*2.2. Data Collection*

The surveys were conducted at ten sites at Serranilla during the "Seaflower Research Expedition 2017". Ten sites were selected to capture the heterogeneity of Serranilla (Table S1). At each site, habitat structure (benthic components and rugosity) and fish assemblage structure and composition were assessed using underwater visual census (UVC) techniques conducted by a paired-diver team along 50 m × 2 m belt transects (n per site = 2 or 3). On each transect, the fish species, abundance, and visual estimated size per individual (interval length) were recorded by the first diver, following established sampling methodologies [35–37]. To highlight the specific relationships of the key fish family of parrotfish (Scaridae) with habitat, the abundance of parrotfish according to the size was also calculated; we distinguished large species (>50.1 cm total length) and small species (<50 cm total length) based on available values from FishBase [38]. Valid species names were verified, according to Fricke et al. [39].

Habitat structure was evaluated as a combination of the percentage cover of the main benthic components and of reef rugosity. To characterize the coral community, we selected the percentage cover of five morpho-functional benthic groups: branching corals, brain corals, submassive corals, spherical corals, and algae. The variables were recorded with a video camera that was held 0.4 m above the seabed while being moved along each of the 50 transects. From each such video transect, 40 frames were selected, and 50 points in each frame were sampled to estimate the percentage of the benthic groups. Rugosity was estimated with the rugosity index (RI), obtained using the chain-link method [40]. Within each belt transect, the second diver laid down a 10 m chain over the seabed closely following the substrate contour and then measured the linear distance occupied by the chain. This was done three times within each transect. RI was then calculated as RI = 1 − d/L, where d is the horizontal distance covered by the chain, and L is the chain's true (i.e., stretched) length. This method is quick, objective, and highly replicable [36].

Fish data for San Andrés were obtained during the same sampling period as the Serranilla surveys (September 2017). Data were collected from transects at three sites on the west side (Luna Verde, Wild Life and Bajo Bonito) and one on the east side (Bahía Honda) of the island. Fish species, abundance, and estimated fish size in San Andrés were recorded following the same UVC described above for Serranilla. To compare parrotfish, we selected 12 transects for each location (Serranilla and San Andrés), all placed parallel to the coast, with comparable values of RI (0.19–0.49).
