St. Eustatius GCRMN Caribbean Final report 2017
Twenty sites across four monitoring zones of St. Eustatius’s marine ecosystem were assessed using the Global Coral Reef Monitoring Network Caribbean Initiative (GCRMN-Caribbean) guidelines between June and July 2017. The protocol extracted data that indicated the biomass of key fish species and overall reef health including but not limited to relative abundance of macroalgae and hard coral, coral disease prevalence, coral recruit density and water quality to name a few. Using the Reef Health Index (McField and Kramer 2007), St. Eustatius’ coral reef ecosystem is in “critical” condition (RHI Score = 1). Coral cover continues to remain stable from previous estimates (2015 = 5.19%, 2016 = 4.99% and 2017 = 4.73%) with macroalgae continuing to dominate (2015 = 27.93%, 2016 = 27.92%, 2017 = 36.6%). Herbivorous fish (Parrotfish/surgeonfish) biomass which aid in keeping macroalgal biomass in check, has suffered a 58% reduction over the last 18 years. The impact of which is observed in the increased macroalgal cover suggesting that parrotfish were the dominant algal grazers in the past since the black urchin (Diadema antillarium) die off. Reports on coral reef surveys done on the island in 1999 described low macroalgal cover in the presence of very high parrotfish/surgeonfish biomass. Grouper/snapper biomass is also poor with no large grouper species being observed on any of our survey dives. Even though these species were observed in relatively frequent numbers in 1999 at similar survey sites. The northern reserve had the highest fish density but lowest biomass suggesting a greater number of smaller fish, possibly juveniles when compared to the other zones. The southern reserve had the greater density of larger individuals when compared to the Atlantic side suggesting that both reserves may be playing a role in maintaining fish biomass and density. The coral species assemblage on the island’s reefs has changed from being dominated by primary reef building corals such as Orbicella spp, Pseudodiploria strigosa and Montastrea cavernosa to smaller boulder corals such as Siderastrea siderea and Porites astreoides. This is probably due to the cumulative impact of the 2005 bleaching event and structural damage by multiple hurricanes, hampering the recovery process. In brief, further studies into identifying the specific drivers of coral reef degradation and potential mitigation measures on the island of St. Eustatius are needed to lessen the blow on these limited and fragile ecosystems.