Galaxias is the common name for a large group of smallish, highly successful freshwater fish in the Galaxias genus and Galaxiidae family. Galaxias are restricted to the southern hemisphere, and generally only occur in temperate latitudes; only one species is known from sub-tropical habitat (McDowall, 2006). Before European settlement, galaxias were the dominant group of native freshwater fish in New Zealand, and, along with the Percichthyidae, one of two dominant groups of native fish freshwater in south-eastern Australia.
Galaxias are cool water species, with many wholly freshwater species specialising in high altitude upland streams (including very small streams), rivers and lakes. Some galaxias species include a marine stage in their life cycle where larvae are washed out to sea where they develop and return to rivers as juveniles. These species are consequently also found in low altitude habitats, but frequently migrate to high altitude reaches of river systems in their adult stage.
Wholly freshwater galaxias species are gravely threatened by exotic salmonid species, particularly exotic trout species, which prey heavily upon galaxias and compete with them for food. This is a major concern as exotic trout species have been recklessly introduced to many different landmasses (e.g. Australia, New Zealand), with no thought as to impacts on native fish such as galaxias, and no attempt to preserve some exotic-trout-free habitats for native fish.
In most situations, wholly freshwater galaxias species show a complete inability to survive in the presence of exotic trout species, and many wholly freshwater galaxias species now occur only in the rare trout-free habitats still available to them. Numerous localised extinctions of wholly freshwater galaxias species (i.e. mountain galaxias) have been caused by the introduction of exotic trout species (including ongoing illegal stockings) and a number of wholly freshwater galaxias species are threatened with overall extinction by exotic trout species and other exotic salmonids (McDowall, 2006).