Micro‐X‐ray fluorescence image analysis of otoliths to distinguish between wild‐born and stocked river‐spawning whitefish captured in the Baltic Sea

Abstract

Strontium concentrations are low in fresh waters compared to seawaters. Therefore, wild‐born river‐spawning and stocked freshwater‐reared whitefish Coregonus lavaretus L. display regions with low concentrations of strontium in the centre of their otoliths. Strontium in otoliths from wild‐born river‐spawning whitefish ascending the River Tornionjoki, river‐spawning whitefish stocked as one‐summer‐old fingerlings caught ascending the River Kemijoki, and sea‐spawning whitefish caught near the Åland Islands was mapped using µ‐XRF. The strontium‐depleted regions in the centre of the whitefish otoliths, measured using ImageJ, had mean sizes of 0.18 ± 0.2 mm 2 (River Tornionjoki) and 2.3 ± 0.3 mm 2 (River Kemijoki), whereas the otoliths from whitefish caught at sea lacked a strontium‐depleted region altogether. Measurement of the area of the strontium‐depleted region in whitefish otoliths provides a convenient method with which to differentiate between whitefish of different provenance and to determine the origins of whitefish in mixed catches.

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