Coral reef survival: depth, marine protected areas, and seascape structure are key
Living corals are more likely to survive and thrive when found in deeper water, within Marine Protected Areas (MPAs), or in compact reef patches, according to a new study by UBC researchers published in People and Nature.
Coral reefs are among the most diverse and valuable ecosystems on Earth. Living corals create shelter for hundreds of marine species, like an apartment building underwater. When corals die, this shelter is lost, leading to sharp declines in fish, invertebrates and biodiversity. Worryingly, climate impacts, overfishing, and untreated sewage pollution have taken a serious toll on these seascapes.
This newly published study focused on Danajon Bank in the Philippines, one of only seven double barrier reefs in the world. This remarkable structure, 135 km long, supports livelihoods and food security through small-scale fishing. The region is in the Coral Triangle, which is the global center of marine biodiversity, and is of huge biological, social and economic importance.
The study authors used modelling to identify factors associated with living versus dead corals, using satellite maps, local knowledge, and long-term fishing records. Such multidisciplinary methods allowed them to disentangle the complex relationships between human impact and ecological resilience.
Reefs are damaged by fishing
“We found that every one and a half years of fishing at a site reduced the probability of living coral by 3.2%,” said Dr. Jennifer Selgrath, lead author on the paper who was a PhD candidate at UBC’s Institute for the Oceans and Fisheries (IOF)’s Project Seahorse during the study, and is currently a research scientist with NOAA Channel Islands National Marine Sanctuary and the California Marine Sanctuary Foundation. “And fishing has long-lasting impacts. Areas that had been heavily fished 10 to 30 years ago still have lower coral today.”
“We were disappointed to find only about 30% of the reefs we studied were dominated by living coral, so it was useful to discover that both ecological context and human activities shaped coral distribution,” noted Dr. Amanda Vincent, senior author on the study, Professor in the IOF and Director of Project Seahorse. “We found that the way the seascape was arranged mattered most, with living corals more likely in deeper areas, marine protected areas, and compact reef patches.”
“Worryingly, destructive practices like blast fishing – using explosives to catch fish – have greatly reduced coral cover. However, we were initially surprised to find that a slightly higher probability of corals in areas with blast fishing near human population centers”, shared Selgrath. “We then learned that those areas are full of mushroom corals. These free-living corals do not provide much shelter for other marine life, but they seem to be adapted to the unstable habitats created by blast fishing.”
Unlike most corals, mushroom corals can move to survive in unstable conditions, which helps explain why they persist even in heavily disturbed, blast-fished areas.
Depth, marine protected areas and seascape features help reduce damage
“Our finding supports evidence that depth helps corals persist under multiple stressors, including hurricanes, marine heatwaves, and fishing. We identified an 8% increase in probability of living corals per 2.25 m of depth. We also found a positive relationship between small community-managed marine protected areas (MPAs) and the presence of living coral. Corals were 18% more likely to be found inside MPAs than outside them,” said Selgrath. “Communities in the Danajon Bank and our partners with ZSL Philippines have worked very hard to set up a network of MPAs. It is inspiring to see that these MPAs are working to protect corals and our oceans.”
Vincent concurs. “Protected areas are working, but they face an increasing array of regional and global stressors, including marine heat waves and disease. Climate stressors should be included in future MPA planning for oceans, and for the people who depend upon them.”
“Beneficial landscape characteristics like depth and patch compactness can be incorporated into marine spatial planning, restoration, and monitoring,” added Vincent. “For example, new MPAs could be placed in sites with deeper waters and compact coral patches, while restoration could focus on expanding existing coral areas.”
“We show that conservation outcomes depend on both reducing harmful stressors and strengthening the natural features that help corals survive,” shared Selgrath. “Key priorities include reducing spatial overlap of destructive activities, protecting reef configurations that support living corals, and addressing stressors that have delayed impacts such as legacy fishing.”
The study, “The influence of multiple stressors on the spatial distribution of corals” was published in People and Nature. Article DOI: 10.1002/pan3.70208