Jellyfish Overpopulation – A Threat To The Oceans?
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.
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.
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
Copyright © 2009 by Marine Science Today, a publication of OceanLines LLC
