New Zealand longfin eel
(Anguilla dieffenbachii)

The New Zealand longfin eel (Anguilla dieffenbachii) is a species of freshwater eel that is endemic to New Zealand. It is the largest freshwater eel in New Zealand and the only endemic species – the other eels found in New Zealand are the native shortfin eel (Anguilla australis), also found in Australia, and the naturally introduced Australian longfin eel (Anguilla reinhardtii).

Longfin eels are long-lived, migrating to the Pacific Ocean near Tonga to breed at the end of their lives. They are good climbers as juveniles and so are found in streams and lakes a long way inland. An important traditional food source for Māori, longfin eels numbers are declining and they are classified as endangered, but over one hundred tonnes are still commercially fished each year.

Description

The easiest way to identify the longfin eel is by the length of its fins: the dorsal (top) fin is about two-thirds the length of the body and starts significantly further towards the head than the anal (bottom) fin. In the shortfin eel the fins are of similar length. When a longfin eel bends its loose skin wrinkles distinctively inside each bend, where a shortfin eels skin is smooth. A longfins mouth extends past its eyes, further than a shortfins.

Female longfin eels are larger and longer-lived than males. Males average 66.6 cm but reach up to 73.5 cm in length, with an average age of 23 years (12–35 years).Females are considerably larger, ranging from 73–156 cm with an average length of 115 cm. Females attain an age of 20 to 60 years before migrating to sea to breed. Longfin eels in the North Island migrate at a younger age and thus have faster generation times.

Life cycle
Like other members of the Anguillidae, longfin eels have a rather unusual life cycle: they grow and mature into fertile adults in fresh water, then migrate to the sea to breed. This catadromous breeding system also ensures randomised mating, creating a panmictic population.

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.

They have the slowest growth rate of any eel species studied, just 1–2 centimetres a year.

Longfin eels life cycle like other species of Anguillidae eels is rather complex consisting of four distinct life stages which remained a mystery for many decades and still is not fully understood.

New Zealand longfins breed only once at the end of their lives, making a journey of thousands of kilometres from New Zealand to their spawning grounds near Tonga. Their eggs (of which each female eel produces between 1 and 20 million) are fertilized in an unknown manner, but probably in deep tropical water. The mature eels then die, their eggs floating to the surface to hatch into very flat leaf-like larvae (called leptocephalus) that then drift along large oceanic currents back to New Zealand.