The Baltic cod - An unexpected journey

Worrying trends such as deteriorating condition, decreasing growth rates and diminished distribution range have been observed in the eastern Baltic cod stock in recent years. However, the mechanisms underlying these changes are still poorly understood. In an effort to better understand the observed changes in this stock, since 2016 >23,000 eastern Baltic cod have been externally tagged with T-bar anchor tags and released into the Baltic Sea, as part of the international project TABACOD.

In addition, 1260 cod have also been tagged using data storage tags (DSTs) which record high-resolution measurements of temperature and pressure. The conventional external tagging provides new data on the growth and movement of recaptured cod, but perhaps the most intriguing insights into Baltic cod ecology come from recaptured DST tagged cod.

Although the number of returned DSTs to date is still rather low (N=23), each one provides a wealth of valuable new data. The unexpected results which have already been revealed by the returned DSTs include information on seasonal migration patterns which would not have been identified from the conventional external tagging alone.

Additionally, we found examples of cod vertical movement patterns that aligned with moon phase, potentially following lunar-related changes in prey distribution. We have also gained direct evidence of the predation on one unlucky cod by a cormorant, through a DST recovered in a colony by an ornithologist.

This DST indicates very shallow, inshore movements of Baltic cod. Such discoveries provide new pieces of the puzzle, which will help us to better understand Baltic cod ecology, formulate new hypotheses, and guide future research.