Study on the abundance of bottlenose dolphin (Tursiops truncatus) in the Revillagigedo Archipelago (Mexico)

Abstract

The main objective of this study was to identify and catalog the population of bottlenose dolphins (Tursiops truncatus) in the Revillagigedo Archipelago (REV), Mexico, to obtain a population estimate, as well as to analyze some ecological aspects, including the presence of calves and if there are movements of individuals between the different islands that make up the archipelago, San Benedicto Island, Socorro Island, and Roca Partida Island. In REV, field work with photo-identification was carried out for the period 2018-2024. A total of 191 sightings of bottlenose dolphins were made, of which 140 could be photographed. 90% of the sightings were of groups of 15 or fewer animals, and the presence of calves was documented in 25 of the sightings (13% of the total), all of which occurred in waters adjacent to Socorro Island. A total of 324 dolphins were identified, of which 15% were recaptured between two and five years, suggesting philopatry for several of the dolphins. Of the 324 dolphins identified, only two were seen on different islands. The discovery curve of new individuals did not stabilize, while the population estimate using the Lincoln-Peterson method yielded an estimate of 237 individuals, ±60 SE. The results suggest that two population units may coexist in the adjacent waters to the archipelago. This study highlights the waters of the archipelago as an important area for dolphins and suggests evidence of a "resident-island" population, that requires systematic monitoring and comprehensive research, including molecular ecology, trophic ecology, social structure, acoustics, and morphology approaches, to determine the status of the population.