eDNA and UVC field sampling
In February 2020, we collected a total of 20 water samples, from 10 stations, with two filtration replicates per station, in the two investigated coastal areas. Each station consisted of a transect of 2km at an overall constant distance from the coast. We recorded the GPS coordinates at the start and end of the transect, which we used to map the transect positions (Figure 1). We conducted eDNA sampling by using a filtration device composed of an Athena® peristaltic pump (Proactive Environmental Products LLC, Bradenton, Florida, USA; nominal flow of 1.0 L.min−1), a VigiDNA® 0.20µM cross flow filtration capsule (SPYGEN, le Bourget du Lac, France) and disposable sterile tubing for each filtration capsule. We performed two filtration replicates in parallel on each side of a boat, at each station, for 30 minutes corresponding to a water volume of ~ 30 L of water filtered by each capsule. At the end of each filtration, the water inside the capsules was emptied, and we filled the capsules with 80 mL of CL1 Conservation buffer (SPYGEN, le Bourget du Lac, France) and stored at room temperature. We followed a strict contamination control protocol in both field and laboratory stages (Goldberg et al. 2016, Valentini et al. 2016). Each water sample processing included the use of disposable gloves and single-use filtration equipment.
Fish composition data from UVC were collected in 2015 by the Carmabi institute. UVC sampling sites were approximately 700 meters apart along the entire island’s southern protected coast. Researchers surveyed fish composition by scuba diving at each of the sites. At each site, researchers conducted a total of five transects that were 30 meters in length. For each transect, divers quantified the number, size and identity of all fishes. All transect lines followed a constant water depth of 8 to 12 m. Survey times per transect were limited to approximately 6 minutes per transect for a total of 30 minutes of surveys, a sampling duration equivalent to the eDNA surveys. The data from the five transects were pooled to provide the final assemblage of all fish species at each site. We selected the sites occurring in the coastal areas of focus, Willemstad and Valentijnsbaai.