Natural reefs are a beautiful riot of colour that are usually formed of jagged rocks and corals. These phenomenal habitats thrive in a delicate balance of clear, sunny, shallow waters.

Unfortunately human activities easily upset this balance. Increasingly, warming waters are causing coral bleaching, while ocean acidification is making it harder for corals to grow. On top of this, overfishing, assorted forms of pollution, sedimentation, and physical damage from anchoring, mining and other chronic human activities are depleting the bustle of marine life inhabiting reefs.

Cambodian reef © Paul Colley.

There are few sights as beautiful, or as fascinating as a healthy coral reef. Credit: Paul Colley.

This can quickly paint a dystopian, deserted seascape of coral skeletons overridden by seaweed, so it is no surprise that quick solutions are being sought to bolster coral communities and regenerate degraded reefs.

One of these strategies is the construction of artificial reefs, which are human-made underwater structures.

However, replicating the successes of a natural reef is no small task. Aside from their beauty, natural reefs are also an integral component of marine ecosystems. Their rugged rocks and coarse corals protect coastal communities from waves and storms. Their nooks and crannies are refuges for vulnerable animals like juvenile fish, and their structural complexity supports thousands of species.

In turn, these abundant communities of fish and invertebrates are a vital protein source for over a billion people worldwide, not to mention providing us with countless new medicines, such as the secosteroid enzymes used to treat asthma and arthritis.

So how well does a human-made reef fare against a natural reef?

Pick your target

Although man-made reefs are artificial, there is nothing artificial about the threats that they face. Both artificial and natural reefs are susceptible to the same pressures, so you have to be very clear about what you want to achieve with an artificial reef.

1. To boost fisheries

Some artificial reefs can help to attract commercially valuable fish, thereby becoming hotspots for fisheries. Most commonly these are interesting structures like submerged shipwrecks, which can also be exciting diving sites for tourism.

An unintended consequence of this type of artificial reef, however, is that the aggregated fish can become vulnerable to overfishing. If this happens, benefits can therefore be short-lived and worsen the situation in the long-term.

Materials vary greatly and affect a reef’s durability. Steel hulled ships are far more resilient than wooden ones, which rot quickly and are not durable enough for the formation of corals. Smaller, lighter structures like cars and helicopters have shorter lifespans of no more than 10 years, and have a higher risk of damaging the natural habitat if they are lifted by stormy waters.

Credit: SCUBATISTA/Marine Photobank.

It can be challenging to find a reef structure that is heavy enough to withstand storms and currents but small enough that it doesn’t pose a risk to passing vessels. Credit: SCUBATISTA/Marine Photobank.

Furthermore, fragile components like car wing mirrors can easily detach and become pollutants, carried far and wide by ocean currents. Vehicles and machinery are also often contaminated with pollutants, including petroleum, heavy metals and anti-fouling paint, and therefore also need to undergo thorough cleaning before use.

2. To boost tourism

When an artificial reef succeeds in attracting marine life and promoting coral growth, it can make an attractive tourist site – particularly when the sunken object in itself makes an interesting underwater display for divers. In theory, this could reduce pressures on natural reefs by enticing divers away.

However, whilst there is some evidence to support this, contradictory studies also show that creating artificial reefs can increase overall diver numbers rather than diverting divers away from the natural reef.

Credit: Rebecca Weeks/Marine Photobank.

Artificial structures can provide an interesting diving experience. Credit: Rebecca Weeks/Marine Photobank.

What’s more, an increase in tourism can also add to pressures on natural reefs indirectly (e.g. through pollution, degradation of coastal habitats), which could worsen the long-term tourism and fisheries prospects.

An additional problem is that sturdier structures such as ships, which might be more suitable for turbulent waters, need to be placed in deeper waters. This makes them generally unsuitable for coral growth or diving.

3. To prevent trawling

A more controversial use of artificial reefs is to place structures in the sea to prevent trawling in the area. In the western Mediterranean, this had succeeded in protecting seagrass meadows and helped to eliminate trawling in some areas. However, this was not seen as a panacea for solving issues with nearshore or illegal bottom trawl fishing.

While this may decrease some pressure on natural reefs and associated habitat, it may be offset by the damage caused when deploying the artificial reef. Unsurprisingly, this can also be dangerous to the crew on trawlers and sparks animosity between commercial fisheries and those responsible for the artificial reef. Furthermore, the nets that tear in the process become detached, adding to the problem of “ghost fishing” whereby lost nets continue to entangle and kill marine life.

4. To compensate for damage or to achieve biodiversity gains

In emergency events like a natural disaster or an oil spill it may be necessary to generate new reef in order to try to compensate for the biodiversity loss at a site. However, it is crucial to remember that artificial reefs cannot replace natural reefs. They should only ever be seen as a short-term or secondary measure.

Interestingly, it is much more expensive to construct an artificial reef than to protect or restore a natural one. According to Emeritus Professor Alasdair Edwards of Newcastle University, for the same price you can build 0.01 km2 of artificial reef or rehabilitate 1 km2 of degraded natural reef. With funding to support reef conservation already scarce, there are real questions around whether we should be redirecting these resources to more costly and less efficient solutions.

Proceed with caution

With potential fishery, tourism or conservation benefits, it is easy to see why an artificial reef might appear to be an attractive proposition. However, before proceeding down this path, some basic questions need to be asked.

First and foremost of these is: what are you trying to achieve, and is an artificial reef the best way to do this? You also need to look at the potential complications – for example, a reef designed to promote tourism might harm local fisheries and vice versa. How will you manage this?

Beyond these fundamentals, you need to look at practicalities: do corals need to be transplanted from natural reefs? If so, how sustainable is this? How well will corals and other reef animals grow on the new structure? Is the structure stable enough to weather currents and storms? Placement and material of these artificial reefs is equally important. Clay, silt, or loose sand should be avoided, as they create murky waters that will smother and kill reef organisms. Placement must also be considered within the context of, and be compatible with, other existing uses for the area (e.g. conservation zones, fishing zones).

Credit: Steve Spring, Reef Rescue Palm Beach County/Marine Photobank.

Tires were dumped off Florida’s coast in the 1970s in an attempt to establish an artificial reef. They ended up scattering for miles. Credit: Steve Spring, Reef Rescue Palm Beach County/Marine Photobank.

In general, purpose-built concrete and limestone structures riddled with holes and cracks are preferable for building artificial reefs. However, the choice of material will depend on the environment and the aims of the project.

Conservation, naturally

If conservation is your goal, then it is important to understand that the natural environment does its job far better than we can. Healthy reefs degraded by natural disasters like tsunamis can recover without our help. However, reefs affected by human pollution, overfishing, and other chronic stressors cannot bounce back with the same vigour. With artificial reefs far more costly, and ultimately facing the same threats as natural reefs, would it not be a better use of time and energy to better protect and restore what nature has already provided? Food for thought.

For more information, read the references for this article.

Main image credit: SCUBATISTA/Marine Photobank