Marking the otoliths of hatchling cod: A method comparison

The Eastern Baltic cod (Gadus morhua) stock has declined severely due to decades of overfishing, habitat degradation, and eutrophication. Restocking has been proposed as one potential measure to support recovery, but effective evaluation of such efforts requires reliable larval marking techniques. This study investigated three otolith marking methods; strontium immersion, alizarin complexone immersion (30 and 50 mg/L), and thermal marking, for newly hatched Eastern Baltic cod larvae. We assessed mark visibility as well as potential effects on larval survival, length, and deformities. Strontium produced clear and detectable marks, assessed using micro-XRF analysis, without adversely affecting larval development. Alizarin also created distinct marks at both concentrations, but the 50 mg/L treatment showed a possible tendency toward reduced survival and increased deformity severity. Thermal marking failed to produce visible otolith marks under light microscopy and was associated with lower survival and elevated developmental issues. This study identifies safe and effective marking methods for early life stages, offering crucial guidance for developing traceable and biologically sound stocking strategies for the vulnerable Eastern Baltic cod stock. The results highlight strontium and low-dose alizarin as effective and biologically safe options for marking larvae, with strontium emerging as the most promising method for large-scale application. These tools provide practical means to evaluate restocking success in future management strategies.