Fish surveys

For the fish surveys, a belt transect with an underwater visual censuses sampling technique (snorkeling) was used. A 25 m linear transect was placed at the center of a 2 m-wide belt. All fish seen along the transect were identified to the species level. On the first transect pass, the observer recorded highly mobile fish that entered the transect but usually fled as a snorkeler approached. On the second pass, less mobile, cryptic, and site-attached species were targeted with more detailed examinations of crevices. For substrate surveys, the cover proportions of live hard corals, dead corals with algal turf, macroalgae, sand, rubble, and others (e.g. anemones, shells, soft corals) were sampled using the Point Intercept Transect method every 50 cm over 25 m of the belt transect used for the fish survey. Sampling was conducted along three transects (three replicates) separated by 25 m at each site. The three belt transects were set up in the middle of each shallow reef flat (depth: 1 m).

A total of 221 fish species were identified on the fringing reef of all MPA or non-MPA sites at Moorea³⁵. The classification of these species as predators or competitors of S. nigricans was based on direct observations made during previous research at these locations^11,39–43. When research on a particular species was not available in the literature, we relied on expertise provided by Prof. Galzin, who has worked on coral reef fishes at these locations continuously since 1974^39–43. Among the 221 recorded species, those within the families Serranidae (Epinephelus hexagonatus, E. merra and Cephalopholis argus), Muraenidae (Gymnothorax javanicus, G. meleagris, G. undulates and Echidna nebulosa) and Scorpaenidae (Scorpaenopsis diabolus, Pterois antennata and P. radiata) were considered to be predators of S. nigricans. Likewise, the species identified as competitors of S. nigricans (i.e., able to eat the algal turf farmed by S. nigricans) were Ctenochaetus striatus, Acanthurus nigrofuscus, A. triostegus and some Scaridae species (Chlorurus spilurus, Scarus frenatus, S. globiceps, S. oviceps, S. rubroviolaceus, and S. schlegeli). Moreover, some other territorial damselfish such as Chromis viridis, Dascyllus aruanus, and Pomacentrus pavo may compete with S. nigricans for space. As only adult fish are likely to be competitors/predators of S. nigricans, we only considered S. nigricans, predators, and competitors that reached their adult sizes in our analyses⁴⁴.