Human activities have a big effect on coral reefs, causing their worldwide decline. Harmful activities include mining coral, pollution (both from living things and non-living materials), overfishing, using explosives to fish, and digging canals or access points to islands and bays. Other dangers include disease, harmful fishing methods, and rising ocean temperatures. Also, the ocean’s role in absorbing carbon dioxide, changes in the atmosphere, ultraviolet light, ocean acidification, viruses, dust storms that carry harmful materials to distant reefs, pollutants, and algal blooms are factors that affect coral reefs. It is important to note that the danger to coral reefs is not limited to coastal areas. Climate change, especially global warming, causes ocean temperatures to rise, which leads to coral bleaching—a serious threat to coral ecosystems.

Scientists estimate that between 2020 and 2040, 70 to 90% of all coral reefs may disappear, mainly due to warmer ocean water, ocean acidification, and pollution. Some believe that large-scale coral deaths during this time may lead to the growth of coral that can survive in warmer water, but this could reduce coral genetic diversity. In 2008, a global study found that 19% of coral reefs had already been lost. Less than 20% of the world’s reefs are currently in good health, and about 75% may be at risk because of human activities. The threat to reefs is especially severe in Southeast Asia, where 95% of reefs are endangered. By the 2030s, 90% of reefs are expected to be at risk from human activities and climate change. By 2050, all coral reefs are predicted to be in danger.

Issues

In the Caribbean Sea and tropical Pacific Ocean, direct contact between coral and common seaweeds causes coral tissue to bleach and die due to competition from chemicals in the seaweed. Lipid-soluble extracts from these seaweeds also cause coral to bleach quickly. At these locations, bleaching and death only occurred where seaweed or its extracts touched the coral. After coral died, seaweed expanded to take over the empty spaces. However, as of 2009, only 4% of coral reefs worldwide had more than 50% coverage by algae, showing no recent global trend of algae taking over coral reefs.

Competitive seaweed and other algae grow well in water with high nutrients when there are not enough herbivores to eat them. Herbivores include fish like parrotfish, the sea urchin Diadema antillarum, surgeonfish, tangs, and unicornfish.

Overfishing, especially when certain types of fish are caught more than others, can disrupt coral ecosystems by allowing coral predators to grow too much. Predators that eat living coral, like the crown-of-thorns starfish, are called corallivores. Coral reefs are made from stony coral, which has a wax called cetyl palmitate in its tissues. Most predators cannot digest this wax. The crown-of-thorns starfish, which can grow up to one meter wide, has long, venomous spines that protect it. Its body breaks down the wax in stony coral, allowing it to eat the coral. The starfish is hunted by the giant triton sea snail, but this snail is often overfished because its shell is valuable. This lack of predators allows crown-of-thorns starfish to grow in numbers, harming reefs.

• The giant triton eats the crown-of-thorns starfish.
• The crown-of-thorns starfish eats coral.

Some marine aquarium fish can reproduce in captivity, like the Pomacentridae family. However, most (95%) aquarium fish are taken from coral reefs. Intensive harvesting, especially in maritime Southeast Asia (such as Indonesia and the Philippines), harms reefs. This is worsened by harmful fishing methods, like using cyanide or explosives. In the Philippines, most (80–90%) aquarium fish are caught using sodium cyanide. This chemical is mixed with seawater and released near fish hiding places. It puts the fish to sleep, making them easy to catch. However, many fish die from liver damage within months, and many non-marketable fish also die during the process. It is estimated that over 1,000,000 kilograms of cyanide have been used on Philippine reefs, with about 150,000 kilograms used yearly. Poverty in fishing communities is a major reason for cyanide fishing, as many people in the Philippines live below the poverty line.

Dynamite fishing is another harmful method. Explosives like dynamite, grenades, or homemade bombs are set off underwater. This kills fish and other reef animals in the blast area and destroys coral, removing the reef’s structure and habitat. Muro-ami is a method where nets are placed over reefs, and heavy stones are dropped to scare fish into nets. This breaks and kills coral. Muro-ami was banned in the 1980s.

