The Department of Conservation (DOC) is installing trout barriers in some South Canterbury streams to stop the decline of critically endangered native fish.

DOC biodiversity senior ranger Dean Nelson said so far five trout barriers had been installed in some spring-fed streams to help stop the decline of three lowland longjaw galaxias species.

The lowland longjaw galaxias species were ranked "nationally critical" under a conservation status of threatened and were found only in the Waitaki catchment.

Supplied A juvenile lowland longjaw galaxias.

Nelson said DOC noticed the population decline as part of its annual fish monitoring in 2010, and installed its first trout barrier in a small spring-fed tributary of Fraser Stream, near Twizel.

It was an "experiment" to see if it worked and what would happen when the trout were removed.

"We noticed an immediate response."

About 80 millimetres to 90mm long, the lowland longjaws were "very rare" and were on the same level of threat as birds such as the black stilt and takahe, he said.

Predation by trout was the main issue without the barriers "the lowland longjaw galaxias will potentially go extinct".

"The springs provide clean water and associated invertebrates that the fish needs to survive and the loose [shingle] that it requires to burrow into for spawning."

Nelson said the trout barriers cost between $10,000 and $15,000 each, and so far had been funded by Environment Canterbury's (ECan) Immediate Steps Fund and DOC.

The barriers had also been installed in streams near Ahuriri and Hakataramea. They would also benefit two other threatened Waitaki River species - bignose galaxias and upland longjaw galaxias.

Two more barriers would be installed in Corbies Stream, which eventually runs into Lake Benmore, and the Fork Stream.

However DOC was still waiting to hear back about a new "global" resource consent being drafted by ECan, which would allow it to carry out similar work anywhere in Canterbury, Nelson said.