New Zealand Longfin Eel

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 eel's skin is smooth. A longfin's mouth extends past its eyes, further than a shortfin's.

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 to 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.

Anguilla dieffenbachii is endemic species to New Zealand, widely distributed through the country's lakes and rivers, including the Chatham Islands.

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. Aiding in their inland distribution is their climbing ability in elver (juvenile) stage, under 12 cm in length. These migration events frequently coincide with increased temperate, water flow and low light conditions. At this size the elvers can travel up to 130 km inland over a summer, and have been observed climbing near-vertical surfaces up to 43 m tall. This feat is accomplished through a combination of surface tension (with the water) and friction.

Longfin eels have an omnivorous diet and are opportunistic feeders. Their diet as small eels largely consists of insect larvae, such as dragonfly nymphs (Procordulia grayi). As the eels grow, they also feed heavily on Lymnaea snails and fish, including galaxiids, the upland bully, and yearling trout which are taken by larger eels. There are reports of these eels eating small waterfowl, such as ducklings.

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.

In 2003 biologist Don Jellyman predicted longfin numbers would drop by between 5 and 30% in the following decade. The species was classified in 2014 and again in 2018 by the Department of Conservation as "At Risk: Declining" under criteria "C(2/1) Total area of occupancy > 10 000 ha (100 km2), predicted decline 10–70%". The DOC panel concluded though that the commercial fishery of the species is stable or increasing, and noted "that public discourse on the longfin eel portrays the species as being severely threatened despite data that indicate otherwise." This conclusion has been seriously questioned, repeatedly, both before and since then, throwing its validity into doubt.