The revelations come after a four-year project collecting and analysing tiny brown trout otoliths (ear bones), to determine which spawning grounds fish caught originated from.
Research was completed at the University of Otago using a precise laser and mass spectrometer. The analysis was completed by Olivier Raven, a visiting master’s student from Wageningen University in the Netherlands.
He analysed about 200 otoliths, of which just over a third yielded data about the fishes’ birthplaces.
The catchment extends from below Roxburgh Dam to the sea, and includes historically renowned trout rivers such as the Pomahaka and Waipahi.
"To best manage the fishery and ensure sustainability, understanding the migratory patterns of brown trout is crucial," Mr Raven said.
"We identified the birthplaces of adult trout caught by anglers in locations often significant distances from where the fish had originated."
He said the otoliths built up in layers, much like tree trunks.
The chemistry of the layers related to the chemical makeup of the waterways the fish lived in at different stages of its life, allowing for accurate tracking.
The key spawning streams identified in the project were Benger Burn, Heriot Burn, Coal Creek, Crookston Burn, Carsons Creek and Jimmy’s Creek.
"A next step will be to make protection of these streams a priority," Mr Raven said.
"Further analysis of the streams may provide valuable insights and strategies for enhancing other tributaries."
University of Otago zoology professor Gerry Closs said he was pleased the project had helped identify the key spawning sites.
"Knowing which tributaries supply the most recruits assists Fish & Game with protection and enhancement of key spawning reaches and gravels in these streams, and improves understanding of how brown trout migrate and disperse through the extensive Clutha River catchment."
The project was funded by Contact Energy as part of mitigation for the Clutha hydro-electrical dam scheme.
