Jellyfish sightings surge in Baltic Sea as Aurelia aurita populations rise

In the past week, jellyfish sightings have increased in the marine areas of the Baltic Sea, specifically the harmless Aurelia aurita or moon jellyfish. According to Samuli Korpinen, a research director at the Finnish Environment Institute (SYKE), this surge occurs as the jellyfish rise to the surface waters to reproduce at the end of summer.

There are two additional factors contributing to this phenomenon. Korpinen explains that southerly winds have been persistent, allowing jellyfish from further south to drift into these waters. Moreover, the increase in jellyfish populations is linked to their search for food, as they find abundant animal plankton in the nutrient-rich surface waters of the Baltic Sea. This region has experienced eutrophication for decades, sustaining high jellyfish populations.

Recent sightings, such as a large group of jellyfish near Suomenlinna, confirm their presence. Korpinen notes that while water temperature impacts many marine species, it does not significantly affect jellyfish numbers.

Jellyfish typically emerge as polyps from the sea floor in spring and may die as they age and after reproduction. Korpinen emphasizes that these moon jellyfish do not pose a threat to humans, as their stinging cells are too weak to harm people. While dangerous jellyfish species exist globally, the low salinity of the Baltic Sea acts as a barrier to their spread.

Currently, there are no concerns regarding hazardous jellyfish in Finland, even as researchers acknowledge the potential movement of freshwater and brackish-water species via ships. Measures, including international agreements, are in place to prevent the spread of invasive species through maritime transport.