Summary Of Bycatch In WCPFC Purse Seine Fisheries At A Regional Scale, 2003–2022

The Western and Central Pacific Fisheries Commission (WCPFC) has a responsibility to assess the impact of fishing on non-target species. In this report, we estimate the bycatch of the large-scale equatorial purse seine fishery operating in the WCPFC Convention Area for the period 2003 to 2022. These large vessels, typically with greater than 500 tonnes carrying capacity, have been responsible for approximately 85% of the purse seine catch of tropical tunas in the WCPFC Convention Area in recent years, with an average annual catch of 1.6 million tonnes since 2010. The estimates cover the full range of finfish, billfish, elasmobranch, marine mammal and sea turtle species that have been recorded in purse seine observer data. The estimates do not cover domestic purse seine fisheries in the western sector of the WCPFC Convention Area, or purse seine fisheries in temperate waters off Japan and New Zealand. In this iteration, a spatiotemporal modelling framework was used to estimate catch rates. These catch rate models should allow better separation of temporal effects from other covariates, improving the utility of the catch rate models for screening of species that may warrant more targeted analyses due to unexplained temporal trends. The inclusion of spatial effects also allows consideration of the spatial distribution of estimated catches. The taxonomic resolution of catch estimates was also improved for marine mammals, to allow more meaningful monitoring of catch and catch rate estimates for these species. The report concludes with recommendations to the Scientific Committee: • Note the estimates of bycatch of the large-scale equatorial purse seine fishery in the WCPFC Convention Area; • Note that the bycatch estimates should be interpreted as the bycatch that would have been recorded by observers with 100% coverage of fishing events; • Note that other studies suggest that shark bycatch estimates are likely to be underestimates, due to underestimation of captures by observers; • Note the refinements to the estimation approach, including the implementation of spatiallyexplicit catch rate models. This should improve the utility of catch rate models for identification of species that may warrant additional targeted analyses; • Note the impacts of recent reductions in observer coverage on the precision of catch rate and catch estimates, and the extent to which they can be used to monitor for temporal trends; • Note the refinements to the taxonomic resolution of catch estimates for marine mammals.