It is globally recognised and understood that the state of our ocean is dire and continuing to decline. Biological terms such as ‘overfishing’, ‘climate change’ and ‘ocean acidification’ are regularly applied to try and pinpoint the culprits of such crimes against our vulnerable seas.

There are many lines of evidence that support the notion that the state of our ocean is declining, the perceived increase in Jellyfish blooms from the 1990’s to early 2000’s is one of them. Jellyfish blooms occur when ocean currents congregate large numbers of individuals into swarms or blooms that consist of thousands of these gelatinous animals.


The increase in jellyfish can indicate poor ocean health because of the specific optimal conditions that jellies require in order to thrive. Warmer ocean temperatures enhance jellyfish production, their feeding and their growth. The overfishing of their predators also means that more jellies survive. Increased water input and nutrients from land (eutrophication) leads to increased supply of plankton into the sea = lots of Jellyfish food!

The question is: have jellyfish blooms really been increasing? The evidence that supports this comes from only a few regions of the world (Benguela current, Sea of Japan and Bering Sea). Other research points toward the possibility that these blooms are part of the natural jellyfish lifecycle. This means that fluctuations/ oscillations in the life history of jellyfish results in periods of high abundance.


This paper by Condon et al. (2012) looked into the occurrences of jelly blooms between 1790 and 2011, investigating 37 datasets of such events. Condon et al. identified two main patterns in jellyfish populations. The first was a weak but significant overall increase in bloom events since the 1970’s.

Secondly, they found that jellyfish experienced recurring oscillations. What was originally perceived as a rise in jellyfish numbers may therefore have actually been part of a natural cycle. This is supported by some examples from around the world. The Bering Sea experienced a rise in jelly blooms in the 1990’s that was accounted to climate change, but the numbers have decreased substantially since then. The same was true for the Black Sea, Sea of Japan and some seas in Denmark.


It is being realised that the jelly blooms are being affected by both global changes and decadal oscillations, but that these are happening on different scales. On a regional scale, influences from humans (such as eutrophication) are causing these jelly populations to increase. At the same time, broad scale oscillations are constantly taking place. This results in compounded blooms that can be terribly harmful when peaks in oscillations and peaks in locally influenced blooms occur at the same time.

Condon et al. concluded that there was no significant increase in jellyfish abundance over the observed over the study time. However, there weren’t enough locations that had jellyfish bloom data over a long time series to contribute to this study. Oscillation events also occur over long periods (~20 years), so a extensive and detailed sample set would be necessary to analyse the changes of these oscillations. This study therefore didn’t have a sufficient basis to reject the claim that jellyfish populations have NOT increased significantly globally.


Regardless of what the ultimate cause of jellyfish booms might be, they do exist. Regarding the increase in jelly numbers that took place in the 1990’s and 2000’s, it is most likely that the rise of a long term oscillation met a local population increase to cause booms. Being aware of these fluctuations and realising the damage they can have both locally and on a large scale should secure our attention. Further research should be undertaken, human impacts should be minimised and mitigation measures should be in place for fishermen, industry and tourism in case detrimental outbreaks occur.

A positive element of this study was the realistic nature with which it was undertaken. Whilst the research was solid, there were recognised gaps in the study such as the lack of long term data and location sites. These weaknesses were taken into account when the possible causes were discussed, as per good scientific practise.

This blog was based on the paper by Condon H., et al. (2012). Recurrent jellyfish blooms are a consequence of global oscillations. PNAS. Vol 110 pp.1000-1005.







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