Fisheries-Independent Evidence of Longline Fisheries Impact on the Threatened Mediterranean Blue Shark Prionace Glauca in The Waters Around the Balearic Islands (Spain)

The Mediterranean Sea is one of the seas most affected by the collapse of shark populations due to overfishing. Considered ‘vulnerable’ by the IUCN on a global scale, the blue shark Prionace glauca is classified as ‘Critically Endangered’ in the Mediterranean Sea. A fisheries-independent study was conducted through the implementation of 14 filming sessions, lasting between 5 and 10 days, during winter and summer seasons between July 2017 and February 2020, in semi-pelagic waters around the islands of Mallorca and Menorca (Balearic Islands, Western Mediterranean). Using 24-hour BRUVs, we collected 1685 hours of video that allowed us to observe 10 blue sharks, six males and four females, estimated between 1.2 and 2.5 m in total length (TL). In addition to their low abundance (average of 0.02 shark ±0,025 observed per hour), 50% of animals presented residual hook, sometimes with a nylon leader several meters long, potentially jeopardizing its survival. In addition to other threats, the frequent presence of residual hooks may contribute to a cryptic mortality of this species in relation to pelagic fisheries, calling into question the future of this species in this region of the world.