Fish assemblages on shipwrecks versus natural reefs in Colombo, Sri Lanka

Shipwrecks provide important habitats for reef fishes, but few studies have addressed how fish assemblages on wrecks compare to natural communities on nearby reefs in terms of species composition, diversity, richness, and density, particularly in the Indian Ocean. To fill this knowledge gap, we conducted standardized diver-operated video transect surveys on three shipwrecks and three nearby natural sandstone reefs in Colombo, Sri Lanka. The shipwrecks provided a habitat that is structurally more complex than the surrounding reefs. A total of 2918 fishes from 20 families and 30 genera were recorded, with 749 observed on reef sites and 2169 on the wrecks. A higher mean density of fishes was observed on wrecks [mean (SE) = 17.2 (5) per 125 m2] than on natural reefs [11.9 (4) per 125 m2]. This difference was predominantly due to the snappers (Lutjanidae), which tended to aggregate in large schools in and around wrecks and constitute a resource for local artisanal fisheries. Wrecks and natural reefs presented similar levels of diversity at the family and genus level and shared 86.7% of genera. They nonetheless showed significant differences in community composition at both the family and genus level. Higher abundances of snappers, cardinalfishes (Apogonidae), and fusiliers (Caesionidae) were recorded on wreck sites while the natural reefs presented higher abundances of damselfishes (Pomacentridae) and barracudas (Sphyraenidae). These results differ from previous similar studies, indicating that differences in fish communities between wrecks and natural reefs can be idiosyncratic. This study highlights the role of shipwrecks as artificial reef structures and their relevance for small-scale fisheries and SCUBA diving tourism.