Oxygen and carbon isoscapes for the Baltic Sea: Testing their applicability in fish migration studies

Conventional tags applied to individuals have been used to investigate animal movement, but these methods require tagged individuals be recaptured. Maps of regional isotopic variability known as isoscapes offer potential for various applications in migration research without tagging wherein isotope values of tissues are compared to environmental isotope values. In this study, we present the spatial variability in oxygen (delta O-18(H2O)) and dissolved inorganic carbon (delta C-13(DIC)) isotope values of Baltic Sea water. We also provide an example of how these isoscapes can reveal locations of individual animal via spatial probability surface maps, using the high-resolution salmon otolith isotope data from salmon during their sea-feeding phase in the Baltic Sea. A clear latitudinal and vertical gradient was found for both delta O-18(H2O) and delta C-13(DIC) values. The difference between summer and winter in the Baltic Sea delta O-18(H2O) values was only slight, whereas delta C-13(DIC) values exhibited substantial seasonal variability related to algal productivity. Salmon otolith delta O-18(oto) and delta C-13(oto) values showed clear differences between feeding areas and seasons. Our example demonstrates that dual isotope approach offers great potential for estimating probable fish habitats once issues in model parameterization have been resolved.