Between September 26 and 28, 2025, large numbers of dead fish washed up on the beaches of Nienhagen, Warnemünde, and Markgrafenheide. Back in January 2025, the IOW had installed measuring devices off Nienhagen that regularly record data on, for example, temperature, salinity, oxygen content, and turbidity of the water. One aim of the measurements is to investigate the development of oxygen-depleted areas in the still relatively unexplored shallow coastal waters of the Baltic Sea.

The IOW's measurements off Nienhagen showed that from September 24 to 28, oxygen saturation at the seabed fell to below 1 %. At the same time, the measuring instruments recorded a drop in temperature and an increase in salinity, a clear indication of a different water mass. The bottom water off Nienhagen was displaced by another water mass without oxygen, which originated from greater depths below the thermocline and had moved into shallow water. As a result, oxygen saturation on September 27 was below 70 % even at a water depth of one meter. Normally, it is 100 % or higher here. From September 23 onwards, wind conditions with wind directions shifting from north to east and later to southeast led to a situation that was initially dominated by coastal downwelling and then changed to a coastal upwelling scenario due to east wind. This pushed the surface water into the open Baltic Sea and replaced it with oxygen-depleted deep water. This “upwelling” meant that the fish no longer had access to enough oxygen to breathe and they therefore suffocated.

The Baltic Sea is heavily polluted with nutrients, which remain in the Baltic Sea for a long time due to the low water exchange. The nutrients cause the formation of algal blooms, whose microbial decomposition consumes oxygen. This results in oxygen-depleted marine sediments, known as “dead zones.” However, the development of such “dead zones” in the shallow coastal waters of the Baltic Sea is still largely unexplored. For this reason, the IOW installed measuring devices off Nienhagen in January 2025. Measurements are taken at four depths and the data is then sent to the IOW.

According to initial findings, the fish kill observed at the end of September 2025 is a consequence of increasing overfertilization of the Baltic Sea (eutrophication), the resulting biomass formation, decomposition processes of this biomass under oxygen consumption, and weather conditions that caused oxygen-depleted deep water to rise to the surface. The extent to which such events will develop in terms of frequency and intensity is being researched at IOW.