mind blowing. Tuna are a significant and highly valued taonga species. Their importance is reflected in place names, whakataūki (proverbs), legends, waiata (songs) and art.
Māori give tuna different names depending on their size, colour, where they’re found and the time of the year. Some names are used by just one iwi while some are common throughout Aotearoa.
Traditionally smoked, tuna were an important source of food and are still treasured for serving to manuhiri (guests) and at hākari (feasts).
7) Glass eels aren’t fussy about the river they come home to
Brittany: Salmon are famous for their homing instinct – they travel back to exactly the river where they were born. (They’re thought to navigate by smell, magnetic fields, the moon and stars.)
Eels will go into any healthy stream or river. They love the smell of decomposing leaves (yum!) and other signals of bushy habitat upstream. Those are the rivers they head for.
Drained wetlands, polluted water, introduced fish and culverts they can’t climb have a negative impact on eel populations.
These factors are likely to be behind the At Risk – Declining conservation status of longfin eels in New Zealand. I’m seeing that in the Rakahuri study, with fewer longfin glass eels each year. But the good news is that if we can improve water quality, access and habitat in rivers, eels will move back in!
8) New Zealand eels have been exported for more than 50 years
Marine: Most eels caught commercially in New Zealand are exported: smoked, frozen whole, filleted, skinned and even live, depending on demand. They go to Asia, North America and Europe (the Brits love their jellied eel!).
In the 1960s, while eel stocks elsewhere had started falling, New Zealand eels rocketed to become one the country’s most valuable fish exports. At its height in the 1970s, more than 30 factories supported the industry, processing up to 2,000 tons of shortfin and longfin eels.
Commercial catches are now declining. The downward trend varies from region to region with catches around 200–300 tonnes, valued at less than $3 million a year. The commercial eel fishery is highly regulated and managed through the Quota Management System by Fisheries New Zealand. Every year, regulators, fisheries scientists and fishers meet to discuss the state of the fishery and agree on the new season’s rules to ensure a fair and sustainable activity.
9) Hydro dams can bring eel migration to a dead stop
Marine: Eels and longfins in particular, can be very long-lived – some even touching a century. Some old girls are living in lakes and rivers above hydro dams. They would have swum up as youngsters before the dams were built and been there ever since, waiting for the call to return to the sea.
For many, that call will spell disaster. They can get stuck, injured or killed by the dams’ turbines and spillways. This has been going on for decades.
Lucky eels are picked up by very dedicated people, often iwi, who run trap-and-transfer schemes to collect and move them safely downstream past the dams. Some are volunteers. Some are paid as part of resource consents for operating the dams.
Many different fixes have been tried and tested – screens, eel bypasses, ‘fish-friendly’ turbines and more – but solving this problem is technically difficult and costly. In the meantime, we need everyone to be on board to safeguard eel passage. I believe there is huge power in structure operators, local government, manawhenua and community groups teaming up for the welfare of eels.
10) There used to be WAY more eels here
Marine: Eels used to be plentiful everywhere. Māori tell stories of bountiful tuna harvests until the mid-twentieth century, like this account from Wairarapa kaumātua Wiremu Aspinall.
“Here in the Wairarapa, goodness gracious I’ve never seen so many eels,
…a mass of eels going out to sea… Big wide drains, about 12 feet wide. The drains were thick with eels. You could hear them at night like ducks taking off and you know they’re running.”
There is a powerful concept in environmental science that explains how our perception of ‘normal’ changes over time, often without us realising. Each generation accepts the environmental conditions they see as the norm, even if those conditions are degraded. Little by little, we lower our expectations for biodiversity and abundance and accept a ‘new normal’. This collective memory loss is called shifting baseline syndrome.
Today, it’s really hard to picture what that was like, how today’s cities, agricultural land and cultivated forests used to look, sound and smell. Try and imagine a beach covered in so many seals that you can barely see the sand, the sun almost blocked out by millions of swirling seabirds, and birdsong so loud from the forest nearby you can hardly hear yourself think.
But perceiving a species as abundant, even if its population is a meagre remnant, can lead to overconfidence or complacency in the way we manage and conserve species.
This is unfortunately the case with eels. Today longfin eels have a threat status of At Risk-Declining and they’re only found in Aotearoa. What a change in just over 100 years.
My work at DOC includes research and advocacy for both species, and rangers like Brittany are working around the country to learn more about and protect these amazing animals through the Ngā Ika e Heke – migratory fish programme.
Go naturing with eels
There’s lots to love about eels and watching them in real life is definitely the best way to learn about and appreciate them. You can see eels in their natural environment or at an aquarium in quite a few places around the country (Google it).
Some practical things you can do help restore eel populations are:
- Get involved in community stream restoration projects.
- Advocate for better eel passage at dams in resource consent processes.
- Choose an energy supplier that works with iwi to trap and transfer eels.
Online resources
There are some excellent resources online. We’ve drawn on these for this blog:
