Sharks are under siege from humans and the consequences are far reaching. It turns out that you can’t pull a top predator out of an ecosystem and expect everything else to remain the same.

A recent article in the Age (Taste for delicacy puts sharks at risk) canvasses the issues. A quote or two…

A THIRD of the world’s open-water sharks — including the great white and hammerhead — face extinction, according to a conservation survey that singles out overfishing as the main culprit.

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The report identified the great hammerhead and scalloped hammerhead shark and the giant devil rays as endangered. The smooth hammerhead, great white, basking and oceanic white-tip sharks are listed as vulnerable as are two species of makos.

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Dr Huveneers said sharks were important in the ecosystem, especially those at the top of the food chain such as the white, great white and great hammerhead. Reducing their numbers could prompt an increase in the number of cownose rays, which consumed scallops.

That last quote reminded me of a story I’d recently come across about an explosion in the number of giant Nomura jellyfish in Japan. (These critters are up to 2 metres across and can weigh up to 220kg.)  So I did a bit of research and turned up a picture and a backgrounder here.

But there was an even more interesting feature on the New York Times site entitled Stinging Tentacles Offer Hint of Oceans’ Decline

From Spain to New York, to Australia, Japan and Hawaii, jellyfish are becoming more numerous and more widespread, and they are showing up in places where they have rarely been seen before, scientists say. The faceless marauders are stinging children blithely bathing on summer vacations, forcing beaches to close and clogging fishing nets.

But while jellyfish invasions are a nuisance to tourists and a hardship to fishermen, for scientists they are a source of more profound alarm, a signal of the declining health of the world’s oceans.

“These jellyfish near shore are a message the sea is sending us saying, ‘Look how badly you are treating me,’ ” said Dr. Josep-María Gili, a leading jellyfish expert, who has studied them at the Institute of Marine Sciences of the Spanish National Research Council in Barcelona for more than 20 years.

The explosion of jellyfish populations, scientists say, reflects a combination of severe overfishing of natural predators, like tuna, sharks and swordfish; rising sea temperatures caused in part by global warming; and pollution that has depleted oxygen levels in coastal shallows.

They’re talking sharks over on the RealSurf forums too…