Thermal marking of rainbow trout (Oncorhynchus mykiss) otoliths

A small‐scale experiment was done to test the feasibility of thermally marking hatchery‐reared rainbow trout (Oncorhynchus mykiss Walbaum) fry that are released into rivers and impoundments in New South Wales (NSW), Australia. Fry of rainbow trout were exposed to two different 48‐h thermal cycles each of a cold and warm water period. One thermal regime consisted of a cold water period during which the temperature was reduced from 14 to 8°C for 18 h followed by a return to 14°C for 30 h. For the second thermal treatment, water temperature was reduced to 4°C for 10 h followed by a period of 38 h at 14°C. Thermal cycles were repeated 4 and 8 times for each thermal regime, respectively. Following a growth period after treatment, obvious marks were visible on all treated otoliths as distinct from control otoliths. The 10°C differential treatment created the most visible patterns and growth of these fish was not significantly different from control fish. This marking method could be applied to normal hatchery practices to evaluate the effectiveness of large‐scale rainbow trout stockings in NSW.