Human activities have a big effect on coral reefs. These activities are causing coral reefs around the world to decline. Harmful actions include mining coral, pollution (both from living things and non-living materials), overfishing, using explosives to catch fish, and digging canals or pathways to islands and bays. Other dangers include diseases, harmful fishing methods, and rising ocean temperatures. Also, the ocean’s ability to absorb carbon dioxide, changes in the atmosphere, ultraviolet light, ocean acidification, viruses, dust storms that carry harmful materials to distant reefs, pollutants, and algal blooms all affect coral reefs. It is important to know that the problems facing coral reefs are not limited to areas near coasts. Climate change, especially global warming, causes ocean temperatures to rise, leading to coral bleaching—a dangerous condition that can kill coral ecosystems.

Scientists predict that between 2020 and 2040, 70 to 90% of coral reefs may disappear. The main reasons are rising ocean temperatures, increased ocean acidity, and pollution. Some experts think that large-scale coral deaths during this time might 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-related activities. The danger to reefs is especially severe in Southeast Asia, where 95% of reefs are in danger. By the 2030s, 90% of reefs are expected to be at risk from both human activities and climate change. By 2050, all coral reefs may 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 because of competition using chemicals. Extracts from seaweed that harm coral also cause quick bleaching. At these places, bleaching and death only happened where seaweed or its extracts touched the coral. After coral died, seaweed took over the empty spaces. However, as of 2009, only 4% of coral reefs worldwide had more than 50% seaweed coverage, meaning there is no recent global trend showing seaweed taking over coral reefs.

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

Overfishing, especially when certain fish are caught too much, can harm coral ecosystems by letting coral predators 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 their tissues. Most predators cannot digest this wax. The crown-of-thorns starfish is a large starfish (up to one meter) with long, venomous spines. Its body has enzymes that break down the wax in stony corals, allowing it to eat the coral. The starfish has its own predators, like the giant triton sea snail. However, the giant triton is hunted for its shell and has been overfished. This has caused crown-of-thorns starfish populations to grow too much, damaging reefs.

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

Some marine aquarium fish species can reproduce in aquariums (like Pomacentridae), but most (95%) are taken from coral reefs. Heavy harvesting, especially in maritime Southeast Asia (including Indonesia and the Philippines), harms reefs. This is made worse by destructive fishing methods, such as cyanide and blast fishing. Most (80–90%) aquarium fish from the Philippines are caught using sodium cyanide. This toxic chemical is mixed with seawater and poured into areas where fish hide. It makes the fish sleepy, making them easy to catch. However, most fish caught this way die within months from liver damage. Many fish that are not valuable also die during the process. It is estimated that over 4,000 Filipino fish collectors have used more than 1,000,000 kilograms (2,200,000 pounds) of cyanide on Philippine reefs alone, about 150,000 kilograms per year. A major reason for cyanide fishing is poverty in fishing communities. In countries like the Philippines, where cyanide fishing is common, more than 30% of the population lives below the poverty line.

Dynamite fishing is another harmful method. Sticks of dynamite, grenades, or homemade explosives are set off in the water. This kills fish and other reef animals in the blast area, along with corals. The blast destroys the reef's structure, removing habitat for fish and other reef animals. Muro-ami is a method where nets are placed over reefs and large stones are dropped on top to scare fish into fleeing. The stones break and kill the 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 branching coral and kill it by getting tangled. When fishermen drop lines near coral reefs, the lines wrap around the coral. The fisher cuts the line and leaves it attached to the reef. The discarded lines scratch coral polyps and their top layers. Corals can heal from small injuries, but repeated or large damage makes recovery harder.

Bottom dragging gear, like beach seines, harms corals by scraping and breaking them. A beach seine is a long net (about 150 meters) with small mesh (3 centimeters) and a weighted line to hold it down while it is dragged across the ocean floor. It is one of the most damaging types of fishing gear for Kenya's reefs.

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 on reefs. In the 1980s, "rock-hopper" trawls were made with large tires and rollers to help nets move over rough surfaces. Studies show that 55% of Alaskan cold-water coral damaged by one pass of a bottom trawl had not recovered after a year. In the Northeast Atlantic, scars from trawling can be up to 4 kilometers (2.5 miles) long. In Southern Australia, 90% of the surfaces on coral seamounts are now bare rock. Even in the Great Barrier Reef World Heritage Area, seafloor trawling for prawns and scallops is causing some coral species to disappear in certain areas.

Reefs near human populations face poor water quality from land and marine sources. In 2006, studies showed that about 80% of ocean pollution comes from land activities. Pollution reaches the ocean through runoff (water that flows over land), wind, and deliberate actions like drainpipes. Runoff carries sediment from erosion, nutrients and pesticides from farming, wastewater, industrial waste, and trash washed away by storms. Some pollutants use up oxygen, causing eutrophication, which kills coral and other reef life.

More people live in coastal areas now. Without proper care, development (like buildings and roads) increases the amount of water that flows into the ocean as runoff by reducing how much water the land can absorb.

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

Reefs near human populations may face local problems, like poor water quality from land-based pollution. Copper, a common industrial pollutant, has been shown to interfere with the life cycle and growth of coral polyps.

Besides runoff, wind carries material into the ocean. This material can come from nearby or faraway places. For example, dust from the Sahara moves to the Caribbean and Florida. Dust also travels from the Gobi and Taklamakan deserts to Korea, Japan, and the Northern Pacific to the Hawaiian Islands. Since 1970, dust deposits have increased due to droughts in Africa. Dust transport to the Caribbean and Florida varies each year, with more dust during certain weather patterns. The USGS links dust events to reduced health of coral reefs in the Caribbean and Florida, mainly since the 1970s. Dust from the 1883 eruption of Krakatoa in Indonesia was found in the growth bands of the reef-building

Oil

Causes and Effects of Oil Spills

Oil spills can occur due to two main types of causes: natural and human-made.

