Mass-Marking Rainbow Trout and Brown Trout Fingerlings with Oxytetracycline

ABSTRACT

We immersed fingerling rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) in a 500-mg/L solution of oxytetracycline (OTC) for 6 h to determine marking efficacy. The technique marked 100% and 92% of rainbow trout and brown trout otoliths, respectively. In a blind test using known marked and unmarked otoliths, one reader correctly identified 99% of rainbow trout and 95% of brown trout otoliths as either marked or unmarked. Reading errors for rainbow trout (1 out of 78) and brown trout (8 out of 167) were of omission. Mortality of OTC-marked trout was less than 3.5% and statistically similar to control groups. Thirty-five thousand fingerling brown trout stocked into two Tennessee tail waters were treated on the hatchery truck with 500 mg/L OTC and all of the fish in a subsample (n = 95) that was held for 30 d were marked. When fish were recaptured up to 14 months post-stocking, OTC marks were subsequently detected on all brown trout otoliths known be treated (n = 23). We concluded that OTC immersion is an effective technique for mass marking fingerling rainbow trout and brown trout during short-term stocking evaluations.