An artificial reef (AR) is a man-made structure in freshwater or the ocean. It is usually placed in areas with flat or bare ocean floors to help marine life. These reefs can be built to control erosion, protect coasts, block ships, stop the use of fishing nets, help restore natural reefs, improve fish farming, or make scuba diving and surfing better. People in ancient times, like the Persians and Romans, created early artificial reefs.

Some artificial reefs are made from objects that had other uses, such as sinking oil rigs (through the Rigs-to-Reefs program), sinking ships on purpose, or using rubble and construction waste. Shipwrecks can also become artificial reefs if they remain on the ocean floor. Other reefs are made from materials like concrete, which can be shaped into special forms, such as reef balls. Green artificial reefs use natural materials like plant fibers and seashells to help the environment by reducing pollution and energy use. In some cases, artificial reefs are built as art pieces.

Artificial reefs provide hard surfaces where algae and creatures like barnacles, corals, and oysters can grow. They also create spaces for fish of different sizes to hide. As marine life grows on these reefs, it forms structures and food sources for groups of fish. How an artificial reef affects the environment depends on where it is placed, how it is built, and the types of animals and plants nearby. While artificial reefs can help corals grow, they may change ecosystems because different species grow at different rates. Studies show that fast-growing seaweed, algae, and corals often grow faster on artificial reefs than they would on natural ones.

Scientists are studying how to build artificial reefs and their effects. Many materials used in the past are no longer considered good choices. A review from 2001 found that about half of the reefs studied met their goals. Long-term planning and regular care are important for success. A recent study from 1990 to 2020 showed that artificial reefs can help restore ocean ecosystems if they are carefully designed to match the needs of their location.

History

Artificial reefs have been built for a long time. Historian Diodorus Siculus recorded that during the First Punic War around 250 BC, the Romans blocked the harbor of Lilybaeum to stop the Carthaginians. They used stones, construction materials, and large timbers with anchors to create an artificial reef in the harbor’s channels. The Persians also built an artificial reef by blocking the Tigris River’s mouth to stop Arabian pirates.

Artificial reefs were used to grow fish or algae as early as the 17th century in Japan, where rubble and rocks were placed to help kelp grow. The first recorded artificial reef in the United States was in the 1830s, when logs from old huts were placed off South Carolina’s coast to improve fishing. In the Philippines, a traditional fishing method called fish nests (also known as gango, amatong, or balirong) uses rocks and waterlogged wood to build mounds in shallow tidal waters. These mounds attract fish and crustaceans. During low tide, people surround the mounds with nets, dismantle them, and collect the fish. The mounds are rebuilt after each harvest. Fish nests have been used since before 1939 to catch young groupers, which are used as seeds for aquaculture.

Before the 1840s, U.S. fishermen used twisted logs to build artificial reefs. More recently, items like old refrigerators, shopping carts, abandoned cars, and broken vending machines have been used in informal reef projects. Official projects have also used old ships, subway cars, tanks, armored vehicles, oil rigs, and beehive-shaped reef balls.

Purposes

Artificial reef structures (ARs) have many purposes. They help protect and improve marine ecosystems, restore damaged areas like kelp forests and coral reefs, and support activities such as fishing, diving, and surfing. In fisheries, artificial reefs can increase the number of fish that are useful for recreation or business, help fish populations grow, and protect underwater habitats from harmful fishing practices. They can also stop coastal erosion and support eco-tourism by creating places for activities like scuba diving and surfing, while reducing pressure on natural coral reefs.

The way artificial reefs are built depends on where they are placed and what they are meant to do. Some reefs may not work well for certain purposes. Early attempts to build artificial reefs often failed or had mixed results. However, recent studies from 1990 to 2020 show that properly designed artificial reefs can help restore marine ecosystems if they match the needs of the environment. Experts suggest more research comparing artificial and natural reefs, better monitoring of reefs over time, and attention to how reefs are shaped and arranged.

Artificial reefs designed to improve ecosystems often develop in predictable stages. First, ocean currents create areas with lots of plankton near vertical structures, attracting small fish like sardines and minnows, which draw in larger fish like tuna and sharks. Later, fish that need shelter, such as grouper and eels, move in, followed by predators like jack and barracuda. Over time, algae, corals, and other sea life grow on the reef.

An electrified reef uses a small electric current on metal structures underwater to help limestone form on the reef, allowing coral to attach and grow faster.

3D printing has been used to make molds for artificial reefs and to create ceramic or concrete reefs directly. Researchers are also developing eco-friendly materials, such as Archireef’s terracotta Reef Tiles. These tiles are safe for coral, break down naturally, and match the pH of seawater. In Hong Kong, these tiles had a 95% coral survival rate after three years, much higher than older methods.

Restoring artificial reefs can include activities like moving coral to new areas, releasing coral larvae, or growing corals in nurseries. For example, the Coral Restoration Foundation in Florida raises corals in nurseries and plants them on damaged reefs. A 2023 study said artificial reefs can help restore marine ecosystems if they are carefully designed for each location.

