An artificial reef is a man-made structure placed in freshwater or ocean environments on the ocean floor. These structures are often built in areas with flat, featureless bottoms to help marine life grow. They can be used to control erosion, protect coasts, block ships, stop the use of fishing nets, help restore natural reefs, improve fish farming, or create better conditions for scuba diving and surfing. Early artificial reefs were made by the Persians and Romans.

Some artificial reefs are made from objects that were not originally meant for this purpose, such as old oil rigs (through the Rigs-to-Reefs program), sunken ships, or construction materials like rubble. Shipwrecks can also become artificial reefs if they remain on the ocean floor. Other reefs are made using concrete shaped into forms like reef balls. Green artificial reefs use natural materials like plant fibers and seashells to reduce pollution and energy use. In some cases, artificial reefs are created as art pieces.

Artificial reefs provide hard surfaces where algae, barnacles, corals, and oysters can grow. They also create spaces for fish of different sizes to hide. As marine life attaches to the reef, it forms structures and food sources for fish. How well an artificial reef works depends on its location, how it is built, and the types and ages of species living nearby. While artificial reefs can support coral growth, they may change ecosystems because different species grow at different rates. Studies show that fast-growing algae, bacteria, and coral grow differently on artificial reefs compared to natural ones.

Scientists are studying how to build artificial reefs and their effects on the environment. Many early materials used for reefs are no longer considered good choices. A 2001 review found that about half of the reefs studied met their goals. Long-term planning and management are important for success. A recent study of reefs worldwide from 1990 to 2020 concluded that artificial reefs can help restore marine ecosystems if they are carefully designed to fit the needs of their specific location.

History

Artificial reefs have been created for a long time. Historian Diodorus Siculus wrote 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 build an artificial reef. Similarly, the Persians built an artificial reef to block the mouth of the Tigris River and stop Arabian pirates.

Artificial reefs were used to increase fish populations and grow algae as early as the 17th century in Japan. People placed rubble and rocks in the water to help kelp grow. The first recorded artificial reef in the United States was in the 1830s. Fishermen used logs from old huts off the coast of South Carolina to improve fishing. In the Philippines, a traditional fishing method called fish nests (known by names such as gango, amatong, or balirong) involves building mounds with rocks and waterlogged wood in shallow tidal waters. These mounds attract fish and crustaceans. During low tide, people surround the mounds with nets, dismantle them, and harvest the fish. The mounds are then rebuilt after each harvest. Fish nests have been used since before 1939 to catch young groupers, which are later used to start aquaculture projects.

Before the 1840s, American fishermen used interlaced logs to create artificial reefs. In recent years, people have used items like old refrigerators, shopping carts, abandoned cars, and retired vending machines to build reefs. Official projects have also included decommissioned ships, subway cars, battle tanks, armored vehicles, oil rigs, and beehive-shaped reef balls.

Purposes

Artificial reef structures (ARs) are used for many purposes. They help protect and restore marine ecosystems, such as kelp forests and coral reefs, and support human activities like fishing, diving, and surfing. Artificial reefs can help fix damaged environments, bring back lost habitats, and increase the variety of life in the ocean. In fishing, they can help grow fish populations that are important for recreation and trade. They can also stop harmful fishing practices, reduce coastal erosion, and support eco-tourism by creating places for people to enjoy activities like scuba diving and surfing.

The way artificial reefs are built depends on where they are placed and what they are meant to do. Some reefs work well for one purpose but not for others. Early artificial reefs often failed or had limited success. 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 comparing artificial and natural reefs before and after construction, monitoring reefs over time, and focusing on the shape and structure of the reef materials.

Artificial reefs designed to improve ecosystems develop in stages. First, ocean currents create areas rich in plankton near vertical structures, attracting small fish like sardines and minnows, which draw larger fish like tuna and sharks. Next, fish that need shelter, such as grouper and eels, move to the reef. Later, predators like jack and barracuda appear. Over time, algae, corals, and sponges grow on the reef.

An electrified reef uses a small electric charge on underwater metal structures to cause limestone to form. This helps coral larvae attach and grow faster.

3D printing is used to make molds for artificial reefs and to create ceramic or concrete reefs directly. Some materials, like Archireef’s terracotta tiles, are safe for the environment and help corals grow. In Hong Kong, these tiles had a 95% coral survival rate after three years, much higher than older methods.

Restoring artificial reefs can include moving corals, resettling coral larvae, and growing corals in nurseries. For example, the Coral Restoration Foundation in Florida grows corals like elkhorn and staghorn and replants them on damaged reefs. A 2023 review said artificial reefs can help restore ecosystems if they are designed properly for each location.

Artificial reefs may help store carbon to fight climate change. Mangroves, seagrass, and algae are natural carbon sinks, and scientists hope artificial reefs can add to this. In the Gulf of Mexico, the RGV Reef is being studied for its ability to capture carbon. Near Juehua Island, M-shaped reefs improved water flow for carbon storage, but local species were limited. Experts suggest adding the right species and managing the area to improve results.

In the Caribbean, placing artificial reefs near seagrass meadows helps fish find shelter, and the fish fertilize the seagrass, making it grow better. This creates a cycle that supports more life and stores carbon.

