First Use of Free-Diving Photo-Identification of Porbeagle Shark (Lamna nasus) off the Brittany Coast, France

Abstract

A large number of pelagic shark species have declined significantly in recent decades due to overfishing, bycatch, and habitat degradation. Whereas porbeagle sharks have become scarce due to a reduction in their populations around the world, recent stock evaluations are giving positive signals about the evolution of the North-Eastern Atlantic stock size. The porbeagle shark (Lamna nasus), an offshore pelagic species with a wide distribution, is designated by IUCN as Globally Vulnerable and Critically Endangered for Europe and subject to various international conservation conventions. An increasing number of observations are reported off the Brittany coast of Trégor. The ecological role of this area for the species is still unknown and greater knowledge is needed to develop and apply sustainable management measures on a local and international scale. This study represents the first use of photo-identification on porbeagle sharks in order to improve the ecological knowledge of the species in the Trégor area. These results confirm the effectiveness of this method, with 19 of the 131 individuals identified being re-sighted, indicating an interesting degree of site fidelity and showing a sex ratio of 100% females. Observations of individuals over several years allowed the researchers to discuss the relevance of the different types of marks. The findings suggest that the Trégor area off the Brittany coast serves as a seasonal residence for female porbeagle sharks, especially between May and October. This study represents a successful first step in the use of photo-identification for this species. It offers technical support for the sharing of the methodology and provides some biological knowledge allowing researchers to discuss potential sustainable management measures for the conservation of porbeagle sharks in the study area and their habitats while needed.