A group of exotic species belongs to the family Poeciliidae, sometimes known as the livebearers because they give birth to live young rather than lay eggs. There are five small livebearers in natural habitats in New Zealand – one (gambusia) was introduced for biological control, the others for aquariums.
Gambusia (Gambusia affinis) were introduced, probably in the 1930s, in the hope they would help control mosquitoes. This is unproven in New Zealand, and they may be no more useful than native fish. Gambusia have become increasingly widespread since the 1980s, and are now common from Waikato and Bay of Plenty northwards. A few populations have been reported in the northern South Island, where efforts have been made to exterminate them as they may be detrimental to native fish.
Resembling gambusia, caudo (Phalloceros caudimaculatus) have been reported from waters around Whangārei, but it is uncertain if there are wild populations. Aquarists have stocks that could become established if released into suitable habitats.
Common guppy, sailfin molly and swordtail
Three poeciliids liberated into the wild by aquarists now live in geothermally heated waters in the central North Island. The common guppy (Poecilia reticulata) has been found in streams near Reporoa, the sailfin molly (Poecilia latipinna) in a wetland at the southern end of Lake Taupō, and the swordtail (Xiphophorus helleri) in a stream near Taupō. None of these species is likely to become established in waters of normal ambient temperature, so their geographical range will be restricted. Some were once in the Waipāhīhī Stream, near Taupō, but when water flows were manipulated, temperatures rose and the fish died out.
North American brown bullhead catfish (Ameiurus nebulosus) were introduced for unknown reasons in the 1870s. They became abundant in the lower Waikato River, and were not found elsewhere for many decades apart from small numbers in Lake Māhinapua, near Hokitika, the only confirmed South Island population. Their range increased from the 1980s – perhaps spread by commercial eel fishers who caught them in their fyke nets (large, funnel-shaped nets that trap fish). Intermittent catfish populations are now widespread in northern waters, including Lake Taupō, although how they reached this lake is uncertain. They have spread from Lake Taupō to the series of hydro lakes in the Waikato River.
European perch (Perca fluviatilis) are a desired angling species, brought to New Zealand in the late 1860s. They are classed as ‘sports fish’, so anglers need a licence to fish for them. Perch are widespread but intermittent around the country. Concentrated populations are in the western and southern North Island, around Hokitika, and in the eastern and southern South Island. The fish has firm, white, tasty flesh, and is much the best eating of the exotic non-salmonid species.