Untangled catch: Assessing the impact of industrial tuna fishing on threatened sharks and rays

Background/Question/Methods

Oceanic sharks and rays (elasmobranchs) are experiencing alarming declines due to fisheries, yet clarity about where to focus management and conservation actions is needed. Industrial and semi-industrial tuna fisheries, which catch 78% of the tuna captured globally, both target and incidentally catch threatened and endangered elasmobranch species in large numbers, with potentially significant and long-lasting impacts on regional and global populations. However, due to limited fine-scale data collection and availability, the contribution of these fisheries to shark and ray mortality is often anecdotal, region-specific, and poorly understood. Here, we use multiple approaches to triangulate pelagic elasmobranch catch in tuna Regional Fisheries Management Organizations to describe the scale and impact of industrial tuna fisheries on 13 threatened oceanic shark species and 10 oceanic ray species. We convert and harmonize reported bycatch data to show that describe the quantity, composition, and potential impact of reported elasmobranch catch and bycatch in these fisheries. Because reported data is likely to underestimate total catch, we supplement this approach with a synthetic literature review approach to estimate unreported catch. We pair this catch data with existing life history data, stock assessments, and conservation designations to estimate the proportion of catch that is vulnerable, overfished and/or threatened.

Results/Conclusions

Our results show that industrial tuna purse seine and longline fisheries report an annual mean of ~2.6 million identified pelagic elasmobranchs over the last decade (2007-2017), corresponding to one elasmobranch reported for every five tonnes of tuna. This catch is dominated by longline fishing gear, which accounts for 97.9% of reported elasmobranch catch, despite providing less than 20% of the world’s tuna. However, catch rates published in the scientific literature suggest that true capture for both gears is likely much higher than the reported data we present here. Populations that are overfished or experiencing overfishing represent 31.7% of the reported catch, though only a small fraction (19%, n=18) of the populations we examined are assessed. Our findings suggest that catch rates are extremely variable among elasmobranch species, and as a way forward, we produce a conceptual framework for prioritizing categories of species at risk from tuna fishing for each ocean.