Some researchers are studying whether artificial reefs can help store carbon to fight climate change. Coastal plants like mangroves and seagrass, as well as algae and phytoplankton, are known to store carbon. Increasing life at artificial reefs might add to this storage.

In the Gulf of Mexico, a reef called RGV Reef is being studied for its ability to capture carbon. Another study near Juehua Island in China found that certain reef shapes helped create carbon storage areas, but local fish were not available to support this. Scientists suggest introducing the right species to improve carbon storage.

In the Caribbean, researchers found that placing artificial reefs near seagrass meadows can help both the reefs and the seagrass. The reefs provide shelter for fish, which then help the seagrass grow by adding nutrients. This combination also protects seagrass from being damaged by fishing nets and increases the amount of life in the area.

Some artificial reefs are built to stop coastal erosion. Their shape and how they interact with water are important for this purpose. Some reefs reduce wave energy before it reaches the shore, while others trap sand to keep beaches stable. Reefs are usually designed to fit the specific needs of each area.

Some reefs, like those used for surfing, are not mainly built to help ecosystems. Early surfing reefs, such as Hoppy’s Reef in California, did not work well. Later examples include reefs in Australia, Queensland, California, and England. These reefs use materials like concrete, rock, and sand-filled bags. However, some materials, like sand-filled bags, break down faster than expected, and others, like stone, may pose safety risks for surfers.

Artificial surfing reefs are sometimes built to improve surfing, protect coasts, or study the ocean. While habitat improvement is sometimes a goal, the materials used for surfing reefs are often different from those used for reefs that help ecosystems. A 2012 study found that many artificial surfing reefs did not meet their goals for improving surfing conditions.

Environmental concerns

Environmental concerns about artificial reefs include possible harm to natural areas during installation. These reefs might change how marine life lives by bringing in non-native species or drawing fish, eggs, and larvae from nearby natural habitats. They can also gather fish in one place, making it easier to catch them and leading to overfishing and harm to fish populations over time. Materials used in artificial reefs might break down or release harmful substances like paint, oil, and plastics, which can damage natural habitats. Parts of the reef might also break off and become ocean waste or wash onto natural reefs and beaches.

Many marine animals move frequently. Fish that gather near artificial reefs vary depending on the reef's age, size, and structure. Different species and life stages prefer different habitats. For example, young Red Snapper (Lutjanus campechanus) are more likely to live near vertical artificial structures, but older Red Snapper return to muddy or sandy areas as adults. Understanding how marine life interacts with its environment is important for managing resources and studying how artificial reefs affect ecosystems. When placing artificial reefs, it is important to consider nearby natural habitats and the needs of species at different life stages, such as where they reproduce or grow.

Using old shipwrecks or oil rigs as artificial reefs creates a new food chain for local ecosystems. Artificial and natural reefs often have different structures and diversity. Unless artificial reefs are designed to look like natural ones, they may not develop the same variety of life over time. For example, the Sint Eustatius reef, which is about 200 years old, has a healthy ecosystem but has fewer coral species than a nearby natural reef.

Artificial reefs can sometimes upset natural ecosystems by attracting non-native or invasive species that harm local environments. In 2008, at Palmyra Atoll near Hawaii, iron from a shipwreck caused an increase in algae and a type of sea anemone, which covered and damaged coral, creating a "black reef."

Artificial reefs can quickly increase fish and algae populations nearby. However, scientists debate whether these increases come from fish moving from other areas to the reef (the attraction hypothesis) or from more fish growing on the reef itself (the production hypothesis). Some experts, like biologist James Bohnsack, say artificial reefs do not create more fish but instead act like tools that draw fish from nearby areas. Others argue that artificial reefs can also support fish growth. A 2022 review said the attraction-production question depends on each reef's specific conditions and must be studied after installation.

Gathering fish in one place makes fishing easier. This can lead to overfishing and harm to fish populations, affecting both small-scale and large-scale fishing practices.

There are concerns that artificial reefs might be used as a way to illegally dump waste. Rules have been created by the U.S. and other countries to prevent misuse, but they may not fully protect the environment.

Some artificial reefs are not as strong as expected and break apart, becoming ocean waste or damaging natural reefs and beaches. In the 1970s, tires were used to build artificial reefs, but tropical storms later destroyed the structures, washing tires onto beaches and harming nearby coral reefs. At Osborne Reef near Fort Lauderdale, Florida, storms broke the straps holding tires together. As of November 2019, 250,000 of an estimated 700,000 tires had been removed. France has also started removing its tire reefs. The Ocean Conservancy now includes tire cleanup in its annual International Coastal Cleanup in September. Since 2021, 4Ocean has added tire removal to its cleanup efforts.

Some artificial reefs built for surfing have also caused problems. Early surfing reefs used sand-filled bags that degraded faster than expected. For example, Pratte's Reef in California and Mount Reef in New Zealand required large cleanup efforts, which sometimes cost more than the original construction. Critics say this method of building reefs is flawed.

Artificial reefs made from materials not meant for the ocean can break down and harm natural habitats. If the wrong materials are used, they might stop algae from growing, which is a food source for coral, causing coral to die.

