Dr. Cathy Lucas, University of Southampton – The Cycle of Jellyfish Blooms

ellyfish have ancient origins and have been around for over 500 million years. They are found throughout the world’s oceans from surface sunlit waters to depths greater than 16 000 ft. When conditions are right, jellyfish have the ability to form spectacular blooms, some of which affect our beaches, fishermen and power plants.

Over the last decade, increased reports of jellyfish blooms in the media have fuelled a perception that jellyfish are on the rise and will “take over” our seas. Even the scientific literature regularly reports that jellyfish are increasing globally and are a symptom of a degraded ocean, but often these conclusions are extrapolated from local or regional studies. So, are blooms of jellyfish really increasing throughout the world’s oceans?  

The Global Jellyfish Group researched this question using datasets stretching back more than 200 years. We found that globally, jellyfish populations undergo synchronous oscillations with multi-decadal periods of rise and fall, including a rising phase during the 1990s and early 2000s that we believe has contributed to the current perception of a global increase in jellyfish abundance. The previous peak of the 1970s went largely unnoticed, probably because fewer people studied jellyfish, there was less awareness of global-scale problems, and without the internet, there was less capacity to share information. Whether the hint of a small rise in recent years is a continuing upward trend or part of an oscillation, time will tell, but we now have a solid baseline from which to assess future changes.
 
While there is little evidence for a global rise in jellyfish, there are regions of the world where blooms have increased, including the Sea of Japan and parts of the Mediterranean. For these regions, sustained increases in jellyfish populations will continue to present problems for coastal industries.