June 11, 2008

Shark Population Declining in the Mediterranean

Scientists recently revealed evidence to show that populations of sharks in the Mediterranean are declining, by as much as 99 percent in some species.

The data indicates that fishing is the primary cause of the shark's decline.

The study, published in the journal Conservation Biology, brought together logs from fishermen to plot population trends of five top predatory sharks. There are currently 447 species of sharks in the Mediterranean, 20 of which are top predators.

Over-fishing combined with slow growth and reproduction rates amounted to a visible decline in sharks and their close relatives, the rays.

"There is a long history of fishing in the Mediterranean, especially coastal fishing," said study leader Francesco Ferretti from Dalhousie University in Halifax, Canada.

Ferretti has been working in the Mediterranean with the Lenfest Ocean Program.

"And until recently, these species were not valuable - they were caught as bycatch by boats chasing important species such as tuna - so they were declining without anyone noticing," he said.

The records collected from fishermen indicated that they tended to regard sharks, leading to an increasing number of tuna traps set to catch the predatory fish.

For five of the 20 top predators, the records showed that catches had been large enough to produce a substantial decline.

The hammerhead shark, for example, has declined by more than 99.99 percent over the past 200 years. Records show hammerheads largely vanished from coastal waters around 1900; in the last 20 years they have barely been seen in pelagic zones either.

The blue shark and the two mackerel sharks have also apparently vanished from coastal waters. Threshers are occasionally still caught in tuna traps; even so, their numbers across the Mediterranean have fallen by more than 99.99%.

The collected data was not complete enough to indicate a trend in remaining top 20 predators. Ferretti suggests that may be because their decline began even earlier, when records were even less inclusive.

Studies on historical populations are rare, he said; but when they do plot declines, that should lead to listing as a threatened species.

"This study will hopefully contribute to a greater threat status for hammerheads and blue sharks, and other assessments in the Mediterranean," he said.

Conservationists have long campaigned for better protection for sharks and rays, which have not traditionally been considered by the organizations that regulate fisheries.

"Historically, [sharks] didn't have high economic value, and as resource priorities and management are linked with the economic value of fisheries, sharks have never been managed - they slipped under the radar," commented Rebecca Greenberg, a marine scientist with the conservation group Oceana.

"Now, many larger shark-catching nations are taking advantage of the fact that they're not regulated; and along with the negative image that many people have of sharks, that's led to the desperate situation we have today."

But conservationists are concerned that the declining predator population may lead to unexpected changes in the sea such as a drastic rise in the number of prey like jellyfish, which may be partly due to falling numbers of predators such as bluefin tuna and turtles.

Conservation groups hope to enact measures to protect sharks, but especially in the Mediterranean - the "most dangerous area in the world" for them, according to Rebecca Greenberg - is long overdue.

These would include the policing of bans on finning - which removes fins for the lucrative eastern cuisine market - measures to reduce bycatch, and the setting of regional and global catch limits.

Image Courtesy Wikipedia


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Conservation Biology

Lenfest Ocean Program

Dalhousie University


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