Elevated boron in cod otoliths from the low-salinity Åland Sea: A unique marker for a new population in the Baltic Sea?

Objective The purpose of this study was to understand the possible provenance of a unique population of Atlantic Cod Gadus morhua that is found in the low-salinity Åland Sea region of the northern Baltic Sea. This population consists of large, healthy individuals, in contrast to the Atlantic Cod in the rest of the Baltic Sea. Methods We used laser ablation inductively coupled plasma mass spectrometry to measure levels of boron (as B:Ca) in the otoliths of Atlantic Cod in regions throughout the Baltic Sea. We examined both lifetime chronologies and concentrations in the core region that corresponds to birth and early life. Results We found that B:Ca concentrations were 31 to 348 times higher in the otoliths of cod that occupy the Åland Sea, including in the core region. These concentrations were much higher than expected given that boron is linearly, positively proportional to salinity, which is higher in the southern Baltic Sea, and other populations displayed very low concentrations by comparison. Conclusions Based on the otolith B:Ca as a unique marker, we suggest that the cod that are sampled in the Åland Sea may be a separate population from those that inhabit the rest of the Baltic Sea. This would not prevent it from mixing with other populations but could point to a separate spawning area. The source of the elevated boron is currently unknown, but the widespread occurrence in cod otoliths from the Åland Sea indicates an extensive nonpoint source.