Photo-Identification Catalogue of the Blue Shark (Prionace glauca) in the Cap de Creus Submarine Canyon (2022 to 2024): selecting the diagnostic characters for intra-year monitoring sighted individuals.The blue shark (Prionace glauca) is a keystone predator within the Mediterranean marine ecosystems. Despite its wide global distribution, its population in the Mediterranean is experiencing a severe decline due to bycatch and targeted fishing, being considered as “Critically endangered” according to the UICN.
The Cap de Creus Submarine Canyon has been identified as an important area for the species, acting as a parts, mating and nursery grounds. The aim of this study was to develop a photo-identification catalogue for intra-year individual sharks monitoring. A total of 186 observations of sharks were collected during 164 chumming sessions carried out between 2022 and 2024. 10 of them were young of the year and in 22 cases no photos or films could be obtained during direct observation sessions. For 47 other observations we were unable to capture images of all or most of their body.
For these reasons we focused on 117 well recorded sharks, including 30 from 2022, 67 from 2023 and 20 from 2024. Images allowed us to compare the observed sharks, focusing on sex, size, scars, cuts, hooks, signals, wounds, abrasions, pigmentation patterns, amputations, tissues necrosis and other morphological anomalies and characteristics.
Firstly, we concentrated on sharks observed in 2023, the year with the highest number of records. It enabled the classification of marks into three main types:
- Type 1 (medium-large orsevere and permanent marks)
- Type 2 (small, mild and potentially temporary marks, that may disappear over time)
- Type 3 (other marks with different characteristics).
Subsequently, only
Type 1 marks were used to identify the individuals. We selected the most useful marks for intra-year photo-Identification, based on their frequency and reliability to minimize identification errors.
This initial review has allowed the documentation of a preliminary set of individuals, with future revisions planned to ensure no individual is overlooked and with an inter-year identification. This is a valuable tool for understanding movement patterns, site fidelity, and potential threats, contributing to the conservation of the blue shark in the Mediterranean.
