Northern pike (Esox lucius) inhabit the coastal areas of the Baltic Sea. A large part of the fishes show anadromous
behaviour and spawn in streams and rivers but spend most of the time foraging in the sea. We examined
spawning migration in four streams in the southwest part of the Baltic, situated within a radius of 50 km. Using
juvenile pike in the streams as references, otolith analysis by microPIXE revealed unique elemental patterns (Sr,
Zn, Br, Co and Mn) for the juveniles in each of the four different streams. The strontium signal in the otolith
of the juveniles was used as an indicator of freshwater origin and the time spent in the stream (size of juveniles).
Adult marked pike in their migrating spawning phase were caught in each of the streams and otoliths were
analysed. Defining earlier freshwater origin by the Sr signal from the otolith core to the increase in Sr when the
fish as juvenile pike migrated to the sea, element composition was determined. A principal component analysis
showed that the elemental fingerprint during the freshwater phase several years back was similar for adult fish
and juveniles inhabiting the stream today. The results indicated native homing of the adults to a specific stream,
a process further corroborated by results from electronic marking (Pit-tags) with the return of adult individuals
over several consecutive years. We interpret the results as evidence that pike in the Baltic Sea consists of several
sub-populations and are developed by homing to specific spawning streams. The results of the study may have
implications for fishery management as pike in the Baltic Sea cannot be seen as homogenous “stock“, but instead
consists of different, unique populations similar to the pattern demonstrated in salmon (Salmo salar).