New Zealand longfin eel

The New Zealand longfin eel (Anguilla dieffenbachii) is New Zealand's only endemic freshwater eel.^[1][2] The other species of eel found in New Zealand are the native Shortfin eel (Anguilla australis) which is also found in Australia, and the sporadically naturally introduced Australian Longfin eel (Anguilla reinhardtii).^[1][2][3][4]

The easiest method for identification of the New Zealand longfin eel is by the length of its fins: the dorsal (top) fin extends further forward towards the head than the anal (bottom) fin. In the Shortfin Eel the fins are of similar lengths.^[5]

The New Zealand longfin eel is classified as a threatened species in gradual decline by the Department of Conservation.^[6]

New Zealand longfin eels also have gained some notoriety, particularly after being featured in the 16th episode of River Monsters, in which they were shown to have attacked and even killed humans.

Description

Male and female Longfin Eels differ in length and in the age at which they migrate with males averaging 666 mm but reaching up to 735 mm in length with an average age of 23 years (12 – 35 years).^[5][7] Female Longfin Eels are considerably larger with an average length of 1156 mm, but can range from 737 to 1560 mm. These females range in age from 20 to 60 years before migrating.^[7] The average age of migration varies between the North and South Islands of New Zealand with the North Island eels having younger migration ages and thus faster generation times.^[3]

Life cycle

Like other members of the Anguillidae family Longfin Eels have a rather unusual migration system where they grow and mature into fertile adults in freshwater water systems then migrate to the sea to breed; a catadromous breeding system.^[8]
This breeding system also ensures that their mating system is randomised (panmictic population).^[5][9]

The New Zealand longfin eel is a very long lived fish with records of females reaching 106 years old and weighing up to 24 kg.^[7][10] Longfin Eels have the slowest growth rate of any eel species studied, growing between 1 - 2 centimetres a year.^[11][12]

Longfin eels life cycle like other species of Angullidae eels is rather complex consisting of four distinct life stages which remained a mystery for many decades and still is not fully understood.^{[5][7][13][14]}

New Zealand longfins breed only once at the end of their lifecycle,^[11] making a journey of thousands of kilometres from New Zealand. They migrate to their spawning grounds in Tonga.^[13][15] their eggs (of which each female eel produces between 1 and 20 million^[5][16]) are fertilized in an unknown manner but is thought to occur in deep tropical water.^[5] The mature eels then subsequently die, with their eggs floating to the surface hatching into very flat leaf-like larvae (leptocephalus larvae) that then drift along large oceanic currents back to New Zealand.^[5][13][17] This drifting is thought to take up to 15 months.^[5] There have been no recorded captures of either the eggs or larvae of Longfin eels.^[13] Upon arrival back in New Zealand the larvae undergo a transformation (metamorphosis) into glass eels, which resemble small adult eels but are transparent.^[5] These glass eels occupy estuaries during which time they develop colouration^[5] and become elvers, resembling small adult Longfin eels.^[5][14] The elvers (young eel) migrate upstream where they develop into adults.^{[5][14][18][19]}

Recruitment

The recruitment of glass eels into the freshwater river networks in New Zealand is a very variable process which is thought to be affected by the El Niño and La Niña Southern Oscillations.^[15] This is a major reason for the failure of Longfin eels aquaculture farms in the 1970s.^[2][5]

Sex determination

The determination of the sex of Longfin eels is a difficult process as their sexual organs are not determined until they are over 450 mm in length.^[9] The only method to determine Longfin eels sex is through internal examination and only becomes easy to distinguish when the eels mature and migrate.^[5][9]

Diet

Longfin eels have an omnivorous diet and are opportunistic feeders.^[20] Their diet as small eels largely consists of insect larvae.^[5][20] When eels become larger, they also feed on small fish including galaxiids and trout.^[5][20]

Habitat and distribution

External video

The Longfin Eel Migration –  River Monsters

The New Zealand longfin eel is an endemic species to New Zealand with a very wide distribution through New Zealand's freshwater waterways including the Chatham Islands.^[3][5][21]

Longfin eels are often found great distances inland (up to 361 km) along fresh waterways and in high country lakes which are connected to the sea.^[3][5][10] Aiding in Longfin eels inland distribution is its climbing ability when it is in its elver (juvenile) life stage and is under 12 cm in length.^[12][19] These migration events frequently coincide with increased temperate, water flow and low light conditions.^[5][12][22] At this size the elvers (young eel) can migrate up to 130 km inland over a summer and has been observed climbing near vertical surfaces up to 43 m tall.^[5] This feat is accomplished through a combination of surface tension (with the water) and friction.^[19]

The in-stream distribution varies depending on the life stage of the Longfin eel. As elvers (juveniles) they prefer shallow water (under 0.5 m deep) with coarse substratum and faster than average stream flow (such as that found in stream riffle).^[3][4] Adult Longfin eels tend to be found next to or under large pieces of debris and undercut river banks.^[4][5]

Significance for Māori

The Longfin eel is an important resource for Māori because it provided an important food (protein) source.^[12] Reflecting this significance for Māori they had extensive knowledge of Longfin eel's migratory routes.^[5][12][16]

Longfin eel fishing

The commercial fishing of Longfin eels started to gain momentum in the 1960s and had a 2000 tonne yearly catch by the 1970s.^[5][12] But the fishery went into decline in the early 1980s and in the 2000-2001 fishing season only 1079 tonnes were taken.^[23] The commercial Longfin eel fishing was included into the Quota Management System (QMS) in 2000 for the South Island and the North Island in 2004. This set limits as to the minimum and maximum size (220 grams and 4 kg) and the Total Allowable Catch (TAC).^[3][12] As of 2007 the TAC has not been reached in any fishing season since the implementation of the QMS.^[3] In recognition of the significance of Longfin eels to Māori they have a 20 percent allocation of fishery stocks.^[24] The capture and export of glass eels in New Zealand has been prohibited.^[3]

Longfin eel aquaculture

There have been a number of attempts at growing Longfin eels in an aquaculture operation. The first were in the 1970s. These did not remain operational for long with the last farm closing in 1982.^[2] The most common reasons for these failures were economic (the high cost of production vs low price for the eels), variable recruitment of glass eels and the high mortality (death) rates in the farms.^[2] In the early 2000s there has been renewed interest in the aquaculture of Longfin eels due to the increasing knowledge of Longfins biology and the diminishing stocks of European eels (Anguilla anguilla) but no farms have been built.^{[2][8][9][12][16][25][26]}

Conservation

Scientists and conservation groups have growing concern for the survival of the species as they can be legally fished, and have a slow reproduction rate, breeding only once at the end of their lifetime.^[11][27] In 2003 a scientist predicted their numbers to drop by between 5 and 30% in the following decade.^[11]

In June 2012 it was reported that some pet food companies use the nationally threatened eels in their products, sparking outrage by conservationists.^[28]

References

• Froese, Rainer and Pauly, Daniel, eds. (2006). "Anguilla dieffenbachii" in FishBase. February 2006 version.
• Tony Ayling & Geoffrey Cox, Collins Guide to the Sea Fishes of New Zealand, (William Collins Publishers Ltd, Auckland, New Zealand 1982) ISBN 0-00-216987-8

Additional resources

Saving Tuna Documentary

Long Finned Eel (Tuna)