Ear bones will tell spawn sites

Ears tell a tale . . . Masters student Olivier Raven, from Wageningen University in the Netherlands, inspects an otolith extracted from a brown trout caught on January 22 in the Pomahaka River. PHOTO: BRUCE QUIREY

A long-term project to determine the key trout spawning tributaries of the lower Clutha/Mata-Au River is set to enter the next stage.

The brown trout origin project initiated by Otago Fish & Game aims to better direct work to protect and improve the fishery.

Initial stages of the project involved gathering otoliths, or ear bones, from almost 1000 brown trout from the catchment intensively over the past four years.

They include 800 juvenile trout collected from tributaries by Fish & Game and almost 200 adult fish caught by anglers.

Fish & Game ecologist Jayde Couper said the lower Clutha/Mata-Au was a regionally significant trout fishery that supported up to 16,660 angler days a year.

‘‘Trout need clean-flowing streams to spawn.

‘‘The research will connect spawning sites with the adult fish we catch.

‘‘It’s valuable because it will allow us to identify key spawning sites and migration pathways both upstream and downstream.

‘‘Having this knowledge will allow us to protect these key habitats.’’

The research is being carried out by masters student Olivier Raven, from Wageningen University in the Netherlands, in collaboration with Prof Gerry Closs and Dr Malcolm Reid from the University of Otago departments of zoology and geology.

Mr Raven, visiting Dunedin for six months to prepare and analyse the otoliths, said he was excited to be part of the project.

Using a high-powered laser and a mass spectrometer, he would first analyse the otoliths of juvenile trout.

‘‘The otoliths build up in layers, much like a tree trunk, and the chemistry of these layers relates to the chemical makeup of the waterway the fish lives in.’’

His supervisor Prof Closs said because four years of juvenile otoliths had been collected, research could determine whether the trace element signatures in the otoliths were the same from year to year for each stream.

‘‘If they are the same, we can match any adult back to the stream where it was spawned.

‘‘This is achieved by analysing the centre of the adult otoliths and comparing them to the juveniles’ trace element signatures.’’

The Clutha project complements work the university has done over about 20 years on the Taieri River catchment to understand relationships between spawning and adult fish.

The work is funded by Contact Energy as part of mitigation for the hydro scheme on the Clutha/ Mata-Au River.

‘Having this knowledge will allow us to protect these key habitats. ,