Materials like PVCs, plastics, oil, paint, asbestos, and rusting metals can release harmful substances such as PCBs and heavy metals (like lead, copper, and mercury). These toxins can enter the food chain and affect fish and humans. However, eating seafood from artificial reefs is unlikely to harm humans at normal consumption levels, except for urchins and other shellfish that should be avoided.

International guidelines recommend using inert (non-reactive) materials for artificial reefs. The 2009 "Specific Guidelines for Assessment of Inert, Inorganic Geological Material" by the London Convention and Protocol/UNEP state that inert materials should not cause contamination through leaching, physical or chemical breakdown, or biological activity.

Efforts continue to create stable materials from waste for artificial reefs. For example, oil and coal fly ash have been mixed with cement and lime to make experimental reef blocks. However, like tires, these materials still raise concerns about harmful substances leaking into the environment.

In the U.S., best practices for preparing ships to become artificial reefs include removing all toxic materials (like PCBs) to meet water quality standards and clearing debris. If removing harmful materials is too difficult, the Environmental Protection Agency (EPA) may allow their removal to be skipped, as happened with the ex-USS Oriskany.

Examples

Florida has many artificial reefs, some made by intentionally sinking ships, such as Coast Guard cutters Duane and Bibb and the U.S. Navy landing ship Spiegel Grove. In the early 1970s, over 2,000,000 used vehicle tires were placed off the coast of Fort Lauderdale, Florida, to form an artificial reef. However, the tires were not properly attached to the reef structures, and ocean currents moved them away, causing them to crash into the reef and nearby natural areas. Neptune Memorial Reef was first planned as an art project called The Atlantis Reef Project by Gary Levine and Kim Brandell. Burial at sea was used to fund the project. By 2011, about 200 "placements" had occurred. Cremated remains are mixed with concrete and shaped into columns, sea stars, brain coral, or 15-foot (4.6 m) castings of lions or other forms before being placed in the water. In 1921, the U.S. battleship Massachusetts was sunk in shallow water near Pensacola, Florida, and used as a target for testing artillery. In 1956, the Florida Supreme Court declared the ship the property of the state of Florida. Since 1993, the wreck has been part of the Florida Underwater Archaeological Preserve and listed in the National Register of Historic Places. It now serves as an artificial reef and a place for recreational diving. The world's largest artificial reef was created by sinking the 44,000-ton aircraft carrier USS Oriskany off the coast of Pensacola, Florida, in 2006. The second-largest artificial reef is the USNS Hoyt S. Vandenberg, a former World War II troop transport ship that was sunk seven miles off Key West on May 27, 2009, in 140 feet (43 m) of clear water. Supporters hoped the ship would draw divers away from natural reefs, helping those reefs recover from overuse. The ex-USS Spiegel Grove is located on Dixie Shoal, 6 miles (9.7 km) off the Florida Keys in the Florida Keys National Marine Sanctuary. Its coordinates are 25°04′00″N 80°18′01″W. The ocean liner SS United States is planned to be sunk off the coast of Destin and Fort Walton Beach in Okaloosa County, Florida. Once sunk, it will become the world's largest artificial reef, visible from 60 feet to 180 feet deep, with its funnels removed. The ship is currently in Mobile, Alabama, being prepared by removing hazardous materials and valuable items. In 1970, the Georgia Department of Natural Resources (DNR) started building artificial reefs to provide habitats for game fish. Georgia lacks natural coral reefs because the ocean floor is too sandy and flat to support coral growth. These reefs attract many types of marine life. A marine ecologist from Georgia Southern University stated that the variety of life on these reefs is similar to natural coral reefs. L Reef was established in 1976. It is located about 23 nautical miles (43 km) east of Ossabaw Island and lies 55–65 feet (17–20 m) below the ocean's surface. It includes items such as New York City subway cars, M-60 tanks, and concrete culverts. In December 2023, the DNR placed two retired railcars from the Metro Atlanta Rapid Transit Authority into the reef. In August 2024, divers found soft coral growing on the railcars and counted nine species of game fish. The USS Yancey was sunk as an artificial reef off Morehead City, North Carolina, in 1990, as AR-302. It lies on its starboard side at a depth of 160 ft (49 m). The USCGC Spar was sunk in June 2004 by Captain Tim Mullane in 108 feet (33 m) of water, 30 miles (48 km) off Morehead City, North Carolina, where it serves as an artificial reef. The USS Indra was sunk as an artificial reef on August 4, 1992, in 60 feet (18 m) of water. Its coordinates are 34°33′55″N 76°58′30″W. The USS Aeolus was sunk to form an artificial reef in August 1988 as AR-305. It is located 18.3 nautical miles from Beaufort Inlet Sea Buoy in 104 feet (30 m) of water. In the late 2000s, the New York City Transit Authority retired old subway cars to make room for new trains. Each car was cleaned, loaded onto a barge, and sunk in the Atlantic Ocean off the coasts of Delaware, Virginia, South Carolina, Georgia, and Florida. Some