Canaries in the Baltic Sea “Coal Mine”: Cod Otoliths Document Accelerating Climate Impacts

Abstract Anthropogenic deoxygenation of the Baltic Sea caused major declines in demersal and benthic habitat quality with consequent impacts on biodiversity and ecosystem services. We employed Baltic cod as “canaries in the coal mine,” using otolith chemical proxies of hypoxia, salinity, and fish metabolic status and growth to track changes from baseline conditions in the late Neolithic (4500 YBP) and early 20th century to the present. Otolith hypoxia proxies (Mn:Mg) increased with expanding anoxic water volumes, but decreased with increasing salinity indexed by otolith Sr:Ca. Metabolic status proxied by otolith Mg:Ca and reconstructed growth were positively related to dissolved oxygen percent saturation, with particularly severe declines since 2010. This long-term record provides further evidence of a profound state change in oxygen for the worse, in one of the world’s largest inland seas. Spreading hypoxia will likely impair fish populations globally and evidence can be tracked with otolith chemical biomarkers.