The remaining habitat of a critically endangered freshwater fish in the Kosciuszko National Park needs to be fenced off urgently to protect it from feral horses, scientists and campaigners say.

A three-kilometre stretch of the Tantangara Creek is the only place known to harbour the stocky galaxias, a fish that was described as a distinct species only five years ago.

The stocky galaxias is a critically endangered freshwater fish in Kosciuszko National Park. Credit:University of Canberra

The National Parks and Wildlife Services said it has been discussing with the University of Canberra ways the fish and its environment can be protected. "Fencing may be one option," a spokesman said.

Mark Lintermans, a fisheries ecologist with the university, said invasive trout had reduced the galaxias' range to a short stretch of the creek.