Fishing gear harms reefs by physically touching them. These gears are often made of synthetic materials that do not break down in the ocean, causing long-term damage. Gill nets, fish traps, and anchors break coral and kill it by entangling it. When fishing lines are dropped near reefs, they can get tangled in coral. Fishermen often cut the lines and leave them attached to the reef. These lines scratch coral polyps and their outer layers. Corals can heal from small injuries, but repeated or large damage makes recovery harder.

Bottom dragging gear, like beach seines, harms coral by scraping and breaking it. A beach seine is a long net (about 150 meters) with small mesh and a weighted line to hold it down as it is dragged across the ocean floor. This is one of the most damaging fishing methods for reefs in Kenya.

Bottom trawling in deep oceans destroys cold-water and deep-sea corals. In the past, industrial fishers avoided coral because their nets would get stuck. In the 1980s, "rock-hopper" trawls were developed with large tires and rollers to help nets move over rough surfaces. Studies show that 55% of Alaskan cold-water coral damaged by a single bottom trawl pass had not recovered after one year. In the Northeast Atlantic, reef scars can be up to 4 kilometers long. In Southern Australia, 90% of coral seamounts now have no coral left, only bare rock. Even in the Great Barrier Reef, trawling for prawns and scallops has caused some coral species to disappear locally.

Coral reefs near human populations suffer from poor water quality caused by land and marine pollution. In 2006, studies found that about 80% of ocean pollution comes from land activities. Pollution reaches the ocean through runoff, wind, and deliberate discharge (like drainpipes). Runoff carries sediment from erosion, nutrients and pesticides from farms, wastewater, industrial waste, and trash from storms. Some pollutants use up oxygen, causing eutrophication, which kills coral and other reef life.

More people live in coastal areas now. Without proper planning, development (like buildings and roads) increases the amount of water that flows into the ocean, reducing the land’s ability to absorb it.

Pollution can introduce harmful germs. For example, the fungus Aspergillus sydowii is linked to disease in sea fans, and the bacteria Serratia marcescens is connected to a coral disease called white pox.

Coral reefs near human populations face local stress from poor water quality caused by land-based pollution. Copper, a common industrial pollutant, can harm coral polyps’ life cycles and growth.

Besides runoff, wind carries material into the ocean. This material can come from nearby or distant places. For example, dust from the Sahara Desert reaches the Caribbean and Florida, and dust from the Gobi and Taklamakan deserts travels to Korea, Japan, and Hawaii. Since 1970, dust deposits have increased due to droughts in Africa. Dust transport to the Caribbean and Florida changes yearly, with more dust during certain weather patterns. The USGS links these dust events to declining health of coral reefs in the Caribbean and Florida since the 1970s. Dust from the 1883 eruption of Krakatoa in Indonesia was found in the coral Montastraea annularis in Florida.

Sediment covers corals and stops them from feeding and reproducing. Pesticides can harm coral growth and reproduction. Some studies suggest chemicals in sunscreens may contribute to coral bleaching by weakening coral’s ability to resist viruses, but these studies had method flaws and did not test real reef conditions.

Nutrient pollution

Oil

Causes and Effects of Oil Spills

Oil spills can happen because of two types of causes: natural causes and human-caused causes.

Natural causes include oil that leaks from the ocean floor into the water, erosion of the seafloor, and changes in the climate. Each year, about 181 million gallons of oil naturally seeps into the ocean, but this amount can change from year to year.

Human-caused spills happen because of activities like drilling for oil, using pipelines, operating refineries, or during wars. These spills are more harmful than natural spills because they release about 210 million gallons of petroleum into the ocean each year. Human-caused spills can cause sudden and long-lasting changes to ecosystems, making it harder to fix the damage.

When oil spills occur, the effects can last for many years and cause serious harm to ocean life. For example, oil can block sunlight and reduce oxygen in the water, which makes it harder for plants to perform photosynthesis.

