Sharks breed very slowly with some species taking more than 20 years to reach sexual maturity. They may then have only 2 or 3 pups every other year. At present we kill around 100 million sharks a year across all species - faster than they can reproduce. Some scientists estimate that the rate of depletion may be as high as 2% per year for certain species. It has been predicted that if this carnage continues then some species of shark that have existed for 400 million years could vanish within the next 50 to 100 years.