Abundance decline over 25 years in Mytilus galloprovincialis on the mid-shore differs along a latitudinal gradient on the Portuguese coast

Climate change drives range shifts in species distributions, but these shifts may be non-linear, with mesoscale processes influencing their pace and supporting resilience in some populations. We re-surveyed distribution patterns of the mussel Mytilus galloprovincialis (Lamarck, 1819) along the Portuguese mainland coast to compare abundance between 1997 and 2022. Potential biogeographical discontinuities in abundance associated with the Nazaré and Lisboa-Setúbal Canyons and a sandy beach (~50 km south of Setúbal) were also examined. An updated data collection method was validated by comparing the abundance of mussels from traditional in situ quadrat surveys with drone imagery. Spatial analysis across 30 sites in four regions (North, Centre, Southwest, South) in 2022 revealed a clear North–South decline in mussel abundance, with highest mean cover in the North (36.1%). A marked decline was observed south of the Nazaré Canyon, but no discontinuity was observed related to the Lisboa-Setúbal Canyon or sandy beach. Compared to 1997, the northern region maintained consistently high abundance of mussels, while the Centre and South showed significant declines, matching the Southwest region which remained low. The North region has significantly lower sea surface (SST) and air temperatures in both summer and winter, and higher significant wave height during winter. No clear temporal trends (1992 to 2022) were observed in average SST, air temperature, significant wave height, or atmospheric and marine heat waves. However, the southern region showed an increased dispersion in summer SST values over the last 15 years, suggesting greater climatic variability and more frequent extreme events. This variability may explain the sharp mussel decline from 36.5% to 1.6%. North of the Nazaré Canyon mussel populations seem resilient over 25 years. The vulnerability of southern populations suggests a worrying trend linked to decline of mussels and range shifts elsewhere in the world.