Developing novel means for unravelling population structure, provenance and migration patterns of European whitefish Coregonus lavaretus s.l. in the Baltic Sea

We sought to investigate catch composition, provenance and migration patterns of an economically important coregonid, the European whitefish (EW, Coregonus lavaretus s.l.). EW displays complex life history and is heavily stocked in the Baltic Sea. By combining otolith geochemistry (Sr:Ca, Ba:Ca, 87Sr:86Sr) and gill raker counts we developed an assignment framework for determining the general ecological form (anadromous or sea-spawning) and provenance (wild or stocked) of EW adults caught in the mixed stock fishery. Among the adults sampled from the Estonian coastal sea (N = 82) 50% were stocked as fingerlings in Finland (1% were stocked as fingerlings in Estonia), and 35 and 14% were of wild anadromous and sea-spawning origin, respectively. Young-of-year individuals (N = 55) sampled from four geographically separated rearing facilities in Finland displayed highly distinct otolith natal fingerprints, therefore demonstrating great potential for pin-pointing the provenance of adult individuals that were stocked as fingerlings. Among the adults that were stocked as fingerlings 13 different otolith natal fingerprint groups were identified, however 81% of them originated from seven distinct sources. This study underscores the prevalence of stocked individuals in the EW mixed-stock fishery, and quantitatively demonstrates the low share of wild individuals. It is suggested that current stocking practices may lack the desired effect on wild populations, and they only serve to supplement the catches. The presented results will allow to monitor the structure of EW stocks in both local and regional scale.