The rare whio or blue duck has had a population boost after 31 captive-raised birds were released on the West Coast this season (File photo).

The odds of seeing the rare duck on our $10 note have gone up on the West Coast, after a successful captive breeding season.

Thirty-one whio (blue duck) have been released into two West Coast sites this season. Nationally, there are up to 3000 of the birds spread across both islands, making them rarer than kiwi. They are considered nationally vulnerable.

This week, 19 captive-reared whio were released into the Southern Alps near Hokitika, where an extensive trapping programme has been operating for nearly 15 years. It started on the Styx River and has been expanded to include the Arahura, Taipo and Kawhaka rivers, with the aim of growing whio number in the area to 50 pairs.

DOC/SUPPLIED DOC staff release rare cative-reared whio (blue duck) into the Southern Alps near Hokitika.

Whio reared in the captive breeding programme have also been released into two sites in the Paparoa Ranges north of Greymouth.

READ MORE:

* Man's best friend helps endangered native whio to survive

* In search of the elusive whio, NZ's endangered blue duck

* Whio numbers increase 48 per cent in Kahurangi National Park

The elusive birds can live only in areas with fast-flowing rivers of good water quality and with good supply of plants and underwater insects. Their presence is an important indicator of ecosystem health.

DOC/SUPPLIED The released whio were reared in a South Island captive breeding programme, and originate from four breeding pairs.

Whio are vulnerable to stoats, ferrets and feral cats, and are most affected during nesting time and when females are in moult and cannot fly. Because they need fast-flowing freshwater to live, they cannot be moved to predator-free islands.

Pairs have territories spanning about 1 kilometre of river, which they are very protective of. This means they need a lot of river to support a large population.

Captive breeding is effective in increasing the fragile whio population. In the North Island, it has been used to repopulate Mt Taranaki.

Some conservation work for whio is funded by Genesis Energy through the Whio Forever programme. Environmental coordinator Cam Speedy said it was gratifying to see so many whio released this season.

"Hopefully the birds released this season will find mates, breed, and continue to grow the population of these endangered native birds."