Algae and sediment pollution on beach near reef Maui; photo by CORAL staff.

Direct Threats

Unsustainable fishing

Overfishing is a pervasive threat, thought to affect more than 55 percent of the world’s coral reefs. When people overharvest fish on a reef, the entire food web is affected. On healthy reefs, algae are kept at low levels thanks to intense grazing by herbivorous fish like parrotfish and surgeonfish. When these fish disappear, the delicate balance of the coral reef ecosystem is disrupted, and seaweed-like algae (called macroalgae) can grow unchecked, eventually smothering reefs.

Globally, there is a shift away from sustainable small-scale harvests towards ones that use indiscriminate fishing methods (e.g., gill and trammel nets) to remove large quantities of biomass, regardless of size or species. When reefs are overfished, fish populations decline and fishers respond by intensifying their effort in an attempt to catch something. Their increased efforts can lead to the collapse or near-collapse of fish stocks, which not only threatens the economic stability and food security of local communities but puts coral reefs at significant risk.

Water pollution

Land-based sources of pollution: Clean water is vital for both human communities and coral reefs. Around the world, water pollution from land causes severe damage to coral reefs, poses risks to human health and threatens the tourism industry. Directly discharged sewage and inadequately treated wastewater from cesspools and septic tanks allow high levels of nutrients, bacteria, chemicals and pathogens to enter the marine environment. Other land-based activities—like farming, logging, road construction, animal husbandry and mining—produce pollutants such as sediments, fertilizers and pesticides which run off the landscape when it rains and end up in the ocean. An overabundance of nutrients in marine environments upsets the delicate balance of coral reef ecosystems. Excess nutrients promote the growth of algae, which can kill corals by smothering them, blocking their access to sunlight and promoting the growth of harmful bacteria. Likewise, sediments can harm corals by blocking their access to sunlight, which the algae that live inside them depend on to photosynthesize effectively.

Marine debris: Marine debris, also known as human trash, can harm or kill coral reefs and the many animals that live in them. Marine debris can get to the ocean from land or from boats and ships. Floating trash hooked on reefs can block the sunlight reefs need for their symbiotic algae to photosynthesize. Lost or discarded fishing nets (called “ghost” fishing gear) can get caught on reefs and entangle fish, sea turtles and marine mammals. Sea turtles often mistake plastic bags for jellyfish and eat them, causing harm to their digestive tracts and even death.

Habitat destruction

The coral reefs we see today are hundreds – sometimes thousands – of years in the making. Like trees, coral reefs are living structures that can take many years to regenerate once destroyed. Since most corals species grow less than an inch per year, reef destruction can have long-lasting consequences. Unfortunately, many human activities directly damage or destroy coral reefs and associated habitats (e.g., mangroves, lagoons). Below are some examples:

Coral mining: Coral pieces are removed in large quantities for use as bricks or road-fill. Sand and limestone derived from coral reefs are used to make cement for new buildings. In some places where coral reefs were heavily mined, the reefs have still not made a comeback.

Construction: In many places, developers have constructed piers and other structures directly on top of coral reefs, destroying them and any possibility of regeneration.

Coral collecting: Particularly beautiful coral species, like black and red coral, which are used to make jewelry, are heavily harvested. Branching corals are often broken off and sold as souvenirs or beachy home décor. As is the case with sea stars and sand dollars, this type of harvesting can cause rapid declines in target species. By purchasing jewelry or other souvenirs made from coral or other once-living marine life, tourists and other consumers often unknowingly contribute to the destruction of reefs.

Destructive fishing methods: Blast fishing and cyanide fishing use dynamite and poison, respectively, to stun and trap fish. Blast fishing can destroy an entire reef in one act. Fishermen use cyanide to stun fish and capture them for the live aquarium trade. While large fish can metabolize cyanide, smaller fish and other marine animals, including coral polyps, are poisoned by the chemical cloud created by this activity. Bottom trawling, a fishing method that drags a weighted net along the sea floor, destroys virtually everything in its wake during a single fishing event.

Boat anchors: Boat anchors are often dropped directly onto reefs, causing significant damage to reefs and disturbing marine life. One easy solution is to install permanent mooring buoys which float on the water and can be used by fisherman or tourists operators as a place to safely anchor their boats.

Unsustainable tourism: When tourism is not carried out responsibly, it can harm reefs. Snorkelers or divers who grab, walk on or kick their fins on coral, can destroy coral reefs and stir up sediment on the sea-floor bottom (which affects water quality). In places where tourism is high, tourists can unwittingly cause additional damage by wearing sunscreen, which leaves chemicals in the water that are harmful to reefs.

Mangrove destruction: Mangrove forests are being rapidly destroyed through logging for firewood and clearing to create open beaches and aquaculture farms. Since these ecosystems normally filter the amount of sediment reaching the ocean, their disappearance has led to an increase in the amount of sediment reaching coral reefs. Mangroves also serve as important nursery habitat where reef species like fishes grow up.