A need for more research

Artificial reefs are man-made structures that are designed to imitate the physical structure of natural reefs to create a habitat for fish and other marine organisms. They should also emulate some functions of a natural reef, like protecting, regenerating, concentrating, and/or enhancing populations of living marine resources.

The objectives of an arti cial reef may further include the protection, restoration and regeneration of aquatic habitats, and the promotion of research, recreational opportunities, securing coastlines, and educational use of the area.

A source of some controversy

Arti cial reefs are not recent developments. ey have existed for centuries all over the world but became more common in the late 1900s and much more complex regarding their purpose and the materials used to make them. e move towards complexity has resulted both in very successful arti cial reef projects and in outcomes that have been di cult to quantify with certainty. e development of novel 3D structures of arti cial reefs has provided good insights into their possibilities for enhancing diversity in the area they are placed. Arti cial reefs can provide unique bene ts in addition to some of the functionality of natural reefs. ey can provide a sustainable habitat for marine life, act as wave-breakers, and help to increase sh populations in and around the reef. Despite being designed to mimic natural reefs, arti cial reefs are still not the same as their natural counterparts and there are still many debates on certain characteristics of arti cial reefs. Some researchers believe that arti cial reefs increase growth and survival of organisms, while others consider that any increase in population seen as a result of arti cial reefs is simply due to aggregation. Additionally, some scholars believe that changing the environment with the presence of arti cial reefs will cause more harm than good, as it can have unintended consequences on local communities (e.g. in some cases arti cial reefs provide a habitat for invasive species).

There are some factors that need to be investigated each time a new artificial reef is considered. The environmental, ecological, social and economic conditions, and intended purposes related to a reef are often unique, so there is no one-size-fits-all solution that can simply be replicated for each new artificial reef.

When the construction of a new artificial reef is considered, possible effects on biodiversity, abundance and distribution of aquatic plants and animals should always be dissected. As coastal reef habitats face diverse threats, like overharvesting, habitat destruction, pollution, and climate change, and as artificial reefs are planned to mitigate them, there is no general solution for their construction as a tool for habitat enhancement. Artificial reefs should be deployed strategically based on scientific assessments fully considering the specificities of a location and resources available to maximise benefits. Artificial reef deployment strategies should therefore be guided by local contexts rather than global or regional generalities.

Do artificial reefs really increase fish abundance?

Even though an arti cial reef may have a high density of sh, it is also important to consider how this has developed. ere are two basic arguments regarding the abundance of sh at articial reefs: Some researchers suggest that this is mainly the result of attraction and arti cial reefs simply redistribute shes without signi cantly extending their numbers. Others, however, are of the opinion that they provide new habitat in a previously saturated environment. is means that there will be room for more sh to settle and survive. Even though it is most likely that arti cial reefs both attract and produce sh biomass, it is still critical to understand more precisely, how arti cial reefs a ect the reef system, to be able to better evaluate their use in management, e.g. de ning shing quotas or identifying areas closed to shing. However, currently not enough research data is available on the topic.

e placement of any arti cial reef should only be undertaken once there is a thorough understanding of the local weather conditions and the environment, including waves and currents, sediment transport, the seabed, water and sediment quality, and biological communities. Such environmental information will assist in determining whether the reef is likely to meet its objectives, since local conditions will potentially a ect the stability of the reef, and whether it will provide a suitable environment for target species.

Size, construction material with special focus on e.g., crevices, and the distance between artificial habitats, all have considerable influence on both the environment and on the biodiversity of an area. These characteristics strongly influence the biomass that can settle on the reef, the abundance and age composition of species, but also on the appearance of non-indigenous, invasive species, about which many researchers and other stakeholders are concerned. Deployment of artificial reefs in most cases involves the introduction of a hard substrate to a soft bottom environment, which alters both the abiotic and biotic properties of the environment. A thorough understanding of such effects, together with possible impacts of local hydrodynamics that also distribute nutrients, or of weather events like hurricanes is fundamental to ensure the highest level of success without polluting or degrading the marine environment.

Scour protection measures can also form artificial reefs

Not all arti cial structures placed on the seabed are constructed to serve as arti cial reefs (e.g. oshore wind farms, oil rigs). An arti cial reef can also be established to ful l multiple tasks (e.g. shore protection). is adds additional elements to the construction which should also be considered. When the main aim is something other than just providing a new habitat for marine organisms, the need for scour protection needs to be analysed as well. Scour protection is a measure used to prevent the erosion of seabed sediment around the foundations of arti cial structures. is occurs when a steady current (e.g. tide or wave activity) encounters a vertical structure on the seabed causing local increases in ow speeds and turbulence levels and ultimately leading to the creation of a scour pit around the structure. e magnitude of scouring is a ected by the current speed, water depth, and the sediment type. Scour protection often consists of rocks that are positioned on the seabed to prevent erosion. As such, it may resemble a marine rocky reef and could have important ecosystem functions. However, there are no appropriate data available. us, this area also needs to be further investigated. When additional functions of an arti cial reef are considered they should not contradict the main purposes of the reef.

Arti cial reefs are not only used by marine organisms but also by humans leading to the need to manage interactions between di erent human user groups. Groups with di erent interests may all use arti cial reefs, sometimes damaging the interests of other groups or the reefs themselves. Research shows, arti cial reefs can increase catch rates in the short-term, however, increased harvests could eventually lead to over shing and/or stricter regulations and shorter harvest seasons. Local businesses could bene t from arti cial reefs as they can attract tourists to deployment areas, but if tourists are attracted to one area, it may decrease tourism in other areas. If shers and divers use the same locations, lost shing gears can disturb the experience of divers, moreover, they also have a negative e ect on the abundance and biodiversity of species. All these should be considered when planning a reef, to avoid more serious problems and challenges later.

Monitoring an artificial reef to ensure healthy functioning

Even if arti cial reefs are deployed in an environmentally friendly and sustainable way, they might need regular management, maintenance, and monitoring, including proper rules and legislation, to ensure that they develop and function as anticipated. Maintenance work may include, for example, shifting reef construction or removing invasive/unwanted species. In certain areas shing e orts and/or the interest of tourists and divers may also need to be controlled and regulated to preserve healthy reefs.

In conclusion, no universal solution exists for the deployment of arti cial reefs. Best practices of arti cial reef deployments should involve all stakeholders, consider local speci cities, such as site con guration, governance, ecosystem, and availability of human and nancial resources for surveillance, and de ne the volume and design of arti cial reefs based on these parameters. A strong legislative framework, with detailed environmental and social impact assessments for implementation, including considerations of long-term governance is recommended. ere is also a need to increase research on arti cial reefs to help the development of enhanced technologies as well as to raise the awareness of people and di erent interest groups about the bene ts and possible disadvantages of arti cial reefs.

Eva

Kovacs, Euro sh International Organisation, eva.kovacs @euro sh.dk

Christian Philip Unmack, Euro sh International Organisation, christian.unmack@euro sh.dk

Port of Vigo tests an environmentally-friendly alternative to concrete for its facilities