Report release: new findings on sustainability status for Pacific Bigeye Thresher Shark

September 2016 | Wellington. The 2013 listing of five species of sharks and all manta rays by the Convention on International Trade in Endangered Species (CITES) leaves no doubt that elasmobranch conservation is a major global concern. Responding to the urgent need for reliable information on stock status of these and other sharks can, however, present a major challenge for analysts concerned with less common and potentially misidentified species. Thresher shark species not only suffer from these data quantity and quality issues, but also have low productivity and high susceptibility to longline fishing compared with most other pelagic sharks. The bigeye thresher shark (Alopias superciliosus) has the widest distribution and the greatest number of available catch records from the Pacific, but is likely to be the most vulnerable of the three thresher species to fishing pressure. 

Illustration: Bigeye thresher shark (Alopias superciliosus) © Les Hata

The Western and Central Pacific Fisheries Commission (WCPFC), in partnership with the Common Oceans ABNJ Tuna Project, agreed to help develop methods for data-poor shark species by accepting the challenge of assessing the Pacific population of bigeye thresher shark. Based on data from 14 national observer programmes across the Pacific, WCPFC's consultant, the New Zealand National Institute of Water and Atmospheric Science (NIWA), has completed a spatially-explicit risk assessment study (read report here) evaluating whether current impacts from fisheries exceed a maximum impact sustainable threshold (MIST) defined based on population productivity. This approach differs from traditional stock assessments because it evaluates whether the population's ability to withstand fishing pressure is exceeded, rather than evaluating biomass and whether the population is overfished. Analytical scenarios covering 2000-2014 ranged from more to less precautionary and accounted for uncertainty in species distribution, initial population status, maximum density and post-capture survival. Results based on assuming 100% capture mortality suggest impact levels between 40% below and 20% above the MIST, whereas results based on assuming a range of post-capture survival rates suggest impact levels ranging from 60% below to equivalent to the MIST. Considering all scenarios, the annual probability that the fishing impact exceeded the MIST was 20 to 40%. The NIWA study will be submitted to and reviewed by the WCPFC’s Scientific Committee at its next meeting in August 2017.

Sharks heads close-up: difference between bigeye thresher (in the foreground) and another thresher species (in the back))

WCPFC is implementing three ABNJ Tuna Project bycatch components designed to improve data, develop new assessment methodologies and enhance management of sharks within the t-RFMOs. In addition to this latest assessment of bigeye thresher shark, the first global shark assessment – for the southern hemisphere porbeagle shark (Lamna nasus) – is currently underway. Two more Pacific-wide assessments aiming to explore novel analytical approaches for data-poor sharks, open new avenues of collaboration and contribute to the global conservation dialogue will be identified and completed before December 2018.