Status and trends of coral reef health indicators on Saba (Caribbean Netherlands)

Coral reefs are threatened globally and their trajectories of decline are very similar worldwide. In the Caribbean coral cover has declined by 80% since the 1970s. Global factors, such as ocean warming and acidification, play a role in this decline, but local factors, such as pollution, coastal development and overfishing, are the largest cause of this decline. These factors have caused an overall decline in coral cover and increase in macroalgae cover, demonstrating a shift from coral-dominated reefs to macroalgae-dominated reefs throughout the Caribbean. Understanding how much and why reefs are threatened and furthermore, why some reefs are threatened more than others, is important in providing and improving management options.

This study aimed to investigate the current status and trends of coral reef health around Saba (Caribbean Netherlands). Monitoring surveys were conducted on 20 sites around the island, according to the protocols of the Global Coral Reef Monitoring Network (GCRMN). On each site 5 transects were surveyed for the following coral reef indicators: fish biomass and density, coral and macroalgae cover, coral health, density of coral recruits, density sea urchins and cucumbers and water quality. Fish abundance and lengths were recorded by a Diver Operated Video-system (DOV). Fish biomass was calculated by using length-weight relationships. Coral and macroalgae cover and coral disease prevalence were assessed through photographs of the benthic substrate. Coral recruits, sea urchins and cucumbers were counted along the transect. Water quality was measured as water transparency using a Secchi disk.

Statistical analysis was done using a generalised linear mixed model (GLMM). Each indicator was tested for significant difference between the fished and unfished zone of the Saba Marine Park (SMP). Furthermore, the effect of habitat complexity on the reef health indicators was taken into account in the model. The Reef Health Index was used to give an overall score on reef health in Saba.

The overall health of the reefs on Saba is ‘fair’, according to the RHI score. When looking at the different zones separately, the unfished zone scores better (fair) than the fished zone (poor). However, when looking at the coral reef indicators separately this difference in status between zones could not be seen so clearly and no statement could be made on whether this was a result of the protection of fishing. Total fish density was significantly higher in the unfished zone. This is in agreement with other studies, where higher fish abundance and biomass are found in areas protected by MPAs due to a decrease in fishing mortality. Acanthuridae (surgeonfish) biomass and density were found to be significantly higher in the fished area, which could be because of a lack of predator control in the fished area. However no other significant effects on fish assemblage were found, which could be explained by the relatively low fishing pressure. Habitat complexity did not differ between zones and did not have an effect on total fish biomass and density, but it did have a significant effect on Acanthuridae density, with higher abundance at higher levels of complexity. Coral cover was significantly higher in the unfished area, although no evidence was found for an indirect effect of fishing through trophic cascades due to a decrease in herbivores and increase in macroalgae. Although macroalgae cover seemed to be higher in the fished zone, no significant difference was found. Zonation had no effect on other benthic indicators.

There was a 20% decrease of coral cover in the last decades and an increase in herbivore biomass and density, with values around 4 times as high in 2015 than in 1991. This is consistent with trajectories in the rest of the Caribbean. A clear increase is seen for commercial fish (groupers and snappers), with a 2-3 fold increase for density and biomass in the fished zone and a 4 and 12 fold increase for density and biomass in the unfished zone. Indicating a decrease of fishing pressure over the years and the allowance for species to grow bigger in unfished areas. Much higher values for commercial fish biomass is seen in Saba than values recorded for other reefs in the Caribbean. Habitat complexity did not show a clear decrease and does not follow the trajectories of reef flattening in the rest of the Caribbean.

Overall, a combination of factors is most likely responsible for the degradation of Saba’s reefs. Overfishing could have impacted the reefs before the establishment of the SMP in 1987 and these areas might still be recovering from the fishing pressure exerted before the establishment of the SMP. Run-off and sedimentation increase eutrophication and susceptibility to disease. Together with a decrease in key herbivores, caused by the Caribbean wide die-off of Diadema antillarum (long-spined sea urchin) in 1983, this results in algal blooms, which eventually leads to a phase shift from coral to macroalgae dominated reefs. Reefs damaged by these local threats are more susceptible to the global threats of ocean warming and acidification (e.g. coral bleaching and increase in hurricanes) which will cause further degradation on the long-term.