Stereo videography reveals fragility in a high value thresher shark population

Population risk assessments are important tools for understanding the sustainability of shark populations and informing their conservation. In the Philippines, policy makers have made inroads to progress legislative protection for sharks in recent years, yet a ban on targeted shark fisheries has not guaranteed the sustainability of their populations and bycatch and illegal fishing still represent a significant threat to elasmobranchs across the archipelago. Pelagic thresher sharks ( Alopias pelagicus ) are important to the region’s tourism economy but little is known of the status of their populations. We designed, tested, and deployed a remote stereo camera system (stereocam) to survey pelagic thresher sharks in the Central Visayan Sea and investigated their population dynamics over a 4-month period from December 2019 to March 2020. The stereocam was effective and accurate in taking key morphometric measurements from in situ observations that we used to assess the maturity of individual male and female pelagic thresher sharks. We then constructed a continuous four-stage model from our demographic data and best life history parameter estimates to determine the sustainability of A. pelagicus populations in the region. Our model projections showed that pelagic thresher sharks in the Central Visayan Sea would be vulnerable to a hypothetical fishing mortality of 5.3% per annum, with the removal of 15–18 females resulting in a decline in the population. Our study represents the first attempt to characterise the demographics of pelagic thresher sharks with stereo videography and provides a framework for future non-invasive assessments of threatened pelagic species.