Annual bycatch and regional density of the porbeagle shark Lamna nasus in the austral Southwest Atlantic Ocean

In response to growing global concern, various international committees and national and regional action plans have emphasized the need for research focused on the conservation and sustainable use of the porbeagle shark. In the Southwest Atlantic, this species is incidentally caught by commercial longline and trawl fleets, with varying intensities depending on the area and type of fishery. This study is based on data collected by scientific observers onboard Argentine trawl freezer and factory vessels operating south of 51°S between 2006 and 2021. It had three main objectives: to estimate the annual bycatch of porbeagle shark; to estimate standardized catch per unit effort as a proxy for regional species density in the Southwest Atlantic Ocean; and to update the annual standardized bycatch index originally developed for the surimi fleet. Porbeagle shark occurrence frequency increased with depth, showing two distinct peaks around 500 and 1200 m. Higher frequencies were recorded east of 64ºW and south of 54ºS, with monthly peaks during the first half of the year, although interannual variation was observed. Occurrence was higher in surimi vessels, particularly when using semi-pelagic gear. The regional standardized porbeagle shark density index fluctuated over the study period without a clear trend, although notable peaks occurred in certain years. Notwithstanding its limitations, this study provides indicators that represent a useful starting point for assessing the status of porbeagle shark populations in the Southwest Atlantic and for contributing to future stock assessments. This information is particularly relevant because it originates from latitudes located farther south within the porbeagle shark’s distribution than those encompassed by other Atlantic datasets. The study area also holds ecological importance as a feeding ground, particularly for mature, pregnant females that remain in the region during summer and autumn before migrating to mid-latitudes. • Porbeagle shark is caught as by-catch by freezer and surimi trawl fleets operating in the austral Southwest Atlantic Ocean. • Data from onboard observers were used to estimate annual bycatch and regional density of the species. • Bycatch levels and regional densities of porbeagle shark appear lower than in other parts of its range and have remained stable in recent years, with no clear temporal trend. • The standardized density index of porbeagle shark showed interannual fluctuations with no clear temporal trend, likely influenced by changes in fleet operations and fishing effort.