The objective of this study was to test if pectoral fin rays, a widely used structure in ageing studies for live sturgeons, are reliable to assess mass-marking success in endangered sturgeon species, particularly European sturgeon Acipenser sturio. Marking success and thus the proper identification of fish stocked at a young age are critical to evaluate the effectiveness of conservation strategies. In this study, a total of 60 1-month old Siberian sturgeons Acipenser baerii was marked. Thirty specimens were marked by immersion in tetracycline hydrochloride solution at a concentration of either 300 ppm or 600 ppm. 30 additional specimens were immersed in alizarin red S solution at a concentration of either 100 ppm or 200 ppm. Four weeks after marking, the sturgeons were sacrificed and marking success was compared between pectoral fin rays and two other structures: otoliths and dorsal scutes. Marking success in pectoral fin rays was variable depending on the marking protocol, being as low as 7% for one of the marking treatments. In comparison, marking success in dorsal scutes exceeded 90% regardless of the treatment. Similar high success was also seen for otoliths marked with alizarin red S. Moreover, for all treatments, more than 20% of the marked pectoral fin rays marked showed discontinuous marks, which cannot be considered of acceptable quality for reliable mark detection. In comparison, only completely continuous marks were observed in dorsal scutes and otoliths. Auto-fluorescence and ontogenic formation in pectoral fin rays are hypothesized to cause mark detection deficiencies in this hard structure. Thus, pectoral fin rays should not be used to identify hatcheryreared sturgeons exposed to fluorescent markers at a young stage.