Jellyfish Overpopulation – A Threat To The Oceans?

Written by on June 11, 2009 in Jellyfish, Marine Life
Sea Nettle, jellyfish  -  Photo: Mila Zinkova

The numbers of jellyfish seem to be increasing due to human activities - Sea Nettle, jellyfish - Photo: Mila Zinkova

New research led by Dr. Anthony Richardson, a CSIRO Climate Adaptation Flagship and University of Queensland scientist, presents convincing evidence that the medusae (jellyfish) overpopulation is associated with over-fishing and excess nutrients from fertilizers and sewage, a result of human activities.

Jellyfish are found in every ocean, from the surface to the deep sea.  Their presence in the ocean is usually seasonal, responding to the availability of prey, which is seasonal in most places, increasing with temperature and sunshine in the spring and summer.  

The first noticeable outbreaks had been recorded in the Sea of Japan, involving the Nomura jellyfish, which can grow up to 2 m in diameter and weigh 200 kg.  But in recent years jellyfish blooms have been also recorded in the Mediterranean, the Gulf of Mexico, the Black and Caspian Seas, the Northeast US coast, and particularly in Far East coastal waters, indicating a clear tendency of more severe and frequent jellyfish outbreaks worldwide.  

Chrysaora Colorata, jellyfish  -  Photo: Sanjay Acharya

Jellyfish can replace fish in some areas - Chrysaora Colorata, jellyfish - Photo: Sanjay Acharya

“Mounting evidence suggests that open-ocean ecosystems can flip from being dominated by fish, to being dominated by jellyfish,” Dr Richardson says.  In a healthy ecosystem fish keep jellyfish in check through competition, they feed on the same kinds of prey, and predation, but overfishing destroys that balance.  A concrete case can be seen off Namibia, where intense fishing has decimated sardine stocks and jellyfish have replaced them as the dominant species.

Climate change may also favor some jellyfish species by increasing the availability of flagellates in surface waters – a key jellyfish food source.  Warmer oceans could also extend the distribution of many jellyfish species.  

High levels of nutrients provide nourishment for small organism on which jellyfish feed.  In areas with increased nutrients in the water, for example in agricultural areas with runoff, there will also be increased numbers of jellyfish.

Dr. Richardson’s new research, in collaboration with colleagues at the University of Miami, Swansea University and the University of the Western Cape,  has been published in the international journal; Trends in Ecology and Evolution, in June.

Moon jelly  -  Photo: Dante Alighieri

Moon jelly - Photo: Dante Alighieri

Copyright ©  2009 by Marine Science Today, a publication of OceanLines LLC

About the Author

About the Author: Celia is Director of Business Operations for OceanLines LLC and is a frequent contributor to both OceanLines and Marine Science Today. She is a certified diver and her favorite topic is marine biology, especially stories about whales. .


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  1. mikaela says:

    this is so cool! i LOVE jellyfish! favorite animals!

  2. Chloe says:

    Jellyfish are dangerous, but hopefully they can wipe out humans and save the planet for the good of the other species.

  3. Emily says:

    Ha, that might not be such a bad thing. Thanks for the comment!

  4. Robert says:

    Its already to hard to reverse the over fishing but now we have jellyfish making it neer impossible!

  5. laura says:

    soo what the jellyfish are harmless. so its a good idea to leave them be and if they do overpopulate then thats not going to kill us.

  6. not saying says:

    JELLYFISH!? Of all animals in the water, JELLYFISH?!