If trout invasions into the habitat of rare galaxias continue, at least two of Otago's threatened species could become extinct in the next 20 to 30 years, the Department of Conservation says.
That is on top of the 20% of Otago's rare fish already lost in the past 13 years, Doc Otago freshwater ranger Pete Ravenscroft said.
''These losses are alarming.''
The Central Otago roundhead and the Clutha flathead galaxias, of which there were only two secure populations in the region, were ranked nationally endangered and nationally critical (the highest threat classification) respectively.
In all, three species of fish only found in Otago had the highest threat classification, five were nationally endangered, two were nationally vulnerable and one was declining.
''Something has to be done now to prevent future losses.''
Despite measures taken by Doc and other agencies, trout were still moving into new waterways without human assistance.
''We thought after 150 years that they would have moved into all possible waterways but their drive to expand continues, to the detriment of our native fish.''
If those trout invasions continued, those two species especially could be lost within decades, he said. Of Clutha flathead populations, 60% had already become extinct.
Doc had reviewed its three freshwater fish recovery plans to assess where there had been improvements, where there were gaps and what more could be done, he said.
It was working with private landowners, Fish and Game and regional councils in areas where there were threatened galaxiids to remove trout and install barriers to prevent future trout movement, protect habitat and improve water quality and fish passage, he said.
''We are encouraged by the regional council's active management of these fish, which can be outside normal consent conditions.''
However a lot more needed to be done and it was a ''bigger problem than Doc can handle on its own'', he said.
Streams which contained the fish were usually small, less than 1m wide, and provided little recreational benefit to anglers.
Fish and Game Otago operations manager Ian Hadland said Fish and Game Otago had been working alongside Doc to minimise the impact of sport fish on some unique populations and previously assisted with trout removal operations and supported the construction of trout barriers.
If trout were moving into marginal habitats it was possible it was caused by less-than-ideal conditions in rivers.
''As noted by Doc, land-use changes, water abstraction and decreasing water quality are also issues which have an impact of freshwater species and require further effort from all parties to address.''
Fish and Game Otago policy, in place for more than a decade, prohibited the deliberate introduction of sports fish where they were not already present. It would continue to work with Doc to reduce the impact of sports fish on sensitive galaxiid populations. Freshwater ecologist and Working Waters Trust director Lan Pham said it was a tragedy that New Zealand was losing those species before they even got to know them.
''The hope lies in enthusiastic landowners and schools in places like the Pomahaka, Lawrence, Kyeburn and Kakanui, where local communities are deciding to get behind their local specials and are mucking in to ensure their survival.''
Fish on the brink
Galaxiid's new classification areas they occupy
Nationally critical:
• Teviot flathead galaxias, 0.5ha
• Clutha flathead galaxias,11.8ha
• Lowland longjaw galaxias, 18.7ha
Nationally endangered:
• Central Otago roundhead galaxias, 22.4ha
• Dusky galaxias, 7.3ha
• Eldon's galaxias, 5.4ha
• Manuherikia alpine galaxias, 17ha
• Nevis galaxias, 4.7ha
• Pomahaka galaxias
Nationally vulnerable:
• Gollum galaxias, 40.8ha
• Taieri flathead galaxias, 21.5ha
• Declining:Southern flathead, 97.1ha
