-Incidental catch or bycatch represents a significant threat for the conservation of seabird populations. The western Mediterranean is an important fishing area where the Spanish pelagic and semi-pelagic longline fleet targeting swordfish (Xiphias gladius), bluefin tuna (Thunnus thynnus) and albacore (Thunnus alalunga) operates. Bycatch of these fisheries includes several seabird species. Given the importance of conservation of the bycatch species (marine mammals, turtles, sharks and seabirds), an on-board observer program was implemented by the Spanish Oceanographic Institute (IEO); this included collecting data on effort and catch, as well as weight and number of individuals of the main bycatch species. The aim of the present study is to report data on seabird bycatch collected by the on-board observer program of the IEO in the Western Mediterranean. Data on seabird bycatch were collected for the period 2000-2008, throughout the year. Six longline gears targeting large pelagic fish were identified operating in the area of study, but only three had an effect on seabird species. Differences in catch per unit effort (CPUE, birds per 1000 hooks) for each gear, as well as their effect on particular seabird species, are reported in this study. A total of 4 786 466 hooks were monitored, which yielded 182 seabirds belonging to 7 different species. The average CPUE for the studied period was 0.038 birds per 1000 hooks. Cory's shearwater (Calonectris diomedea) and yellow-legged gull (Larus michahellis) were the species the most highly represented in the catch. In contrast, Balearic shearwater (Puffinus mauretanicus) was only present in the longline fishery targeting albacore (CPUE = 0.005 birds per 1000 hooks). Our results suggest that Cory's shearwater is the species the most affected by the longline fishery in the Western Mediterranean, probably due to its biological characteristics, and corroborate the well-established downward trend in its population.Key words: Bycatch / Seabird / CPUE / Western Mediterranean Sea / Pelagic longline Résumé -Les captures fortuites ou accessoires représentent une menace considérable pour la sauvegarde des populations d'oiseaux de mer. La partie occidentale de la Méditerranée est une zone importante de pêche où la flotte espagnole de palangriers pélagiques et semi-pélagiques opère et cible l'espadon (Xiphias gladius), le thon rouge (Thunnus thynnus) et le germon (Thunnus alalunga). Les captures accessoires de ces pêches incluent plusieurs espèces d'oiseaux de mer. Étant donnée l'importance de la protection des espèces capturées accidentellement (mammifères marins, tortues, requins et oiseaux de mer), un programme a été mis en place par l'Institut espagnol d'Océanographie (IEO) : des observateurs étant à bord collectent des données d'effort de pêche et de capture, aussi bien que le nombre d'individus et le poids des principales espèces des prises accessoires. L'objectif de cette étude est de rapporter les données de captures accessoires des oiseaux de mer, collectées par ce prog...

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Hundreds of thousands of seabirds die annually as a result of being bycaught in longline fisheries, and these rates are unsustainable for many seabird populations worldwide. To understand effects at the population level, it is essential to assess sex-and age-biased mortalities, since uneven mortalities may exacerbate bycatch impacts. In the Mediterranean, bycatch is the main cause of population declines for the 3 endemic shearwater species (Scopoli's Calonectris diomedea, Balearic Puffinus mauretanicus and Mediterranean shearwater P. yelkouan), but little is known about population biases of the birds caught in longliners. From 2003 to 2015, we collected 639 shearwater carcasses from Spanish longliners operating in the north-western Mediterranean, determined their age and sex and examined their spatial and temporal patterns and the origin of ringed birds. Most shearwaters caught in longliners were adults, but the proportion of immatures and subadults increased in the late breeding period. Adult Scopoli's shearwaters were mostly caught around the breeding colonies. In contrast, all Puffinus birds were caught on the Iberian shelf. Catches of Scopoli's shearwaters were male-biased, particularly during the prelaying period and close to the breeding colonies. Catches of Puffinus shearwaters were also malebiased during the pre-laying period, but adult catches were female-biased during chick-rearing. Ring recoveries revealed that most birds were ringed in the nearby Balearic Islands, but some Scopoli's shearwaters ringed in France and Italy were also caught during their migration, indicating that the impacts of the Spanish longliners extend well beyond the Spanish colonies. The adult-biased and sex-biased mortality found in this study may aggravate bycatch impacts on populations and highlights the urgent need for conservation action.

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Fisheries by-catch is considered to be a major threat to loggerhead turtles in the Mediterranean Sea. Technical differences in both gear configurations (e.g. hook and bait type) and fisheries operations carried out by the Spanish Mediterranean surface longline fleet could have an effect on by-catch rates and size selectivity. The aim of the present study was to test the differences in by-catch per unit effort and body size of loggerhead sea turtles caught using different gear types in the Mediterranean surface longline fishery. Our results suggest that differences in the gear type used have an effect on catch rates and size selectivity. Thus, surface longliners targeting albacore (LLALB) using smaller hooks tend to capture smaller loggerheads but have the highest by-catch per unit of effort (BPUE), whereas other longlines, such as surface longliners targeting bluefin tuna (LLJAP) and traditional surface longliners targeting swordfish (LLHB), using larger hooks tend to select the larger animals; moreover, LLHB had the lowest BPUE. Disproportionate rates of fisheries-induced mortality on certain size/age-classes can differentially affect sea turtle populations, as each sea turtle age-class contributes differently to current and future reproduction. Thus, fisheries management should not only be focusing on preserving the large juvenile and mature turtles, but also on reducing the total by-catch. Thus, we recommend encouraging the use of LLHB versus other surface gears. We suggest that it is very important to take into account the gear type (and its particular catch rates) when making inferences about the impact of longline fisheries on sea turtle populations.