The prerequisites for sustainable management of fish stocks is knowledge about the stocks geographic distribution and the extent of migration between adjacent management units. In recent decades the use of otolith chemical fingerprints for stock identification purposes has gained increasing interest. Owing to their time-keeping properties and their lifelong record of environmental history, otoliths are a useful tool for studying stock affiliation and individual fish’s migration patterns. Otoliths consist of calcium carbonate, organic matrix and small quantities of trace elements. Trace elements are absorbed primarily from the water across the gill surface and therefore provide a record of environmental conditions experienced by the fish. The chemical composition of the water depends on the geochemistry of the surrounding catchment and therefore provides an area-specific chemical “fingerprint”, which is reflected in the fish’s otoliths. Otolith chemistry has over the last three decades gained increasing attention as a tool for analysing fish stock dynamics, migration patterns, and connectivity between areas, and plays an increasingly important role as a fisheries management tool. In this study we will demonstrate the power of this approach to identify large-scale movement patterns of cod in the transition zone between the North Sea and the Baltic Sea. This area is ideal for such studies because the environmental conditions are dominated by a pronounced horizontal salinity gradient and a progressively more coastal-type environment in the Kattegat, Sound and western Baltic Sea compared to the North Sea. The cod stocks in these waters are known to consist of genetically unique populations with overlapping distribution areas. With the current low stock sizes of cod it is crucial to identify each stocks’ spatio-temporal distribution for sustainable management. Our results demonstrate the need for a stock assessment that takes these complex drift/migration patterns into account
