Ever since humans started building ships, they’ve turned into wrecks. Some shipwrecks are world-famous, the subjects of hit movies and decades-long searches for their final resting place. But most of them aren’t as renowned, as large, or as deeply buried as the Titanic. There’s an estimated three million shipwrecks scattered across the ocean floor, from sunken World War II destroyers to colonial Spanish galleons to small abandoned dinghies. Today, many of them are teeming with marine life, reclaimed by the ocean as homes for coral, eels, snappers and sharks.
It’s not just happenstance and disaster that brings about this phenomenon. Innumerable boats and other human-made objects have been deliberately sunk to the ocean floor—known as artificial reefs —typically in the hope of kicking off reef development and attracting fish. Ancient Roman and Persian sailors even deliberately sunk items to the ocean floor, though usually to trap enemy vessels in naval battles or to divert marauding pirate ships. At some point, humans recognized the fish-attracting powers of reefs and sought to replicate the results in favorable locations. Records from the 1830s show fishermen off South Carolina building log huts in coastal waters to generate higher yields, and offshore rigs became fishing hotspots with the development of oil and gas drilling. Today, industrious individuals sink all manners of refuse in their favorite fishing spots. You can find unsanctioned artificial reefs made out of busted fridges, wheel-less shopping carts or the frames of old cars teeming with red snapper, butterflyfish and stony corals.
In the 1970’s state and federal entities started experimenting with creating sanctioned artificial reefs, typically as habitat restoration projects. “They’re installed to create new homes for a variety of marine life,” said Avery Paxton, a researcher with the National Oceanic and Atmospheric Administration’s National Centers for Coastal Ocean Science who studies the ecology of artificial reefs. “They can also be used to restore degraded habitats.” Research shows that artificial reefs host more fish than natural ones and attract more large predators, like sharks and barracudas, that like to hang out in the water column swirling above the reef.
Reefs are even erected to attract tourists and fishers looking to snorkel or fish among the flocking plethora of marine life. Well-known destinations like the Florida Keys are a playground of artificial reefs. The Florida Keys National Marine Sanctuary is home to 61 artificial reefs, including the vessel Duane, sunk in 1985 after 50 years crawling oceans around the globe. Nearby lies the Hoyt S. Vandenberg, a Navy transport ship loaded with explosives and rocketed to the ocean floor in 2009. And some artificial reefs show up in unexpected places, and in unexpected forms. For example, more than 2,500 decommissioned New York City subway cars sit off the shores of Georgia, South Carolina, Virginia, Maryland, Delaware and New Jersey. The cars were cleaned, hollowed out and sunk into the Atlantic as part of a reef creation program between 2001 and 2010.
When a human-made object sinks, be it an inadvertent wreckage or a purposeful effort in an artificial reef program, the ocean quickly takes over. “From the minute it sinks, it starts to form a new habitat underwater,” Paxton said. “Once you sink a ship, it quickly attracts schools of smaller fish,” echoed Chris Taylor, an ecologist who works at NOAA with Paxton. Over time, as a complex community of fishes gather and colonize the reef, a carpet of algae, corals and other invertebrates grow on the structure’s surface. The currents curling around a sunken structure likely push up clouds of plankton, the essential base of the ocean food web. This buffet attracts small fishes like sardines, which in turn entice sharks and other larger predators to hang around. Eels and groupers move into the holes and dark spaces in the reef, and tuna and barracuda slink in looking for a snack.
Almost anything can become a reef under the right environmental conditions. But many objects can be sources of underwater pollution in their original states. Toxic paints, asbestos, iron and rusting metal particles can seep out of the structure and into the surrounding waters. Because of this, official reef-creation projects include extensive cleaning and decontamination before the structure or object is sunk into place. That’s not possible, of course, in the case of shipwrecks, which often sink and are abandoned in times of duress or bad weather and only are rediscovered as artificial reefs years later.
For example, scientists have noted instances of iron leaching from sunken ships and having adverse knock-on effects on surrounding reef life. In 2008 at Palmyra Atoll, a reef formation and U.S. National Wildlife Refuge south of Hawaii, researchers observed an invasion of a type of sea anemone called a corallimorph around a shipwreck. The vessel, which ran aground in 1991, was moored to the ocean floor by iron chains. Algae need iron to grow, but the element isn’t plentifully available in many parts of the central and south Pacific Ocean, so its sudden abundance prompted an explosion of algae and an unbalanced invasion of the corallimorphs. The overgrowth, dubbed a “black reef” by scientists due to the anemones’ dark coloration, stretched for over half a mile around the shipwreck and smothered the existing coral in the area.
The shipwreck was removed in a multimillion-dollar restoration project spearheaded by the U.S. Fish and Wildlife Service in 2013. In the years since, scientists have had some success with attempts to rid the reef of lingering corallimorphs with pulverized bleach, a treatment that could be used at other similar sites. The Palmyra Atoll is renowned for its biodiversity (it’s home to three times as many coral species as the Hawaiian Islands), and scientists hope the restoration and removal efforts will help retain the area’s abundant marine life.
While pollution shouldn’t be a concern in the creation of intentional artificial reefs, there are still unanswered questions about what impact these structures have on the larger ocean ecosystem. Paxton, Taylor and their colleagues are working to further understand how fish use artificial reefs, and if new human-made reefs prompt fish populations to grow or just pull them from elsewhere. “Are these fish being taken away from nearby natural habitats, or over time are new fish actually being produced?” Paxton said. “It should take a thoughtful approach to both the planning of where these things should go and the potential impacts that they may have on natural habitats,” said Taylor. But one thing is for sure, Paxton said. “Artificial reefs can be incredible fish habitat.”