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, and have been recorded at sizes of almost 2 m in length and more than 50 kg in weight.

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

Knowing the age (i.e. proportion of young vs. old) of eels in an eel population provides an understanding of how fast they grow and may give an indication of the pressures faced by tuna in a particular environment or habitat.

Every year, a proportion of eels mature and migrate to sea to spawn. 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.