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Local Threats to Coral Reefs
Estimated threat to coral reefs from combined local activities such as development, pollution and fishing
Caution: Certain assumptions or simplifications were made about each of the four types of threats considered in this study. The assumptions, listed by category, are described below:
Coastal development:
• Some tourist locations were missed in the modeling process, especially new locations.
• The model did not directly capture sewage discharge, but relied on population as a proxy.
Watershed-based pollution:
• The model used a proxy for sediment, nutrient, and pollutant delivery.
• Nutrient delivery to coastal waters might have been underestimated due to a lack of spatial data on crop cultivation and fertilizer application. However, agricultural land was treated as a separate category of land cover, weighted for a higher influence.
• The model did not incorporate nutrient and pollutant inputs from industry or from intensive livestock farming, which could be considerable.
• Sediment plumes accumulated through inland watersheds and discharged at river mouths can drive reef degradation. After reaching the river mouth, sediment matter disperses according to currents, but the movements of currents were not taken into account in modeling the plume dispersal process. The model designers decided for the sake of simplicity to model the dispersion process as depositing an equal proportion of the plume in successive concentric circles around the river mouth until all of the plume matter has been allocated. This data did not include threats from nutrient deposits released at river mouths, which can cause algal blooms which damage coral reefs.
Marine-based pollution and damage:
• Threats associated with shipping intensity may have been underestimated because the data source was based on voluntary ship tracking, and did not include fishing vessels.
• The threat model did not account for marine debris (such as plastics), discarded fishing gear, recreational vessels, or shipwrecks, due to a lack of global spatial data on these threats.
• The drivers of coral reef degradation include discharged oil, bilge, and ballast liquids from ships involved in world trade. There were no available datasets that measure these discharges, so port and ship traffic density were used as a proxy.
Overfishing and destructive fishing:
• Accurate, spatially referenced global data on fishing methods, catches, and the number of fishers was not available; therefore, population pressure was used as a proxy for overfishing.
• This model didn’t capture the targeting of high-value species, which affects most reefs globally but has fewer ecosystem impacts than wider-scale overfishing.
• Management effectiveness scores were only available for about 83% of the reefs within marine protected areas.
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