Assessment of the vulnerability of sea turtles to IOTC tuna fisheries

Mortality from interactions with fishing gear poses a significant threat to sea turtle populations globally. Within the Indian Ocean Tuna Commission (IOTC) area of competence, semi-quantitative risk assessments in 2012 and 2013 identified specific sub-populations of olive ridley, loggerhead, leatherback and hawksbill turtles to be highly vulnerable to the impacts of fishing. Here, we present an update to these previous risk assessments using a Productivity-Susceptibility Analysis (PSA) within the Ecological Risk Assessment for the Effects of Fishing (ERAEF) framework developed by Hobday et al. (2011). Results revealed that no sea turtle sub-populations were classified as low vulnerability to longline, purse seine or gillnet fisheries – all were classified as either medium or high vulnerability. Sea turtles were found to be more vulnerable to gillnet and longline fisheries than purse seine fishing, due mostly to the large spatial area and depth distribution of longline fishing, and the assumed high post-capture mortality of sea turtles in gillnet fisheries. Within these fisheries, the species identified to be most vulnerable to fishing were green turtles, loggerhead turtles and hawksbill turtles, particularly in the Arabian Sea and Bay of Bengal. Our results were generally consistent with previous assessments, which suggests that there would be minimal gain in repeating a PSA for sea turtles in the short to medium term, unless there is a significant change in the data available for the assessment. It is important to note that the results from the PSA provide only relative measures of vulnerability. Results are also limited by a lack of information and the underlying assumptions of the PSA. Most notable is the lack of effort data for gillnet fisheries, and information on gear selectivity and post-capture mortality of sea turtles from all gear types. Notwithstanding these limitations, management efforts would benefit from prioritising the implementation and enforcement of mitigation measures, particularly for gillnet and longline fisheries. Priority should also be given to improving reporting of sea turtle interactions in all fisheries, and collating and analysing existing data on sea turtle interactions from IOTC member countries to identify factors that contribute to higher interaction and mortality rates. This information is essential to underpin the development and implementation of effective mitigation strategies for sea turtle.