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.
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.
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'.