Longfin Tuna

General Facts/Context

The longfin eel (scientific name Anguilla dieffenbachii) is one of estimated 35 New Zealand native freshwater fish species, almost all of which  are in decline. More than half are on the threatened species list1.

There are only around 16 freshwater eel species in the world, three of which can be found in Aotearoa/New Zealand. What makes the longfin eel important is that it is endemic, which means they are found nowhere else.

Biology

What do they look like?

Longfin eels vary in colour, but are usually dark-brown to gray-black, although  very rarely they are fully or partially yellow. The longfin is distinguishable from its shortfin (non-endemic but still native) cousin by the length of its dorsal (back) fin, which extends to around two-thirds the length of its back . They also have pectoral (side) fins, which have 16–20 bony rays. Like all fish, eels have scales,but as they are very small and embedded in their skin  eels have a slippery and scale-less appearance. Eels can absorb up to 50% of their oxygen needs through the skin, which can get slimy when they are stressed. Their mouths are large, with small, sandpaper-like teeth that point inwards, and they have sensors (that look like white spots around the lips) that detect movement in front of them.

While longfin eels generally have poor eyesight, they have many other senses that help them survive. They have an excellent sense of smell which is enhanced by tubular nostrils that stick out in front of their noses to help in hunting. They also have sensors on their  sides and taste buds on their  heads, and body slime helps breathing and provides skin protection out of water. The skin is also highly sensitive to help it “see”. However, as you will read under the ‘life-cycle’ section, eels undergo significant physical transformations to prepare them for their long seaward breeding journey.

How big do they get?

Longfin eels are sexually dimorphic in that the females grow to be much larger than the males. The females can grow to around 2 metres, although most grow to over 1 metre, which puts them amongst the world’s biggest eels. They also get very heavy, with females reaching weights of around 25kgs with some growing to over 50kgs. Female Longfin eels can achieve ages of between 30 to 100 years before reaching sexual maturity (15 to 45 years for males)2. The age (of a dead eel) can be ascertained by counting the rings on an ear bone (otolith) that tells its age like those of a tree. And as with tree rings, widely spaced rings indicate years of abundant growth.

The growth rate is slow, at around 2.5 centimetres a year at a length of 30 centimetres, (depending on temperature and food availability), and can slow markedly to only 1.5 centimetres a year  in a large eel.“The biggest eels are very old females that, for reasons that are not yet understood, have not yet migrated to sea to breed.”3

Habitat

Where are they found?

Picture by Stephanie Bowman

Longfin eels are widely distributed throughout Aotearoa/New Zealand. They are found  in many freshwater habitats, usually inland from the coast4. They need habitats with adequate water quality, depth and flow, and streamside cover is also important for shade and protection. However, longfin eels are able to survive in many different conditions, including farm drains, large ponds, farm dams and areas with quite murky water. Tuna can access isolated tarns by swimming up underground streams .and can also move between waterways, as long as it is raining or damp,   sometimes being found slithering through damp grass on their passage from one location to the next.Given their size, large female longfins may need up to 400 metres of home territory5.

Feeding

What do they eat?

Longfin eels are a top-level predator in the freshwater ecosystem, which means no predators prey on the large adults (except humans). They are predominantly nocturnal (night) feeders and feed on a variety of species depending on the  age of the eel – such as other fish, younger eels, koura (fresh water crayfish), insects and worms. Large eels have even been known to eat small birds.

Lifecycle

Long Fin Eel Life CycleThe life cycle of eels is astonishing and unique. What is known is that the life cycle begins and ends in the ocean (a cycle known as catadromy). Once the eels  reach sexual maturity they are ready to swim out to sea to return to their ancestral breeding ground.The migration begins around April when the males head off first, followed soon after by the females.

Getting ready for the journey

Before longfin  eels begin their migration  their body undergoes a physical transformation. The eels cease to feed and the stomach shrinks as the sexual organs grow large.  Their yellow belly changes to a silvery colour,  parts of the body (the head,  fins and back) darken and the bulbous head of the female changes as  the   dome behind the eyes  reduces, the head changing  from rounded to sharp and lean and the lips becoming  thin. The pectoral fins and  eyes  enlarge, becoming surrounded by a ring.

Heading off for the breeding grounds

The exact breeding location of the longfin eel is  unknown, however it is thought to be somewhere deep in the ocean trenches near Tonga or east of New Caledonia.  It is not known exactly how long this journey will take them, but one tagged female eel took 161 days from Canterbury’s Lake Ellesmere (Te Waihora) to a point 160 kilometres north-east of New Caledonia7.

There exists a Maori karakia which also suggests that the location is somewhere in the pacific, as it refers to Hawaiki (Tēnā te puna kei Hawaiki, the source is at Hawaiki6).

It is thought an adult female eel releases around 1–20 million eggs, which are then fertilised by the male. After spawning the adult eels die.

Hatching- Larvae

The eggs hatch, and the resulting transparent leaf-shaped larvae (leptocephalus) float within the  South Equatorial Current, taking around 18 months to reach New Zealand’s shores. The absence of food found in the larvae suggests that they absorb nutrients through their skin during this time. Even though they may not eat they do have teeth, and scientists think this may be the storage of calcium to create bones once they begin to grow.

Glass Eels

As the larvae approach the shores of New Zealand they gradually transform into ‘glass eels’ (shaped like an adult eel but transparent). The  glass eels are around 5-6cm long when they reach land  between July to December ( with August to October bringing a swell of numbers). They head towards the freshwater outfalls of rivers.

Elvers

As the glass eels  begin to migrate upstream, and take on a gray-brown colouration, and are then known as elvers.  Elvers possess tremendous climbing abilities, and are able to move vertically up high waterfalls and travel long distances to find a suitable home. They continue to migrate up stream until they settle in a suitable pool to grow. Young eels are sexually indistinguishable and it is thought that environmental factors affect sexual differentiation

  1. Joy, Mike and McEwan, A. 2009. New Zealand Freshwater Sciences Society Newsletter, Number 48, August. ISSN 1178-6906 (online).
  2. Joy, Mike and McEwan, A. 2009. New Zealand Freshwater Sciences Society Newsletter, Number 48, August. ISSN 1178-6906 (online).
  3. http://www.doc.govt.nz/conservation/native-animals/fish/facts/eel/ . Date Accessed 30th March
  4. Potangaroa, Joseph. 2010. ‘Tuna Kuwharuwharu-The Longfin Eel: Facts, Threats and How to Help. ISBN 978-0-473-16583-3.
  5. Potangaroa, Joseph. 2010. ‘Tuna Kuwharuwharu-The Longfin Eel: Facts, Threats and How to Help. ISBN 978-0-473-16583-3.
  6. http://www.teara.govt.nz/en/eels/3 accessed 31 March 2010
  7. http://www.teara.govt.nz/en/eels/3 accessed 31 March 2010

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