Breeding evidence and seasonality of the blue shark (Prionace glauca) on the northern Catalan coast

Many shark species face serious threats that have generally led to a collapse in their numbers at a global scale. The blue shark (Prionace glauca) is one of the most by-caught large shark species in the Mediterranean Sea, where it has been classified as critically endangered. Despite the status of the species in this sea, little is known about its breeding areas. Here, we provide evidence that the Cape of Creus marine canyon and its surrounding areas are a blue shark mating, birthing and nursery area. We carried out a total of 170 chumming pelagic surveys from April to October in 2022, 2023 and 2024, with a total sampling effort of 933 h. In those surveys, we carried out 213 blue shark observations, including young of the year (n=10), some of them newborns, and adults (n=25), one of them a pregnant female. The encounter rate of blue sharks during periods of high adult and newborn presence was estimated at 0.42 individuals per hour, showing a significant seasonal concentration in this area. The peak presence of adults occurred between June and July, while that of newborns was observed in late June, overlapping with the peak of adults. We also registered two young of the year near the head of the Begur/La Fonera submarine canyon. A significant number of juveniles were also found in the study area, showing that this zone is also crucial for them. We conclude that the Cape of Creus, Lacaze-Duthiers and Begur/La Fonera submarine canyons are important areas for conservation of the blue shark in the western Mediterranean Sea and therefore emphasize the need for effective management and conservation strategies.