The Whanganui River could be New Zealand's largest inanga spawning site.
Horizons Regional Council staff have pinpointed sites where inanga have laid eggs along the Whanganui River during a hands-on hunt recently.
Inanga are the most numerous of the five species that make up whitebait.
Horizons senior environmental scientist for water quality Logan Brown said their spawning, or egg laying, site at Whanganui is nearly 13km long and larger than the site discovered at Whirokino on the Manawatu River which was previously considered the largest known spawning site in New Zealand.
Mr Brown says that to ensure whitebait remain available for people to catch, it's important to identify those areas that are used for spawning as they're vital in providing future generations of fish.
"Using old data on spawning areas as a starting point, we have identified a very large area containing spawning sites during egg hunts over the last two years," Mr Brown says.
"Numerous clutches of inanga eggs were located in the grass along the river bank, and we've marked these with GPS. It was great to identify these spawning areas and find a good number of eggs.
"Other spawning sites we have identified are on the Kai Iwi and Mowhanau streams."
Horizons staff will work with land owners to look at options to enhance areas where spawning occurs.
This mainly involves ensuring the right sort of vegetation is present for the fish to spawn on. Inanga are unique in that they migrate inland from the sea, grow from juvenile whitebait to adults in freshwater, and then migrate downstream to spawn within the tidal influenced zone of our rivers.
Their spawning requirements are very specific and are limited to a very small area of our rivers.
A threatened native species, inanga spawn in grasses along the banks of rivers near the coast and above the height of regular tides.
The eggs hatch during the higher spring tides and are carried out to sea.