Assessing the effectiveness of no-take zones on fish populations in the Marine Natural Park of Cap Corse and Agriate, Northwestern Mediterranean Sea

No-take zones (NTZs) are expected to rebuild exploited fish populations, yet their performance is rarely assessed with species-level indicators. We quantified the reserve effect of the Nonza-San Fiurenzu NTZ (24.2 km2) inside the Marine Natural Park of Cap Corse and Agriate (north-western Mediterranean) and simultaneously compared the effectiveness of two underwater visual census (UVC). Using 488 underwater visual-census transects (2018-2022) we monitored densities, size structure and biomass of three emblematic predators-the common dentex Dentex dentex, dusky grouper Epinephelus marginatus and brown meagre Sciaena umbra-and compared trends with neighbouring fished sectors. Gaussian GLMs related log-transformed density and biomass to protection status, season, year, sea-surface temperature (SST) and chlorophyll-a. Protection was the dominant predictor for E. marginatus and S. umbra: mean densities inside the NTZ were 3-4 times higher than outside, and biomass was enriched by factors of six and four, respectively. For the mobile D. dentex densities did not differ, but biomass was greater in the NTZ, indicating size-selective benefits. SST showed a negative effect on D. dentex and S. umbra; chlorophyll-a had no detectable influence. Cross-transect surveys (two divers) yielded density estimates comparable to-or higher than-the logistically intensive comb protocol (four to eight divers), suggesting that simpler designs can suffice for long-term monitoring. Our results demonstrate that a well-enforced NTZ embedded in a moderately protected park can rapidly enhance biomass and restore size structure of vulnerable Mediterranean predators.