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Jellyfish Population Boom Due to Overfishing: Study May 17, 2013
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Japanese fishermen attempt to remove huge jellyfish that overloaded their net in April 2012.
Overfishing of the world’s oceans is the leading cause of an unprecedented jellyfish population boom, a new study claims.

Published in the Bulletin of Marine Science, the report argues that the unrestricted fishing of small open-water fish, such as sardines and herring, has reduced the number of creatures that jellyfish compete with for plankton and other forms of food.

By thinning out these hunting rivals, many of which also feed on jellyfish eggs, the unregulated fishing has eliminated the main checks on explosive jellyfish population growth.

Over the past decade, that growth has been apparent, with jellyfish showing up in bodies of water where they had never before been a problem, such as the Sea of Japan and the Mediterranean Sea.

There, they have clogged fishing nets and driven swimmers onto dry land.

The international team of marine biologists who co-authored the study drew their conclusion after comparing two ecologically similar coastal habitats in southwest Africa.

In the first area, an ecosystem just off the coast of Namibia where local small fish populations had suffered years of overfishing, the team found that jellyfish had filled the ecological niche in surging numbers.

But in the tightly regulated and fish-abundant South African waters studied farther to the south, jellyfish populations were found to be stable.

Because jellyfish will also feed on fish larva, this jellyfish boom will only make it that much more difficult for overfished species to recover, the study warns.

Photo: Shin-ichi Uye