Provenance of whitefish in the Gulf of Bothnia determined by elemental analysis of otolith cores

The strontium concentration in the core of otoliths was used to determine the provenance of whitefish found inthe Gulf of Bothnia, Baltic Sea. To that end, a database of strontium concentration in fish otoliths representingdifferent habitats (sea, river and fresh water) had to be built. Otoliths from juvenile whitefish were thereforecollected from freshwater ponds at 5 hatcheries, from adult whitefish from 6 spawning sites at sea along theFinnish west coast, and from adult whitefish ascending to spawn in the Torne River, in total 67 otoliths. PIXE was applied to determine the elemental concentrations in these otoliths. While otoliths from the juveniles raised in the freshwater ponds showed low but varying strontium concentrations (194–1664 μg/g,), otoliths from seaspawning fish showed high uniform strontium levels (3720–4333 μg/g). The otolith core analysis of whitefish from Torne River showed large variations in the strontium concentrations (1525–3650 μg/g). These otolith data form a database to be used for provenance studies of wild adult whitefish caught at sea. The applicability of the database was evaluated by analyzing the core of polished otoliths from 11 whitefish from a test site at sea in the Larsmo archipelago. Our results show that by analyzing strontium in the otolith core, we can differentiate between hatchery-origin and wild-origin whitefish, but not always between river and sea spawning whitefish.