Opportunistic data for assessing abundance and site fidelity of fin whales (Balaenoptera physalus) in the Faro area, Portugal

Abstract The distribution and migration of fin whales ( Balaenoptera physalus ) remain poorly understood in parts of the North Atlantic. One of these regions is the Algarve in southern Portugal, which is shaped by seasonal upwelling and the exchange of Atlantic and Mediterranean waters. This area represents a potentially critical yet poorly understood habitat for this species. This study presents the first comprehensive assessment of fin whale abundance and site fidelity in the Faro area, located approximately between 36–37° N and 8–7° W. Data was collected from 2020 to 2024 through photo-identification and GPS vessel tracks, in collaboration with a local whale-watching company. A regional catalog of 86 individuals was established, and an open spatially explicit capture-recapture model estimated a population of 1.476 individuals. Sightings peaked in spring, likely linked to phytoplankton blooms and foraging opportunities. While re-sighting rates were low, some individuals returned across years, indicating possible seasonal site fidelity. Our results demonstrate the value of data collected from platforms of opportunity in an under-surveyed region, providing a foundation for ongoing monitoring efforts. Results also highlight the Faro area’s ecological relevance as a seasonal feeding ground for fin whales and offer insights to support future conservation in the eastern North Atlantic.