Reader question: Why do jellyfish wash up on Estonia's shores in early fall?
Where do jellyfish come from, and where do they go? Why are they still so abundant in Estonian waters in September, and is their presence indicative of anything in terms of water temperature, the weather or how long summer will last? University of Tartu (TÜ) marine biologist Jonne Kotta responds.
The Aurelia aurita, known variously as the common jellyfish, moon jellyfish, moon jelly or saucer jelly – and meririst or commonly millimallikas in Estonian – often reaches Estonian waters from the North Sea, which offers them the most suitable conditions.
The waters of the Baltic Sea, while brackish, are still too fresh for jellyfish to complete their entire life cycle, but they are carried into the Baltic by currents and waves regardless.
Unlike fish, jellyfish and other plankton generally can't swim against currents and waves; they simply go wherever the water takes them. If a wave happens to wash a jellyfish up onto the shore, then that's where it will remain. Similarly, the sea often washes large amounts of decaying seaweed onto the shore as well.
Jellyfish typically reach Estonian waters at the end of summer and beginning of fall due to their seasonal life cycle. Over the summer, huge quantities of them occur in the oceanic water column. The more jellyfish that occur in the North Sea during summer, the more are carried by the waves to Estonia by the end of the season. By this time, they are largely at the end of their life cycle.
Their appearance isn't indicative of much in terms of water temperature. Although temperature certainly affects these creatures, jellyfish are often more abundant when the water is saltier. Of course, their numbers are also impacted by the availability of food, among several other factors.
Similarly, jellyfish are not a reliable indicator of the weather. The climate today is so erratic that extremes and exceptions are becoming more frequent; predicting the future based on the past isn't particularly rewarding. They may indeed indicate something about the weather, but moreso about the past than the future.''
It's also worth mentioning that these jellyfish are completely harmless. Some people may worry that these are some sort of invasive species, but the common jellyfish, or meririst, is a naturally occurring species in the region.
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Editor: Rait Piir, Aili Vahtla
