New study finds Baltic Sea cod healthier than expected
A new study of cod in the Baltic Sea found no evidence of thiamine deficiency – a condition that has been linked to increased mortality in several marine species. The findings suggest that Baltic Sea cod might not suffer from thiamine deficiency to the extent previously thought, or that it fluctuates over time.
Thiamine deficiency has been a known issue among Baltic Sea salmon since the 1970s, causing large die-offs of young fish. Over the past decade, several other fish and bird species in the region have also been reported to suffer from the condition. In. 2020, a study reported severe thiamine deficiency in Baltic Sea cod, suggesting that the vitamin shortage might be contributing to the species' decline.
Now, a new study from Linnaeus University indicates that the Baltic Sea cod might not suffer from thiamine deficiency to the extent previously thought. The researchers have analyzed cod from various Baltic Sea locations, where thiamine deficiency has previously been reported, and compared them to cod stocks in the Åland Sea, one of the last places in the Baltic Sea where cod still grow to large sizes, and the North Sea, where thiamine deficiency has not been observed.
"We found no clear differences in thiamine concentrations between the assessed cod stocks. Furthermore, none of the fish we examined showed signs of an impaired metabolism that thiamine deficiency would cause. This is good news for a fish species that has been heavily declining in numbers and size in the Baltic Sea. We urge managers to continue assessing thiamine deficiency in cod over time, since it is known to fluctuate among years in other affected species", says Marc Hauber, doctoral student at the Department of Biology and Environmental Science.
Thiamine deficiency is just one of many challenges for Baltic cod. The populations have rapidly declined in number, size, and condition over the last 50 years due to overfishing, degradation of feeding and spawning grounds, and shifts in the ecosystem. Though strong regulations have been put in place, little improvements have been observed.
"If further studies confirm our findings, we could cross off thiamine deficiency from the long list of struggles that Baltic cod is facing", says Samuel Hylander, associate professor at the Department of Biology and Environmental Science.
More information
Link to the study: Hauber, M. M., V. Todisco, O. Nordahl, et al. 2026. “Thiamine Allocation and Deficiency Status Throughout the Life Cycle of Cod.” Ecology and Evolution 16, no. 1: e72828. https://doi.org/10.1002/ece3.72828.
The study is part of the research in Linnaeus University Centre for Ecology and Evolution in Microbial model Systems (EEMiS), and the Food Web Ecology research group.