Despite covering less than 1 percent of the ocean floor, coral reefs are home to 25 percent of all fish species. Fish harvested from coral reefs feed tens of millions of people in developing countries and reef-related fishing, trade and tourism generates up to $30 billion annually, accounting for more than 15 percent of the GDP of 23 nations. Reefs also help protect more than 93,000 miles (150,000km) of coastline in 100 countries.

Coral reefs are in deep trouble: the World Resources Institute estimates that 95 percent of the reefs in Southeast Asia are at risk and almost 75 percent of the planet’s coral reefs are endangered by a panoply of threats.

(World Resources Institute)

Bleaching: In recent years, coral reefs around the world have suffered severe “bleaching” due to warming ocean waters – a result of climate change and the recurring El Nino phenomenon. Coral reefs are the product of an ecological partnership between coral polyps, which build the physical reef, and microscopic algae known as zooxanthellae. The coral provides a safe home for the zooxanthellae and the algae in turn make sugars and amino acids through photosynthesis that feed the coral. These algae also give corals their Technicolor hues. In nutrient-poor waters, that partnership allows these large ecosystems to thrive. But when water temperatures rise, the algae turn toxic and the coral jettisons them. That leaves corals without access to nutrients and without color. Although bleaching is not necessarily a death sentence for reefs – they can recover if water temperatures fall – it heavily stresses the corals and puts the entire ecosystem at risk.

Great Barrier Reef bleaching 2016-2017. (ARC Centre Of Excellence for Coral Reef Studies)

Back-to-back bleaching events in 2016 and 2017 affected 90 percent of the 1,430-mile (2,300km) Great Barrier Reef in Australia. Surveys show that in 2016 two-thirds of corals in the northern section of the Great Barrier Reef died. In 2015–16, bleaching damaged 40 percent of the world’s ocean reefs. As the oceans continue to warm, scientists warn that bleaching could affect 99 percent of all coral reefs by the century’s end.

Acidification: Oceans absorb about half of all human-related carbon dioxide emissions. When carbon dioxide mixes with water it forms carbonic acid and turns the water more acidic. This lower pH weakens and dissolves the shells of many marine organisms, including the reefs created by corals.

Pollution: Coastal development and agriculture results in the runoff of wastewater, sediment, chemicals and fertilizers into the ocean that can damage reefs and disrupt coral growth, reproduction and feeding. Fertilizers and other nutrients produce algal and phytoplankton blooms that can spawn large populations of coral-eating crown-of-thorns starfish.

Overfishing: Destructive fishing practices, such as bottom trawling, dynamite fishing and cyanide fishing, damage coral reefs while overfishing removes predators and algae-eating fish, allowing the algae to crowd out corals and disrupting the coral ecosystem.