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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Melissa Block.

Picture a foot wide jellyfish with a hideous mass of bright red tentacles hanging underneath it. Now imagine billions of those creatures crowded up against your favorite stretch of coastline. Well that's a scene that's now playing out off the coast of Southwestern Africa, where a colossal jellyfish bloom has lasted several years. Scientists on the scene blame over-fishing for the problem and they say a coastline near you could be next.

Here's NPR's John Nielsen.

JOHN NIELSEN reporting:

Not too long ago the waters off the coast of Namibia were famous for their richness. Trawlers caught up to 18 million tons of fish every year. Miners armed with giant vacuum tubes sucked diamond laden muck up off the bottom.

Then about six years ago the jellyfish invaded and the flow of riches stopped. Oceanographer Andrew Brierley, of the University of Andrews in Scotland, says the unexpected bloom began in a part of the ocean known as the Benguela Coastal Zone, which until then had been best known for its anchovies and sardines.

Brierley says the bloom grew explosively, blanketing the entire coastline of Namibia. Viewed from a plane, he says the scene was almost biblical.

Dr. ANDREW BRIERLEY (University of Andrews, Scotland): You could almost imagine walking across the ocean's surface on jellyfish. They're bunched together in very tight packing densities and you can see the whole of the sea surface for kilometer on end apparently smothered completely in jellyfish.

NIELSEN: Brierley says these creatures jammed up all the mining vacuums, then the heavy jellyfish started ruining lots of local fishing nets.

Dr. BRIERLEY: When a fishing net pulls through them it's effectively like filling the whole of the net with water and so they try and lift this huge net full of water onto the deck and it just bursts. The jellyfish rip through the net and tear it to pieces.

NIELSEN: Brierley's part of a group that's been studying this bloom. In the journal Current Biology they report that 12 million tons of jellyfish appear to be involved making this the largest single bloom on record.

More importantly, while most blooms vanish after just a few months, this one has now been in place for years and shows no signs of disappearing. Brierley says this is true in part because the jellyfish involved have a voracious appetite for fish eggs.

Dr. BRIERLEY: And so we may be in a situation in Benguela where it's impossible to revert to the previously highly productive fish dominated situation.

NIELSEN: Critics of the global fishing industry have theorized for years that changing fishing trends will bring more blooms like these.

Daniel Pauly, a fisheries biologist at the University of British Columbia, says that as big fish have become harder to find, more fishing fleets have started hunting for smaller fry like anchovies and sardines. Pauly says these smaller fish compete for food with so-called trash species like jellyfish.

When the little fish get caught, he says, those trash species have a chance to move in and take over.

Dr. DANIEL PAULY (University of British Columbia): Because they're not controlled anymore and they might outgrow the smaller fish and then you have a sea full of jellyfish.

NIELSEN: To say the least the Namibian bloom has drawn new attention to this theory. But jellyfish experts like Monty Graham of the Dolphin Island Sea Lab in Alabama say they're not sure they see a solid link to over-fishing here.

Graham says the Benguela system is known for the strength and power of its currents, for example. One of them could have triggered the bloom by bringing the jellyfish in along with lots of extra nutrients. Also, since there are no long-term records of jellyfish blooms in existence, this one could turn out to be a natural thing.

Dr. MONTY GRAHAM (Dolphin Island Sea Lab): We may just be experiencing some real long cycles, 20 year, 30 year cycles in jellyfish blooms.

NIELSEN: Brierley agrees that there's no conclusive proof of a link to over-fishing here. Unfortunately that's not a fact that helps the Namibians, who've long depended on their fisheries.

John Nielsen, NPR News, Washington.

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