Mass‐marking of farmed European eels (Anguilla anguilla (Linnaeus, 1758)) with alizarin red S

The Working Group on Eel of the International Council for the Exploration of the Sea (ICES) regularly reports that a significant amount of stocked eels in Europe was pre‐grown in aquaculture farms prior to stocking—so called “farmed eels.” The ICES advices chemical marking of stocked recruits to ensure their traceability throughout all life stages. To date, however, there was a lack of knowledge concerning the most suitable chemical substance and its application on farmed eels. The aim of this study was to fill this gap by presenting successful attempts of marking those eels with alizarin red S (ARS). An ARS concentration of 150 mg L^?1 buffered with 150 mg L^?1 Tris(hydroxymethyl)aminomethane applied as an immersion bath over 9 h was sufficient to mark a total of 3572 kg of farmed eels (6.5–8.0 g mean body weight). The marking success was 100% on otoliths and highest stocking density of up to 67.1 kg m^?3 (corresponding 54.0 kg m^?2) turned out to have no effect on mortality which was consistently below 1%.