Mass-marking of brown trout (Salmo trutta L.) larvae by alizarin:: method and evaluation of stocking

This study describes otolith marking of brown trout (Salmo trutta L.) larvae by immersion in different solutions of alizarin red S (ARS). The best results were obtained after marking with ARS at a concentration of 150 mg L-1. To evaluate the efficiency of stocking with brown trout fry, 10 000 20-day-old larvae were marked in years 2002 and 2003 with ARS and released 2 weeks later into sections of a river with natural brown trout reproduction. Electro-fishing surveys carried out 2 months after stocking in 2002 revealed that only 4.8% of all caught young-of-the-year trout originated from stocking; in 2003 the percentage was 8.9%. Based on the substantial natural reproduction and the low ratio of stocked to wild trout, it was recommended to discontinue stocking.