Tuna - biology and ecology

There are several ways to tell the three New Zealand eel species apart.

Freshwater eels have an unusual life cycle which sees them travelling between the ocean, estuaries and freshwaters.

Glass eels (about 5.5 to 7.0 cm) arrive in fresh water during spring, especially during September and October, although they may be present from July to December.

Once in freshwater, glass eels develop into darker pigmented juvenile eels known as elvers.

Tuna are found in all sorts of habitats (places), including coastal estuaries, lakes, wetlands, rivers, mountain streams and even alpine tarns.

Longfin eels are the largest and longest-lived fish in New Zealand's freshwaters, and where they are present they are the top predator.

New Zealand longfins are one of the largest eel species in the world.

The sex of tuna is not determined genetically, as in some other organisms.

Currently, the most reliable method to estimate the age of tuna is by examination of their otoliths or "ear bones"

Once eels become migrants (also known as tuna heke or tuna whakaheke) they stop feeding, and progressively develop the external features that clearly distinguish them from 'feeders'.

All species of freshwater tuna spawn at sea, although the spawning grounds of only four species are known with certainty worldwide.