Tracking Baltic hypoxia and cod migration over millennia with natural tags.

Growing hypoxic and anoxic areas in coastal environments reduce fish habitat, but the interactions and impact on fish in these areas are poorly understood. Using "natural tag" properties of otoliths, we found significant correlations between the extent of Baltic Sea hypoxia and Mn/Ca ratios in regions of cod (Gadus morhua) otoliths corresponding to year 1 of life; this is associated with elevated bottom water dissolved manganese that increases with hypoxia. Elevated Mn/Ca ratios were also found in other years of life but with less frequency.

Prehistoric versus modern Baltic Sea cod fisheries: selectivity across the millennia.

Combining Stone Age and modern data provides unique insights for management, extending beyond contemporary problems and shifting baselines. Using fish chronometric parts, we compared demographic characteristics of exploited cod populations from the Neolithic Period (4500 BP) to the modern highly exploited fishery in the central Baltic Sea. We found that Neolithic cod were larger (mean 56.