Estimating annual leatherback bycatch in the Pacific Ocean by fishery and country to inform targeted conservation strategies

The Pacific Ocean supports two critically endangered leatherback sea turtle populations, both severely impacted by ongoing fisheries bycatch. Conservation planning includes population viability analyses (PVA), which depend on accurate demographic inputs to yield realistic results for informing management decisions. These projections are currently limited by a lack of fisheries bycatch data, and as population recovery is dependent on reducing leatherback bycatch, it is critical that we know where, when, and how much bycatch is occurring. Here, we undertake a systematic review to aggregate existing estimates of leatherback bycatch within the Pacific Ocean for the purpose of creating the most contemporaneous and comprehensive estimate of Pacific leatherback bycatch. Searches through scientific databases and Google Scholar yielded 29 bycatch and mortality (hereafter: interaction) estimates from WCPFC fisheries that were deemed valid. Validity was determined by aligning article data with the SPIDER search methodology. These estimates covered 11 WCPFC fisheries and came from the years 1993 - 2024. In summation, they recorded the bycatch of 2058 leatherbacks and the mortality of 543, averaging to 66 bycaught leatherbacks and 18 mortalities per year. When linear extrapolation accounts for the countries not represented and fisheries not observed, this becomes an estimate of 381 leatherback mortalities per year attributed to WCPFC fisheries. However, linear extrapolation is not representative of the total fishing effort by each fishery. Therefore, effort per fishery was calculated using Global Fishing Watch estimates of fishing hours in 2020. When incorporating actual fishing hours of the WCPFC fisheries, it was found that mortality could be as high as 1599 leatherbacks per annum, making the literature estimate 1% of the actual value (and the linear extrapolation 24% of the actual value). This highlights the severe data paucity for two populations that are predicted to go extinct within the century. Finally, 2020 mortality estimates were used to identify the Pacific fisheries that are currently most at-risk for causing leatherback mortality....