Decline in shark population worldwide

Sharks are in big trouble on the Great Barrier Reef in Australia and worldwide, according to scientists who claim to have developed the world's first way to measure rates of decline in shark population.

"There is mounting evidence of widespread, substantial, and ongoing declines in the abundance of shark populations worldwide, coincident with marked rises in global shark catches in the last half-century," said lead scientist Mizue Hisano at the ARC Centre of Excellence for Coral Reef Studies.

He added: "Overfishing of sharks is now recognised as a major global conservation concern, with increasing numbers of shark species added to the International Union for the Conservation of nature's list of threatened species.

"First, many countries with coral reefs don't keep reliable records of catches or fishing effort. Second, around 75 per cent of the world shark catch consists of illegal and unreported finning. Third, sharks may be caught, discarded, and not reported when fishers are targeting other species."

The scientists have developed several alternative models, which combined birth rates and growth rates for sharks with a variety of different methods for estimating mortality.

They then used state-of-the-art statistical methods to combine the uncertainty associated with each of these methods and arrive at a more robust long-term population prediction for two GBR shark species -- the grey reef shark and the whitetip reef shark.

As a further check on their results, the scientists used their population projections to see how well their models could explain differences in shark abundances on fished and unfished reefs, based on how long the unfished reefs had been protected.

The team found that results obtained by all methods of assessing shark population were in close agreement that sharks are declining rapidly due to fishing.

"Our different approaches all painted a surprisingly consistent picture of the current state of population decline, but also of the potential recovery of these species if they are adequately protected.

"More broadly, we believe that our study demonstrates that this approach may be applied to a broad range of exploited species for which direct estimates of mortality are ambiguous or lacking, leading to improved estimates of population growth," Hisano said.