Characterizing fish communities must be a priority to safeguard resources and determine critical changes. Here, species richness and the spatial and temporal evolution in the structure of fish assemblages were analysed based on photos taken in underwater free-diving contests. A total of 29 contests held from 2008 to 2015 at four different locations along the northeastern Spanish coast, including a marine protected area were analysed. Contests reward the number of species per participant and photographic quality. Species image frequency from each tournament were standardized to catch image rate. A total of 88 taxa were recorded, including 32 cryptobenthic species, the highest number recorded in the Mediterranean littoral system so far. Cluster analyses yielded four major groups. Catch image rates in the marine protected area were significantly higher for seven species of high commercial interest and for two big labrids of recreational interest, including an endangered species (Labrus viridis). Overall, the study showed that photographic free-diving contest data are a potential tool for determining species richness in littoral systems since contest rules promote competition between participants to obtain maximum fish diversity. We believe that this type of cost-effective data can be applied worldwide as a complementary way of monitoring littoral fish assemblage.