Two ways oil harms wildlife are through toxicity and fouling. Toxicity happens when harmful chemicals in oil enter an animal’s body, damage its organs, and can cause death. Fouling occurs when oil physically covers an animal or plant, making it hard for them to survive.

Oil Impacts on Coral Reef Communities

Oil pollution is dangerous for marine habitats because it contains harmful chemicals. Oil spills can happen naturally or during activities like transporting oil. These spills harm marine and coastal wildlife. When animals are exposed to oil, they may get skin irritation, weaker immune systems, or damage to their stomachs.

If oil floats on the surface of the water above a coral reef, it does not directly harm the coral below. However, when oil sinks to the ocean floor, it becomes a problem. The oil mixes with sediment particles, which can damage coral. This effect is less harmful than if the coral comes into direct contact with the toxic oil.

When oil touches coral, it harms not only the coral but also fish, crabs, and other marine animals. Even small amounts of oil can cause coral reef fish to behave in ways that are dangerous for them and their habitat. Oil can harm the fish’s growth, survival, and ability to find safe places to live. It can also increase the chance of being eaten by predators. Fish that are exposed to oil as larvae may later develop heart problems or physical issues.

Evidence of oil’s harm to coral reefs can be seen near the Macondo well, where coral covered in crude oil chemicals and brown particles began to die within seven months after the Deepwater Horizon oil spill.

Soft corals, like gorgonian octocorals, are especially vulnerable to oil spills. This is because their structure depends on tiny particles in the water.

Corals rely on tiny microorganisms, including helpful ones that protect them from pollution. However, oil spills can damage these microorganisms, weakening the coral’s ability to survive when exposed to pollution.

Methods to Clean Up Oil Spills

Booms are floating barriers used to stop oil from spreading. They are often used with skimmers, which are tools that collect oil from water using sponges or absorbent ropes. Other methods include burning oil in place (in situ-burning) and using chemical dispersants to break oil into small droplets. However, burning oil does not completely remove it and can create harmful chemicals that affect marine life.

Chemical dispersants are mixtures that break oil into small droplets. They are the most common cleanup method but can make corals more vulnerable to stress. Dispersants can also harm coral directly and have been banned in some cases. However, one type of dispersant, Corexit 9427, is still used.

Microbial biosurfactants are a newer, eco-friendly method that uses natural substances to reduce oil’s impact on reefs. However, this method is still being studied and is not always effective.

Threatened species

The global standard for recording marine species that are at risk is the IUCN Red List of Threatened Species. This list helps guide efforts to protect marine life around the world. A species is placed in the threatened category if it is critically endangered, endangered, or vulnerable. Other categories include near threatened and data deficient. By 2008, the IUCN had evaluated all 845 known species of reef-building corals. Of these, 27% were classified as threatened, 20% as near threatened, and 17% as data deficient.

The Coral Triangle, which includes the Indo-Malay-Philippine archipelago, has the highest number of reef-building coral species in the threatened category and also the greatest diversity of coral species. The loss of coral reef ecosystems will cause serious harm to many marine species and to people who rely on reef resources for their livelihoods.

Issues by region

The Great Barrier Reef is the world's largest coral reef system. It is located in the Coral Sea, and a large part of the reef is protected by the Great Barrier Reef Marine Park. Environmental pressures include water runoff from land, changes in salt levels, climate change, outbreaks of crown-of-thorns starfish, overfishing, and spills or improper ballast discharge. A 2014 report by Australia's Great Barrier Reef Marine Park Authority (GBRMPA) stated that climate change is the greatest environmental threat to the reef. By 2018, 50% of the coral on the Great Barrier Reef had been lost.

Southeast Asian coral reefs face risks from harmful fishing methods (such as using cyanide or explosives), overfishing, sediment buildup, pollution, and coral bleaching. Efforts like education, rules, and creating marine protected areas help protect these reefs.

