Incidental Mortality of Black‐Browed Albatross and White‐Chinned Petrel in Argentine Longline Fisheries: A Comprehensive Estimation Addressing Spatial and Temporal Stratification and Cryptic Mortality

ABSTRACT The continental shelf and shelf‐break of the Southwest Atlantic are among the most productive and biodiverse regions globally. Incidental mortality from fishing activities poses a significant threat to seabird populations in this area. The overlap between fishing operations and seabird distributions leads to bycatch, particularly in longline fisheries where seabirds get hooked during setting or hauling operations. This study is the first in Argentina to assess seabird incidental mortality considering cryptic mortality. We estimated potential annual mortalities between 2005 and 2009 for two keystone species that predominantly interact with fishing vessels. For black‐browed albatrosses, the estimated annual mortality was 686 individuals (95% CI: 582–800), whereas for white‐chinned petrels was 2278 individuals (95% CI: 1981–2606). We evaluated the differential impact of three longline fleets, distinguished by their operating areas, gear configurations and target species. For both seabird species, the estimated mortality levels in the fleet targeting kingklip were an order of magnitude higher than the fleets targeting skate and Patagonian toothfish. The highest incidental mortality levels were concentrated in the central continental shelf between 44° S and 49° S near the slope. Identifying bycatch patterns may inform and improve the spatial management of fisheries, which is crucial as their cumulative bycatch significantly impacts the sustainability of seabird populations. We emphasise the need to increase the observer coverage in fisheries and improve data quality. Future research should aim to evaluate the effects of incidental mortality on the sustainability of seabird populations across multiple fisheries.