Ocean Sentry
Sharks in the Mediterranean Sea have undergone a massive decline over the last two centuries, scientists have discovered from historical records.Some species shrunk by more than 99% over the period, mainly due to fishing.
Researchers used fishermens” notes and archive records to plot population trends of five top predatory sharks.The study, in the journal Conservation Biology, comes just weeks after a warning that half of the world”s ocean-going sharks face extinction. Sharks and their close relatives, the rays, are particularly vulnerable to over-fishing as they grow and reproduce slowly.
“There is a long history of fishing in the Mediterranean, especially coastal fishing,” said study leader Francesco Ferretti from Dalhousie University in Halifax, Canada, who 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 told BBC News.
Shark traps
There are 47 species of shark found regularly in the Mediterranean, of which 20 are top predators.Fishermen tended to regard them as pests, according to records amassed by the researchers.
For five of the 20 top predators, the records – from traditional tuna “traps”, commercial boats and fishermen using rods and lines – were good enough to show that catches had been large enough to produce a substantial decline.
The hammerhead population, they conclude, has declined by more than 99.99% over the last 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%.
For the rest of the 20 top predators, records were not comprehensive enough to plot a trend, though declines were evident. Francesco Ferretti suggests that may be because their decline began even earlier, when records were even more sparse.
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.
