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Abstract

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 & Aring;land 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 & Aring;land 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 & Aring;land 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 & Aring;land Sea indicates an extensive nonpoint source.In 2021, we found that high levels of boron, an element that is rarely studied in fish otoliths, could distinguish an unusual population of cod that inhabits the low-salinity waters in the part of the northern Baltic Sea that is known as the & Aring;land Sea. Furthermore, the high boron (measured as boron in ratio to calcium) appears to be recent, as & Aring;land Sea otoliths that were collected in 1948 and 2013 contained much lower concentrations of boron, similar to the levels that were found in other sites in the Baltic Sea. The elevated B:Ca in modern Atlantic Cod otoliths from the & Aring;land Sea does not correlate with salinity, in contrast to seawater B:Ca that does so. Although the source of this unique signal is unknown, it serves as a biomarker for a highly unusual population of brackish-tolerant Atlantic Cod.

Keywords

Baltic Cod; microchemistry; otolith boron

Published in

Marine and Coastal Fisheries
2025, volume: 17, number: 4, article number: vtaf022
Publisher: OXFORD UNIV PRESS

SLU Authors

UKÄ Subject classification

Fish and Aquacultural Science

Publication identifier

  • DOI: https://doi.org/10.1093/mcfafs/vtaf022

Permanent link to this page (URI)

https://res.slu.se/id/publ/143214