*2.1. Study Site*

Serranilla island (15◦50 N, 079◦50 W), officially named Isla Cayos de Serranilla, is located at the northwest end of the Seaflower Biosphere Reserve (Figure 1). Seaflower BR, with an area of 180,000 km2, was declared as a Biosphere Reserve by UNESCO in 2000; within this larger area, the Colombian Government designated 65,000 km2 as Marine Protected Area in 2005 [33]. In fact, Serranilla is not a single island, but a reef complex formed on a submarine mountain, on which emerged cays are scattered. The principal cay is Beacon Cay, located in the south east of the complex (Figure 1c). The underwater seascape consists of a carbonate platform with reef habitats such as reef crests, coral patches, sand flats, hardground areas, and also seagrass beds [34]. This island is located a long way (>400 km) from the most populated island of the archipelago, San Andrés, and lies only 320 km south of Jamaica (Figure 1a,b; Figure S1). San Andrés, in contrast, has a terrestrial area of 27 km2, with a maximum length of 12.6 km and maximum width of 3 km. On the west side of San Andrés there is a submarine hard bottom platform extending seawards for 200–500 m, and a mainly continuous rocky shore, but no barrier reef. On the east side of the island, the platform extends seawards for more than 2 km with some soft sand bottom

areas, small coral patches, mangroves, and sandy beaches on the coast, and, at the east edge of the northern part, a discontinuous barrier reefs of about 9 km in length [33].

**Figure 1.** Maps showing the position of the study area: (**a**) the location of the Seaflower BR in the south west of the Caribbean, (**b**) map of the Seaflower BR showing the San Andres archipelago, including San Andrés, Old Providence, and Serranilla island, and (**c**) the locations of the survey sites at Serranilla (red points: S1–S10) together with (inserted) an aerial image of Beacon Cay, the largest cay on Serranilla (top left).
