Testing a mass marking technique using alizarin complexone on reared European flounders (Platichthys flesus) to quantify restocking efficacy

Since 1993, more than 1.6 million reared European flounders (Platichthys flesus) have been released in the Limfjorden to support stock enhancement. Despite this effort, little is known about their survival, growth and contribution to the local and regional fishery. Tagging hatchery-reared fishes prior to release enables differentiation from wild stocks and assessment of restocking success. Alizarin complexone (ALC) is a fluorescent dye and a well-known mass-marking technique that leaves a lasting mark in otoliths without harming the fish when applied with an appropriate dose. However, the optimal dose varies by species and life stage. While ALC tagging has been used in related species such as Japanese flounder (Paralichthys olivaceus) and turbot (Scophthalmus maximus), no established protocol exists for European flounder. We tested a range of ALC concentrations (0–100 mg/L) on reared age-0 European flounder (n = 136; T_L: 31–110 mm) via 24-hour immersion in ALC solutions to identify the most optimal, non-lethal dose. In addition, a subsample (n = 15; T_L: 79–103 mm) was double-tagged with ALC, 21 days apart, to evaluate the potential of separating close cohorts and tracking growth. Preliminary results indicate that concentrations of 50 mg/L or higher produce clear, detectable marks under fluorescence microscopy, and that double tagging can be used to separate close cohorts. To our knowledge, this is the first study to apply ALC immersion tagging to European flounder, offering a promising and practical tool for long-term monitoring of restocking efforts.