Non Technical Summary

Over 3 million trout are produced annually by NSW's hatcheries to supplement trout populations for recreational fishing. There has been no assessment of the effectiveness of this stocking program and this study was initiated to develop a method for mass-marking hatchery-reared rainbow trout fry. Such a technique would allow one to discern hatchery fish from wild fish and so estimate the survival of hatchery-reared fry in the wild. Thermal induction has been used to mark otoliths of other fish species and involves the exposure of fish fry to alternating periods of warm and cold water. This treatment induces identifiable marks on the otolith (or earbones) of the fish which can recognised throughout the life of the fish.

Two thermal treatments were tested on rainbow trout in this study and both treatments produced clear marks on the earbones. However, the fish treated for the coldest but shortest water temperature periods showed the clearest patterns on the otoliths.

We have demonstrated that thermal induction can be used as a reliable method for marking hatchery-reared rainbow trout fry. This research marking method will be compared with a chemical marking method which has also been trialed. The most practical and cost-effective method will be selected and applied to batches of hatchery-reared fry and used to evaluate the success of stocking rainbow trout in NSW.