We evaluated the ability of oxytetracycline hydrochloride (OTC) and calcein (CAL) to double-mark otoliths in largemouth bass (Micropterus salmoides, LMB) fry and fingerlings. To observe longevity of marks, fish were sampled at six-month intervals for two years. Marks on fry otoliths disappeared rapidly regardless of chemical used; most marks were not visible after 180 days. Marks from CAL on fingerling otoliths were short-lived; 50% were not visible after 180 days. Marks from OTC on fingerling otoliths were visible on 100% of treated fish and were retained throughout the two years of study. Immersion-marking with OTC of fingerlings is a simple, effective, and relatively long-lasting technique for mass-marking LMB for various fisheries assessments. We successfully double-marked fingerling LMB with OTC when we imposed a two-week interval between marks. Alternate marks with OTC and CAL were unsuccessful because readers were often unable to distinguish between the OTC and CAL marks.
