Long-term sea urchin densities (Paracentrotus lividus and Arbacia lixula) and predatory fish biomass in and around the Medes Islands (1991–2023) and Tavolara–Punta Coda Cavallo (2006–2025) marine protected areas, western Mediterranean

This dataset provides long-term annual densities of two co-occurring Mediterranean sea urchins, Paracentrotus lividus and Arbacia lixula, together with sea urchin predator fish biomass, at two Western Mediterranean Marine Protected Areas (MPAs): the Medes Islands (Catalonia, Spain) and Tavolara-Punta Coda Cavallo (Sardinia, Italy). Data were collected at 32 rocky-reef sites on shallow rocky substrates at approximately 5 m depth, to document the long-term population dynamics of the two species across gradients of protection. Sea urchin abundance was quantified by scientific divers using standardized underwater visual census along replicate belt transects, following the long-term monitoring protocols routinely applied at each MPA. For each site and year, mean density (individuals per m²) and its standard error are reported per species (all size classes combined). Predator biomass (g per m²) was estimated from underwater visual censuses of fish known to prey on sea urchins, mainly sparids (Diplodus sargus, Diplodus vulgaris, Sparus aurata) and labrids (Thalassoma pavo, Coris julis). Monitoring began in 1991 at the Medes Islands and in 2006 at Tavolara-Punta Coda Cavallo, with the combined series extending to 2025; the series are not fully continuous, as some years lack data due to logistical constraints or changes in sampling effort. Survey protocols from the two independently developed programs were harmonized under the Managing Sustainable Sea Urchin Fishery and Marine Forest Conservation (MurFor) project, including standardization of transect dimensions, depth strata, and data formats, to enable cross-site and cross-MPA comparison. Each record includes site name, geographic coordinates, reserve status, species, year, and depth.