The Longfinned Eels of Lake Rotoiti 'Giant Tuna'.

The ‘Giant Tuna of Lake Rotoiti’

Visitors to the Nelson Lakes may be stunned to a seething mass of eels right around the dock at the front of St Arnould. 

More commonly known as the longfin eel, these creatures are the top predators in their environment—meaning any living or dead animal is ‘fair game.’ Young eels feed on insect larvae, worms, and water snails.

As they grow larger, their diet expands to include bigger prey like other fish, smaller eels, trout, mice, crayfish, and even ducklings. Incredibly we witnessed an adult eel attack a full-grown mallard duck. 

The duck managed to escape, but we later observed another duck missing a leg and wondered if these deep-water predators were the cause.

The Ancient Longfin Eels of Nelson Lakes

The longfin eels of the Nelson Lakes are likely some of the oldest eels in New Zealand—possibly even the world.

The cold waters slow their growth, with many of the females not reaching full maturity until past the age of 90. This extended lifespan occurs because eels only breed once, at the end of their lives. The fact that these eels are still growing (albeit at a reduced rate) demonstrates their immense age.

Typically, a mature longfin eel might reach 1.2 metres in length, but here in Lake Rotoiti, the average length is just 0.7 metres due to slower growth rates. While these eels look large, they can grow even bigger—some of the largest recorded eels in New Zealand have reached 24 kg in other parts of the country.

The Waters of Lake Rotoiti. A sign in the foreground indicates that fishing for eels is prohibited. 

How Many Longfin Eels Are There in NZ?

The New Zealand longfin eel population is declining, with numbers estimated to be only 20–30% of what they once were due to habitat loss, overfishing, and changes in water quality. They are considered At Risk – Declining by the Department of Conservation. 

Types of Eel in New Zealand

There are three main species of eel in New Zealand:

• Longfin eel (Anguilla dieffenbachii) – Found only in New Zealand and known for their slow growth and large size. This is our only endemic eel and their declining conservation status is of great concern.
  
  
• Shortfin eel (Anguilla australis) – Smaller and more common, also found in Australia and the Pacific. Not threatened and found along much of the New Zealand Coastline but usually at lower elevations to the longfinned eel.
  
  
• Spotted eel (Anguilla reinhardtii) – A rare visitor from Australia, sometimes seen in northern waters. Often called the Australian long finned eel. Not considered threatened.
  
  

Are Longfin Eels Dangerous?

Longfin eels are not considered dangerous to humans, but they are opportunistic hunters with strong jaws and sharp teeth. We spoke to a number of locals who mentioned different injuries curious tourists had suffered over the years. From bitten children, to a tourist who had lost a chunk of their thumb to an eel the general advice was to steer clear as they may bite when provoked and can mistake fingers for food. That said, they do generally avoid human interaction and are unlikely to attack unless provoked.

Several long finned Eels of the lake up close.

The State of the Nelson Longfin Eel Population

In Nelson and the wider Tasman region, the longfin eel population faces challenges due to pollution, habitat destruction, and barriers to migration such as dams and culverts.

While still common in many areas we’ve witnessed the population decline over the fifteen years we’ve been visiting the wider Nelson Region. While not devastating there are noticeably fewer eels in many of the streams where they were once common. This reflects a broader slow decline that the species has experienced over the past century.

Would you like to learn about other New Zealand Animals? Why not spend some time learning about our recently extinct South Island Kokako.

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