Indonesia is home to one-third of the world's coral reefs, covering nearly 85,000 square kilometers (33,000 square miles) and supporting one-quarter of its fish species. These reefs are in the Coral Triangle and have been harmed by destructive fishing, tourism, and bleaching. A 1998 study by Indonesia's LIPI found that only 7% of the reefs are in excellent condition, 24% are in good condition, and about 69% are in poor or fair condition. One source estimated that Indonesia could lose 70% of its coral reefs by 2050 without restoration efforts.

In 2007, Reef Check, the world's largest reef conservation group, reported that only 5% of the Philippines' 27,000 square kilometers (10,000 square miles) of coral reefs are in "excellent condition." These areas include Tubbataha Reef in Palawan, Apo Island in Negros Oriental, Apo Reef in Puerto Galera, Mindoro, and the Verde Island Passage near Batangas. Philippine coral reefs are Asia's second-largest.

Coral reefs in Taiwan are threatened by human population growth. Many corals are harmed by untreated sewage and tourists who take souvenirs, not knowing that this harms habitats and causes disease. Some corals near Taiwan's southeast coast have turned black due to illness.

Coral disease was first recognized as a threat to Caribbean reefs in 1972 when black band disease was discovered. Since then, diseases have become more frequent.

It is estimated that 50% of the Caribbean Sea's coral cover has disappeared since the 1960s. A United Nations Environment Program report warned that Caribbean coral reefs might disappear completely within the next 20 years due to coastal population growth, overfishing, pollution, global warming, and invasive species.

In 2005, the Caribbean lost about 50% of its reefs in one year due to coral bleaching. Warm water from Puerto Rico and the Virgin Islands traveled south, causing this bleaching.

The Mesoamerican Barrier Reef System, located in the Caribbean Sea, is the second-largest coral reef system in the world. It faces growing threats from climate change. Coral bleaching is becoming a major cause of reef damage. A study analyzing data from 2015 to 2017 found that intermediate bleaching events were most common in 2017. Researchers examined 23 factors related to stress and sensitivity, identifying key influences on bleaching severity, such as seasonal warming and heat stress. Their findings suggest that commonly used tools like Degree Heating Weeks may not fully explain bleaching risks. Alternative metrics could improve predictions. The study also found that deeper reefs with diverse coral species are more vulnerable to bleaching than previously thought. This challenges the belief that deeper reefs are safe from climate stress. The authors proposed a new method for assessing bleaching risk using publicly available data, which could improve monitoring and conservation strategies.

Jamaica is the third-largest Caribbean island. If conservation efforts are not made, the Caribbean's coral reefs may disappear in 20 years. In 2005, 34% of Jamaica's coral reefs were bleached due to rising sea temperatures. Jamaica's reefs are also threatened by overfishing, pollution, natural disasters, and reef mining. In 2009, researchers found that many corals are recovering very slowly.

Southeastern Florida's reef track is 300 miles long. Florida's coral reefs are currently experiencing an unusual disease called stony coral tissue loss disease. The disease affects many coral species across a wide area.

In January 2019, scientists confirmed that the disease outbreak extended south and west of Key West. In December 2018, the disease was first spotted at Maryland Shoals near the Saddlebunch Keys. By mid-January, five more sites between American Shoal and Eastern Dry Rocks were confirmed to be affected.

Puerto Rico has over 5,000 square kilometers of shallow coral reef ecosystems. These reefs and their ecosystems are valued at nearly $1.1 billion per year on average.

The U.S. Virgin Islands' coral reefs and their ecosystems are valued at about $187 million per year on average.

Hawaii's coral reefs, such as those in French Frigate Shoals, are important to Hawaii's $800 million annual marine tourism industry. These reefs are being harmed by coral bleaching and rising sea surface temperatures, which lead to coral diseases. The first large-scale bleaching occurred in 1996. By 2004, scientists found that sea surface temperatures had been increasing steadily. If this trend continues, bleaching events will become more frequent and severe.