Comparison between cod from the eastern Baltic Sea caught in the Sea of Åland (left image) and in the southern Baltic Sea (right image). The same person (Maddi Garate Olaizola) appears in both images to give an idea of size. The researchers estimated the large cod to be 13 years old, while the small one is probably around 3 or 4 years old. Photo: Yvette Heimbrand and Olof Lövgren

Healthy cod in the Åland Sea carry a chemical mystery

News published:  15/09/2025

Researchers at the Swedish University of Agricultural Sciences (SLU) have made a remarkable discovery in cod from the Åland Sea. Compared to their relatives in the rest of the Baltic Sea, the Åland cod have extremely high levels of boron in their otoliths. But where it comes from remains a mystery.

The situation is critical for cod in the Baltic Sea. Most of their spawning and nursery grounds have disappeared due to oxygen depletion caused by climate change and eutrophication. Despite the closure of cod fishing in 2019, the species is still in poor condition, with severely reduced growth, and many individuals are skinny and infested with parasites. In the Åland Sea, however, the cod are thriving – many of them are large and fat. The key to their healthy condition may lie in the good oxygen levels that reach down to 300 meters in depth and the availability of the bottom-dwelling crustacean Saduria entomon – known in Swedish as “skorv” – a favorite on the cod’s menu. 

– They are like cod from the good old days, says Yvette Heimbrand, researcher at the Department of Aquatic Resources at SLU.

Together with visiting researcher Karin Limburg, Yvette Heimbrand has analyzed the chemical composition of otoliths from cod in the Åland Sea. Otoliths are small calcium carbonate structures in the fish’s ear that form growth rings, much like the rings of a tree. They store trace elements that can reveal where a fish has lived. To their surprise, the researchers discovered extraordinarily high levels of boron in cod otoliths from the Åland Sea – up to several hundred times higher than in other Baltic cod*. They also observed elevated boron concentrations in the otolith core, which forms when the fish is born and only a few days old.

– Much about this discovery is still unclear, for example where the boron comes from and how the fish take it up. These are questions we don’t yet have the answers to, but which we will continue to investigate, says Yvette Heimbrand.

In connection with their discovery, the researchers also measured boron levels in otoliths from cod caught in the Åland Sea during the 1940s and in 2013, and found very low concentrations.

– This suggests that there is a new, unknown source of boron in this part of the Baltic Sea, even though its magnitude and distribution remain a mystery, says Yvette Heimbrand.

Photo of a cross section of an Åland Sea cod otolith, with its boron data (purple line) superimposed to match. Note the peak of boron in its core (location 0). The laser transect that was made to measure the boron is visible running across the otolith. The otolith cross section is about 10 mm (1 cm).

In a recently published article, the researchers highlight these special cod and conclude that we need to take extra care with this unique group in the Åland Sea to avoid the risk of losing it and its distinctive traits.

*It is important to note that these are still trace element levels measured in ppm (parts per million). The cod are not suffering from boron poisoning.

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

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