1. Most fish stocks worldwide are fished at maximum sustainable yield (MSY) or overfished, as many fisheries management strategies have failed to achieve sustainable fishing. Identifying effective fisheries management strategies has now become urgent.

2. Here, we developed a spatially-explicit metapopulation model accounting for population connectivity in the north-western Mediterranean Sea, and parameterized it for three ecologically and economically important coastal fish species: the white seabream Diplodus sargus, the two-banded seabream Diplodus vulgaris and the dusky grouper Epinephelus marginatus.

3. We used the model to assess how stock biomass and catches respond to changes in fishing mortality rate (F) and in the size of fully protected areas within the existing system of multiple-use marine protected areas (MPAs). For each species, we estimated MSY and the corresponding values of stock biomass (B_MSY) and fishing mortality rate (F_MSY), providing crucial reference points for the assessment of fisheries management.

4. D. sargus is currently in low overfishing, while D. vulgaris and E. marginatus are in high overfishing. Stock recovery to B_MSY for the last two species requires a reduction of current F around 50%. This would guarantee an increase in both stock biomass (around 50 and 75% for D. vulgaris and E. marginatus, respectively) and catch (around 15 and 30%) after a transient time of ~15–30 years. Alternatively, doubling the size of fully protected areas over fishable areas within the existing network of MPAs would lead to positive conservation effects for all three species without substantially affecting the overall productivity of the fishery and the total economic value of the catch.

5. Synthesis and applications. We provide the first assessment of stock status for three coastal species in the north-western Mediterranean and evaluate the ecological and fisheries outcomes of different management strategies. Extending full protection inside existing multiple-use marine protected areas or reducing fishing effort outside can deliver both conservation and fisheries benefits.

Data on population density for the three fish species were collected in June–July 2017 across the north-western Mediterranean, at sites characterized by three different protection levels: fully protected (no-take zone), partially protected (buffer zone) and unprotected (outside MPAs). Data were collected via underwater visual census (UVC) along instantaneous strip transects of 25×5 m. Censuses were performed on rocky substrates where other substrate types, like sand or seagrass, generally represented <10% in cover (both within and around transects). Population density data collected at different sites were averaged over a grid with a 1/16° resolution (keeping different protection levels distinct). In addition, a long-term time series was collected within the Tavolara MPA (Sardinia, Italy), where population density was assessed via UVC between 2005 and 2016.

The first dataset is contained in the files NWMed_xx.csv, where xx indicates the species (DS: Diplodus sargus; DV: Diplodus vulgaris; EM: Epinephelus marginatus). Each file reports longitude and latitude of the centre of the grid cell, level of protection (FP: fully protected; PP: partially protected; UP: unprotected), and population density (individuals/m²). The second dataset is contained in the files Tavolara_xx.csv, which report year, level of protection and population density for each species (where xx is, again, the acronym of the species).