5.4 Restoration artificial reefs

5.4.1.Objectives

This kind of artificial reefs can be used to:

• recover degraded habitats and ecosystems where the interventions aimed to reduce the human pressure causing the degradation have failed;
• compensate the loss of ecologically important habitats caused by some human activities linked, for example, to coastal development and energy production (wind mills, offshore platforms, etc.).
In this case, the basic principle should be that of not creating something that would not naturally exist in the environment.
Particular attention is required in the use of artificial reefs for the rehabilitation of natural coral reefs. In this case, artificial reefs may represent a solution for coral reefs of particular economic value that were damaged by shipping accidents, or for damaged sites used by tourist operators. However, the use of artificial reefs is only recommended to supplement damaged reef areas of a few square meters. Such methods are not considered viable or feasible for coral reef rehabilitation over several square kilometres due to the potential damage that installation operations could cause to adjacent coral reefs and associated ecosystems (ICRI, 2009).

Mitigation reefs should be created as soon as possible once damage on habitat or resources has been observed, as delays can contribute to increase ecological losses.

5.4.2.Design

In this case, natural materials that are as similar as possible to the original ones (boulders, stones, etc.) should be employed. In coral reef rehabilitation, boulders or concrete modules are usually employed and often associated with the transplantation of corals from the impacted areas to enhance the mitigation process.

5.4.3.Practical applications

? Denmark

An example of restoration artificial reef comes from Denmark, where natural cavernous boulder reefs have been extensively exploited for their high concentration of easy?to? excavate large boulders which makes them suitable for constructing sea defences and harbour jetties. In 2008, the Danish Forest and Nature Agency constructed the Laeso Trindel artificial reef (Kattegat) to restore and maintain the local cavernous boulder reef habitat, a site of importance designated as a Natura 2000 site in accordance with the European Union Habitats Directive (Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora). The project consisted in the immersion of around 60 000 m? of boulders of various sizes and weights (1–6 tonnes; Plates 7, 8 and 9).

Plates 7, 8 and 9. Laeso Trindel artificial reef: construction of the reef (left) and benthic colonization on the reef boulders (from Dahl et al., 2009).