Experimental validation of Sr uptake in juvenile pike (Esox Lucius L.) otoliths – from water and food.

• Strontium (Sr) or Sr:Ca ratios in otoliths have been widely used in the last decade to describe migration histories of fish between fresh and marine waters. However, reference experimental studies on particular species and waters are necessary to confirm the underlying assumption and evaluate the usefulness of this tool for field data. We conducted an experiment with the aim to answer these questions for anadromous pike (Esox lucius L.) in the Baltic Sea. Juvenile pike were reared in successively increasing salinities (10 steps from 0 to 7 ‰) for 110 days, and then in constant maximum salinity (7 ‰) for an additional 50 days. Pikes in the experiment were divided into two food treatments, given prey fish from brackish environments (7 ‰) and fish from freshwater lakes. Sr:Ca in pike otoliths were positively related to Sr in water (i.e salinity) in both treatments, suggesting that Sr:Ca ratios may be used to describe migration histories between rivers and the Baltic Sea. A significant difference in Sr.Ca ratios was found between the food treatments, showing that differences in diet may contribute to variation in otolith Sr:Ca, at least for pike in the Baltic Sea. Maximum Sr:Ca values for pike given marine food corresponded with field collected fish from the Baltic Sea.

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