Transnational mortality from Spanish longline fisheries bycatch is shaping the decline of a vulnerable French seabird.

The marine realm suffers from cumulative causes of biodiversity erosion and world seabird community declined by 50 % since 1970. Seabirds routinely transgress regional and international boundaries and threat assessments should be performed at large spatial scales. We studied the demographic consequences of transnational Spanish longline fisheries bycatch on vulnerable Scopoli's shearwater (Calonectris diomedea) populations in the French Mediterranean. First, we assessed space use for 174 shearwaters breeding in the Calanques National Park (NP) using GPS-tracking between 2011 and 2022. By matching this information with a published bycatch risk map for Mediterranean Spain, we found that shearwaters largely overlapped with Spanish longline fisheries. Second, we calculated Spanish demersal longline bycatch on shearwater populations of Calanques and Port-Cros NPs, using bycatch reports, ring recoveries and ringing effort. Annually, Spanish demersal longline fisheries killed 5.3 birds (95 % CI = 0.4–14.2; 0.5 % of the population) from the Calanques NP and 8.1 birds (0.5–21.6; 2.6 % of the population) from Port-Cros NPs. Third, we assessed the demographic consequences of this bycatch using multi-event capture-recapture models and matrix population modeling based on long-term nest monitoring. Adult survival was low (0.84 to 0.92) relative to other Procellariiforms. Even though annual shearwater casualties on Spanish demersal longline seem modest, they may compromise the persistence of several French Scopoli's populations within 50 years. We demonstrate the importance of designing marine conservation at international scales for highly mobile species, to fully embrace the cumulative effects of marine anthropogenic threats on adult seabird survival, the main leverage effect for long-lived vulnerable populations. • Seabirds suffer from the transnational cumulative impacts of fisheries bycatch. • The French Scopoli's shearwater population largely overlaps with Spanish fisheries. • Demersal Spanish longliners kill up to 3 % of French colonies annually on average. • Spanish bycatch therefore accelerates French shearwater population decline. • Highly mobile species require an international approach to marine conservation. [ABSTRACT FROM AUTHOR]