Coral reef A fringing reef forms around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the level of the underlying earth allows, the corals grow around the coast
Deep-water coral extended to . Scientists recently deployed concrete reef balls in an attempt to provide habitat for fish and coral. Scientists estimate that trawling has damaged or destroyed 30 to 50 percent of the Norwegian shelf coral area. The International Council for the Exploration of the Sea, the European Commission’s main scientific advisor on fisheries and environmental issues in the northeast Atlantic, recommend mapping and closing Europe’s deep corals to fishing trawlers. In 1999, the Norwegian Ministry of Fisheries closed an area of containing the large Sula Reef to bottom trawling. In 2000, an additional area closed, covering about . An