Some artificial reefs are built to stop coastal erosion. Their shape and how they affect water movement are important for this purpose. Reefs can trap sand on beaches or redirect wave energy away from the shore. They are often custom-made for specific areas and can support local wildlife.

Some reefs, like surfing reefs, are not focused on helping ecosystems. For example, Hoppy’s Reef in California (1971) was not successful. Other surfing reefs, like Cable Station Reef in Australia and Narrowneck Reef in Queensland, have been built using concrete, rocks, or sand-filled bags. These materials vary in how long they last, and some have issues with durability or safety.

Artificial surfing reefs also aim to protect beaches and study coastal environments. However, they are often made of different materials than reefs designed to help ecosystems. A 2012 review found that many surfing reefs did not meet their goals, like 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 could also gather fish in one place, making it easier to catch them and causing overfishing, which harms fish populations over time. Materials used in artificial reefs might break down, releasing harmful substances like paint, oil, or plastics, or parts of the reef might become ocean waste or wash onto natural reefs and beaches.

Many marine animals move a lot. Fish near artificial reefs vary depending on the reef's age, size, and structure. Different life stages of the same species may prefer different habitats. For example, young Red Snapper (Lutjanus campechanus) are more likely to live in vertical artificial reef 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 existing natural habitats and the needs of species at all life stages, including where they reproduce and grow.

Using shipwrecks and oil derricks as artificial reefs creates a new food chain in the local ecosystem. Artificial and natural reefs often have very different food chains. Artificial reefs may not develop the same variety of life as natural reefs unless they closely resemble natural ones. For example, a 200-year-old reef near Sint Eustatius has a healthy ecosystem but has fewer and different coral species than a nearby natural reef.

Artificial reefs can sometimes upset natural ecosystems by bringing in non-native species that harm local life. In 2008, at Palmyra Atoll near Hawaii, iron from a shipwreck caused algae and a type of sea anemone to grow, covering coral and creating a "black reef."

Artificial reefs can quickly increase local fish and algae populations. However, scientists debate whether these increases come from fish moving to the reef from nearby areas (the attraction hypothesis) or from new fish growing there (the production hypothesis). Some experts, like James Bohnsack, believe artificial reefs do not increase fish populations but act as tools to gather fish from other areas. Others argue that artificial reefs can also support new fish growth. A 2022 study said the question of whether artificial reefs attract or produce fish must be studied on a case-by-case basis 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 efforts.

There are concerns that artificial reefs might be misused 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 always work well.

Some artificial reefs are less stable than expected, breaking apart and becoming ocean waste. In the 1970s, tires were used to make artificial reefs, but tropical storms destroyed the system, washing tires onto beaches and damaging nearby coral reefs. At Osborne Reef near Fort Lauderdale, Florida, storms broke the straps holding tires together. By 2019, 250,000 of 700,000 tires had been removed. France has also started removing its tire reefs. Organizations like Ocean Conservancy and 4Ocean now include tire cleanup in their efforts.

Some artificial surfing reefs have also had problems. Early projects used sand-filled bags that broke down faster than expected. For example, Pratte's Reef in California and Mount Reef in New Zealand required costly cleanup work. Some say this method of building reefs is not reliable.

Artificial reefs, especially those 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 needed for coral survival.

Materials like plastics, oil, paint, and rusting metals can release harmful chemicals, such as PCBs and heavy metals, into the environment. These chemicals 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, which should be avoided.

International guidelines recommend using materials that do not release harmful chemicals. The London Convention and Protocol/UNEP developed rules in 2009 for using inert, non-polluting materials in artificial reefs.

Efforts continue to create safe waste-based materials for artificial reefs. For example, oil and coal ash have been mixed with cement and lime to make experimental reef blocks. However, concerns about chemical leaching remain, similar to tire reef problems.

In the U.S., best practices for preparing ships for artificial reefs include removing all dangerous materials, such as PCBs, and clearing debris. If removing harmful materials is too difficult, the EPA may allow it, as happened with the ex-USS Oriskany.

Examples

Florida is home to many artificial reefs, many of which were created by intentionally sinking ships, including Coast Guard cutters Duane and Bibb and the U.S. Navy landing ship Spiegel Grove.

In the early 1970s, over 2 million used vehicle tires were dumped off the coast of Fort Lauderdale, Florida, to form an artificial reef. However, the tires were not fastened correctly to the reef structures. 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. Gary Levine and Kim Brandell imagined and created the project. Burial at sea became a way to fund it. By 2011, about 200 "placements" had happened. Cremated remains were mixed with concrete and placed inside columns or shaped into sea stars, brain coral, 15-foot (4.6 m) models of lions, or other shapes before being placed in the water.

In 1921, the U.S. battleship Massachusetts was sunk intentionally in shallow water near Pensacola, Florida, to test experimental 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 worked as a spacecraft-tracking ship after the war. It 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 underwater. Its funnels will be removed. The ship is currently in Mobile, Alabama, being stripped of hazardous materials and valuable items.

In 1970, the Georgia Department of Natural Resources (DNR) began 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 organisms. A marine ecologist from Georgia Southern University said, "The variety of life here is similar to natural coral reefs."

L Reef was created 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 used by the U.S. Army, 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