More than any other species humans have landscaped the Earth, altering it to suit our needs.  What may be less obvious is we are also landscaping the oceans – and have been doing so for a very long time.  Historical artefacts suggest that artisanal fisheries in the Mediterranean Sea and Australia were utilizing discarded rocks as fish aggregation devices around 3,000 years ago.  The first recorded modern artificial reef came from Japan 500 years ago, where rubble and rocks were used to grow kelp.  180 years ago, logs were placed in coastal waters around South Carolina as fish aggregation devices.  Today artificial reefs can be found throughout the world, very often placed - if not created - to fill a specific purpose. 

At Australia’s Cable Station Beach an artificial surf reef created by modifying an existing limestone reef with granite rock, has resulted in surfable waves an average of 130 days of the year.  Artist Jason deCaires Taylor created some 450 cement-cast figures for The Silent Evolution at the underwater museum MUSA (Museo Subacuático de Arte) off Cancún Mexico, with the idea of drawing people away from fragile reefs.  Off the south-eastern Iberian Peninsula, anti-trawling reefs have been placed around sensitive seagrass beds to reduce the impact of illegal bottom trawling.  To tackle the problem of coastal erosion in Portugal, researchers have suggested that artificial reefs designed to either dissipate wave energy or rotate waves to reduce longshore currents could prove useful tools. Artificial reefs come in a variety of shapes, sizes, and materials.  Reef restoration projects typically use limestone boulders or concrete/ceramic structures, often textured, shaped, and interlaced with cavities in such a way as to provide a heterogeneous reef-scape to mimic the preferred habitat of a number of different species. 

Not all reefs are purpose-built.  Florida USA is home to the two largest artificial reefs in the world, both created from the sinking of decommissioned vessels (USS Oriskany and USHS Vandenburg).  Rigs-to-reefs programs convert defunct offshore oil and gas platforms into artificial reefs for conservation and fishery purposes as well as reduce rig decommissioning costs.  In well-managed jurisdictions, placing artificial reefs is no simple task.  Many factors need to be taken into account, such as the stability of the environment they are to be placed in, and the impact of the reef to the physical processes as well as the organisms residing in the area in which it is to be placed, and of course any human uses of the area.

On the face of it artificial reefs seem like a win-win scenario, providing benefits for people as well as marine organisms but they are not always as successful as hoped.  During the 1980s and 1990s the extension of an offshore breakwater at Hirakata fishing port in Japan altered sand transportation throughout an adjacent pocket bay, increasing sand deposition in the wave sheltered section nearest to the breakwater, but increasing shoreline erosion further up the coast.  In 2004 a number of artificial reefs were created along sections of the bay to tackle the erosion, but have thus far proved ineffectual. 

In some cases artificial reefs have proved harmful to marine biodiversity.  Off the coast of Florida’s Fort Laderdale, Broward Artificial Reef Inc. with the support of fishers devised and constructed the Osborne Reef.  Created from over 2 million discarded tires clipped together with steel clips and nylon, the artificial reef was designed with two purposes in mind - create an area to which marine organisms could attach and ultimately attract more game fish, and solve a tire disposal problem.  Over time the clips steel and nylon broke down, the tires loosened and the artificial reef separated, effectively destroying what little marine life had settled on the tires.  Hurricanes and large coastal storms spread the tires over a much larger area than the original 150,000m2 the reef was originally placed on including onto natural reefs, destroying areas of delicate slow growing corals.  Recovery (assuming it happens at all) could take decades if not more. 

Despite clean-up operations beginning in 2001 tires remain on the sea floor and are likely to do so for some time. 

The intentions behind Osborne Reef were inherently good, if a little misguided.  The same cannot be said for all reefs created out of discarded items.  Almost anything can– and has been dumped as an artificial reef.  Although the Convention on the Prevention of Marine Pollution by Dumping of Waste and Other Matter, 1972 regulates the dumping of waste at sea, items hazardous to the marine environment have been placed in the ocean as artificial reefs, like refrigerators and engines that have not been sufficiently cleaned of their toxic elements. 

Even in well planned ship sinking, contamination remains an issue.  A year after the USS Oriskanay was sunk off the Florida coast, the Florida Fish and Wildlife Conservation Commission noted that PCB levels in nearby reef fish had risen, though over time levels have declined.  Furthermore when placed on soft bottomed sea floors – sand and mud areas, an artificial reef introduce a hard substrate to an area where there was not any before.  Whilst this boosts habitat opportunity for species that require such habitat, there may very well be a detrimental impact to the species that are associated with soft-bottomed habitats as their habitat is effectively destroyed. 

One of the goals of artificial reefs is to alleviate visitor pressure on natural dive sites which can undergo extensive damage from divers, snorkelers, and boat users.  Questioning divers in Barbados, researcher Anne Kirkbride-Smith revealed that divers enjoyed artificial reefs for the “guarantee of a ‘good dive’”.  Unfortunately enjoyment levels did decline with diver experience.  Given that learner and novice divers are typically the main culprits of accidental reef damage, artificial reefs allow for a good novice dive experience w reduce accidental damage to natural reefs.  How artificial reefs impact overall numbers to natural dive sites is still in question, especially in areas where visitor numbers are increasing.  Whilst themed dive sites such as MUSA’s sculptures certainly draw many visitors, there is no evidence that people are visiting the sculptures instead of the nearby natural reefs.    

 Even the use of artificial reefs as a conservation or fisheries management tool is not quite as clear cut.  A number of studies have demonstrated that artificial reefs can support marine species – even at times at apparently higher density, biomass, and diversity than natural reefs, but the community composition can be quite different to that found on nearby natural reefs.  Furthermore it is unclear if artificial reefs act as aggregation devices, rather than boosting overall productivity.  If habitat is a limiting factor for species, then extending their habitat could indeed boost productivity - or at least provide alternative sites for their persistence if their natural habitat has become degraded.  If the species are scarce because of overfishing/destructive fishing, or other human disturbances, then no number of artificial reefs will boost productivity if those human impacts are not also effectively tackled.  Instead, the reefs concentrate targeted species into one area, increasing ease of extraction. 

Artificial reefs remain a controversial topic.  Some argue that we need artificial reefs to restore biodiversity and support human needs.  Others are less happy with altering the sea floor.  As professor of coral reef ecology Alistair Edwards and marine biologist Dr Edgardo Gomez notes,   “There are estimated to be in excess of 500,000 “reef balls” of varying size deployed worldwide. These will provide at most a couple of square kilometres of topographically complex substrata as a cost of US$ tens of millions. There are an estimated 300,000 km2 of shallow coral reefs in the world, so there is plenty of substrate available. The main problem is that much of it is poorly managed or degraded”.  Perhaps we shouldn’t be asking ourselves how we can make better artificial reefs, but what we should do to take care of the natural ones. 

This story was written for The Marine Professional, a publication of the Institute of Marine Engineering, Science & Technology (IMarEST).