Saturday 1 June 2013

The mysterious ebb and flow of Jellyfish populations.

Many scientists and conservationists are worried about the state of the world's oceans. Many important marine ecosystems are known to be under stress: once prolific fisheries have collapsed; dead zones lacking oxygen, and therefore life, have appeared in the coastal waters of developed Northern Hemisphere nations; other areas are suffering from nutrient excesses from agricultural runoff, provoking blooms of toxic algae; and the oceans are becoming more acidic. One concern that has received significant publicity in recent years is the apparent explosion in Jellyfish numbers since the 1970s, with anecdotal evidence from around the world suggesting that these animals have undergone a dramatic, and unprecedented, rise in numbers, potentially to the detriment of other marine life.

Giant Jellyfish (Nemopilema nomurai) clog fishing nets in Japan. Shin-ichi Uye/

In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 15 January 2013, a team of scientists led by Robert Condon of the Dauphin Island Sea Laboratory in Alabama, examine data on Jellyfish populations for the past century in order to try to quantify the apparent rise in numbers in the last 40 years.

Data on Jellyfish populations used in the study. Areas where populations have been recorded for longer have larger circles. Areas where populations have increased significantly during this time are represented in red, areas where Jellyfish have declined in blue. Condon et al. (2013).

In the course of this study Condon et al. made an unexpected discovery; Jellyfish populations across the globe rise and fall in a 20-year cycle, for reasons which could not be determined in the course of the study. Jellyfish numbers have risen since the 1970s, but not as much has been assumed, since two cycles of natural expansion, in the 70s and 90s, are included in the period over which people have noticed the rise in numbers.

Bloom of Moon Jellies (Aurelia aurita) in Chesapeake Bay. Scott Kupiec/National Science Foundation.

Follow Sciency Thoughts on Facebook.