The Blue duck is a native of New Zealand, where its Maori name is “whio”. It is the only member of the family Hymenolaimus. It inhabits fast-flowing inland streams, nesting along the riverbanks, where predation is a threat. It is a powerful swimmer and generally reluctant to fly.

The blue duck is also known by its Maori name, Whio, and is endemic to New Zealand. It is a small grey-blue duck with chestnut chest feathers and is superbly camouflaged against the back-drop of raw river rocks. It is one of only three ducks world-wide that exclusively live on fast flowing rivers and is well adapted for this environment.

The presence of blue duck indicates good stream health. They can dive under water for up to 20 seconds. Blue duck are a protected species. Blue duck are specialised whitewater feeders feeding mainly at dusk and dawn on aquatic insects. The usual breeding age is two years or older.

The blue duck is 53 to 54 cm (21–21 in) long and varies in weight by sex. Females are smaller than males, weighing 680–870 g (24–31 oz), whereas the males weigh 820–1,077 g (28.9–38.0 oz). The plumage is a dark slate-grey with a greenish sheen on the head, a chestnut-flecked breast.