Fish and Game Southland manager Maurice Rodway told a resource consent hearing yesterday the part of the river passing through the Ashton Flats to Mossburn, about 45km in length, was a brown trout fishery of international importance recognised by a Water Conservation Order.
In his opinion, it ''provided the most outstanding large brown trout fishing in the world''.
Its value strongly related to the surrounding wilderness and a commercial cycleway would significantly downgrade the fishery.
''Native fish and trout spawning areas could be disturbed and damaged by the construction activities required, which are very intrusive in an area of this nature,'' he said.
A proposed bridge over the upper reaches of the Oreti was ''almost as high as a two-storeyed house'' and would be ''pretty prominent'', Mr Rodway said.
The 175km trail will eventually link Kingston with Walter Peak Station, on the western shores of Lake Wakatipu.
It is estimated it could be used by up to 60,000 cyclists annually. Stage 1, from Kingston to Mossburn, opened in November.
The district council needs a resource consent for about 30km of stage 2.
A hearing began before independent commissioner Denis Nugent on Monday and is expected to finish today.
Consent was first sought in 2012 but a commissioner declined part of the application because of concerns about river works.
The new application attracted 226 submissions: 138 opposed, 83 in support and four neutral.
About 35 submitters will be heard.
Fish and Game favoured the trail following the Mararoa River, near the North and South Mavora Lakes, before rejoining the proposed route, Mr Rodway said.
Reports commissioned by the council had investigated the alternative route in a ''superficial way'', he said.
But Fish and Game considered the Mararoa route was comparable for scenic beauty and had the advantage of keeping cyclists, and construction, away from the fishery.
It would also avoid the need for a bridge over the Oreti and the construction of switchback tracks from the river berm up hills in the Three Kings Gorge.
Cyclists on the Mararoa route would be able to use Department of Conservation picnic and toilet facilities at the Kiwi Burn and the swing bridge over the river, Mr Rodway said.
