At 60 d posthatch. larval Atlantic salmonSalmo salarwere immersed in water containing calcein at 125 or 250 mg/L or oxytetracycline at 250 mg/L or in untreated control water. Immersions were static, 48-h treatments. All fish immersed in calcein solutions acquired a chemical mark visible microscopically under long-wave ultraviolet light and manifested as apple-green fluorescence in calcified structures, including fin rays. No fluorescence was detected in fish immersed in oxytetracycline or untreated water when viewed in the same manner. Mortality from the time of immersion through day 10 was significantly higher (P≤ 0.05) in 250 mg calcein/L (10.5%) than in 125 mg calcein/L ( 1.0%). At 234 d postimmersion, the calcein mark was detected non-lethally in samples of caudal fin tissue from over 93% of calcein-treated fish examined. Results suggest that calcein may be a valuable tool for mass-marking larval fish for long-term hatchery product evaluation.
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