Natural causes include oil seeping 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 seep into the ocean, though this amount can vary.

Human-made causes involve activities such as oil drilling, pipelines, refineries, and wars. These activities are responsible for most oil entering the ocean. Human-made spills release about 210 million gallons of petroleum yearly. These spills cause sudden and long-lasting harm to ecosystems, often requiring extended efforts to repair.

When oil spills occur, their effects can last for many years and cause serious harm to aquatic life. For example, oil can block sunlight and reduce oxygen levels in the water, which harms plants that rely on photosynthesis.

Two major ways oil harms wildlife are through toxicity and fouling. Toxicity happens when harmful chemicals in oil enter an animal’s body, damaging internal organs and potentially causing death. Fouling occurs when oil physically coats an animal or plant, making it difficult for them to survive.

Oil Impacts on Coral Reef Communities

Oil pollution harms marine habitats because of its toxic ingredients. Oil spills can occur naturally or during human activities like transportation and drilling. These spills damage marine and coastal wildlife. Exposure to oil can cause skin irritation, weakened immunity, and harm to the digestive system in affected organisms.

When oil floats on the surface of coral reefs, it may not directly harm the coral below. However, when oil sinks to the ocean floor and mixes with sediment, it becomes a problem. The oil-sediment mixture can be more harmful than oil alone.

When oil comes into contact with coral, it affects not only the reef but also fish, crabs, and other marine life. Even small amounts of oil can cause coral reef fish to make poor decisions, which can be dangerous for them and the reef. Oil can also harm fish growth, survival, and behavior, increasing the risk of being eaten by predators. Studies show that fish exposed to oil as larvae may develop heart problems and physical issues later in life.

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

Soft corals, such as gorgonian octocorals, are especially vulnerable to oil spills. Their structure and function, which rely on filtering tiny particles from water, make them more likely to be damaged.

Corals depend on many tiny microorganisms, including helpful ones that protect them from pollution. However, oil spills harm these microorganisms, weakening the corals’ ability to defend themselves against pollution.

Oil Spill Cleanup Methods

Booms are floating barriers used to contain oil spills and stop the oil from spreading. They are often used with skimmers, which are tools that absorb oil from the water. Other methods include burning oil in place (in situ burning) and using chemical dispersants. In situ burning involves burning collected oil, but this process breaks the oil into new chemicals that can still harm marine life.

Chemical dispersants are mixtures that break oil into small droplets, making it easier to clean up. However, these dispersants can weaken corals’ ability to handle stress and can harm coral directly. Some dispersants have been banned because they damage coral and reduce the ability of coral larvae to settle on reefs. One dispersant, Corexit 9427, is still used.

Microbial biosurfactants are a type of eco-friendly cleanup method that uses naturally occurring microorganisms to break down oil. However, this method is still being studied and is not yet a guaranteed solution for cleaning up oil spills.

Threatened species

The IUCN Red List of Threatened Species is the main method for keeping track of marine species that are in danger. This list helps decide where to focus marine conservation efforts around the world. A species is listed as threatened if it is critically endangered, endangered, or vulnerable. Other groups include near threatened and data deficient. By 2008, the IUCN had evaluated all 845 known reef-building coral species. Of these, 27% were classified as threatened, 20% as near threatened, and 17% as data deficient.

The Coral Triangle, which is part of the Indo-Malay-Philippine archipelago, has the most reef-building coral species that are threatened. It also has the greatest variety of coral species. Losing coral reef ecosystems can cause serious harm to many sea animals and to people who rely on reef resources for their jobs and daily needs.

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 challenges include water runoff from land, changes in salt levels, climate change, recurring 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) states that climate change is the greatest 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 practices, such as using cyanide or explosives, overfishing, sedimentation, 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 sq mi) and hosting one-quarter of its fish species. These reefs are in the Coral Triangle and have been harmed by destructive fishing, tourism, and bleaching. Data from 1998 by LIPI shows that only 7% of Indonesia’s reefs are in excellent condition, 24% are in good condition, and about 69% are in poor or fair condition. If restoration efforts do not begin, Indonesia could lose 70% of its coral reefs by 2050.

In 2007, Reef Check, the world’s largest reef conservation group, reported that only 5% of the Philippines’ 27,000 square kilometers (10,000 sq mi) 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. Untreated sewage and tourists taking souvenirs, which harms coral habitats and causes disease, affect many corals. Some corals have turned black from disease along Taiwan’s southeast coast.

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

It is estimated that 50% of the Caribbean Sea’s coral cover has disappeared since the 1960s. A United Nations Environment Program report suggests that Caribbean coral reefs might disappear in 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 and identified key influences, such as seasonal warming rates and heat stress. They found that traditional metrics like Degree Heating Weeks may not fully explain bleaching risks. Deeper reefs with diverse coral species were more vulnerable to bleaching than expected, challenging the idea that deeper reefs are safe from climate stress. The study suggests a new method for assessing bleaching risks using publicly available data, which could improve monitoring and conservation efforts.

Jamaica is the third-largest Caribbean island. If conservation efforts are not made, Caribbean 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 slowly.

Southeastern Florida’s reef tract is 300 miles long. Florida’s coral reefs are currently experiencing a widespread disease called stony coral tissue loss disease, which affects many coral species over a large area.

In January 2019, scientists confirmed that the disease had spread south and west of Key West. In December 2018, the disease was found 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.

The U.S. Virgin Islands’ coral reefs and their ecosystems are valued at $187 million per year.

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 major bleaching event occurred in 1996. By 2004, scientists found that sea surface temperatures had been increasing steadily. If this trend continues, bleaching events will happen more often and become more severe.