Sharks are in big trouble on the Great Barrier Reef and worldwide, according to an Australian-based team who have developed a world-first way to measure rates of decline in shark populations.
"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," say Mizue Hisano, Professor Sean Connolly and Dr William Robbins from the ARC Centre of Excellence for Coral Reef Studies and James Cook University.
"Overfishing of sharks is now recognized 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," they say in the latest issue of the international science journal PLos ONE.
"Evaluating population trends for sharks is complicated," explains Professor Connolly. "The simplest approach of looking at trends in fisheries catches doesn't work well for sharks. 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."
"An alternative is to take estimates of shark growth, birth, and mortality rates, and use these to calculate population growth rates. Estimates of growth and birth rates are easy to get, but it is very hard to get good estimates of mortality in sharks and other large animals," he says.
To deal with this problem, the team 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 lon
|Contact: Sean Connolly|
ARC Centre of Excellence in Coral Reef Studies