Rising herbivory pressure of Sarpa salpa and the emerging threat to Southern European marine forests

Climate change (CC) is driving shifts in marine ecosystems, particularly affecting temperate and subtropical marine forests, which are critical for biodiversity and ecosystem stability. Dominated by canopy-forming seaweeds, these forests are sensitive to CC-induced stressors, such as rising temperatures and tropicalisation, which favour turf-forming algae and increase the abundance of herbivorous fish, threatening ecosystem structure and function. Along the western coast of Portugal in Southern Europe, the herbivorous fish Sarpa salpa plays a key role in tropicalisation, potentially exacerbating the decline of cold-water seaweeds, whilst the Iberian upwelling system may offer localised protection through cooler conditions. This study combines stomach content analysis, subtidal surveys, functional trait analyses, fish landing records, and sea surface temperature (SST) data across the North, Centre, and South regions to assess the ecological role of S. salpa. Results confirmed a preference for brown algae, including habitat-forming species, with clear regional variation: kelps dominated diets in the North, whilst Cystoseira s.l. and turf-forming species prevailed in the Centre and South. Fish landing analyses revealed region-specific dynamics: landings in the North increased with higher SST, those in the Centre were unaffected by temperature, and those in the South were negatively correlated with SST. These findings highlight the potential threat of S. salpa to temperate marine forests under CC.