Engaging non-scientists to survey ecosystems, a process known as citizen science has been adopted worldwide. For the first time, this was applied to monitor fish assemblages in the Kas Peninsula, Turkey, an area known for its important Lessepsian fish populations. For 3 years (2004, 2007 and 2010), fish assemblages were surveyed using underwater visual census by transect method. A total of 29 species was observed, seven of which were Lessepsian species. Results show a significant increase in the Lessepsian species over the study period. In 2004, they represented 34% of the total abundance, increasing to >61% in 2010. Differences were observed in the progression of populations between two invasive herbivores, Siganus rivulatus and Siganus luridus (Siganidae), and two native herbivores, Sparisoma cretense (Scaridae) and Sarpa salpa (Sparidae). The siganids were recorded each year in all sites, whereas S. cretense was regularly observed in fewer numbers, while S. salpa was rarely censused. Abundance of Siganus spp. increased threefold in 6 years while abundances of S. cretense and S. salpa remained stable. S. rivulatus was the most abundant among the four species. Its competitive superiority may be due to its greater adaptability to fluctuating environmental conditions and biological traits, e.g. rapid growth, earlier sexual maturity, high fecundity. Because siganids are herbivorous, their presence can strongly impact algal ecosystems. It is then important to follow their spreading at a large scale, but this can be challenging due to the limited number of scientists. Adopting citizen science can thus be a useful strategy to monitor the spread of invasive species in the Mediterranean.