1 Introduction

Historically, artificial reefs have been used around the world to attract fish and facilitate its capture for human consumption. In the Mediterranean Sea, there is evidence that the first artificial reefs were inadvertently created in the 1500s. At that time, the rocks used to anchor tuna fishing nets were left on the seabed at the end of each fishing season.

These anchors accumulated over time and created new rocky habitats that became inhabited by fish which were subsequently exploited by local fishers between the tuna fishing seasons (Riggio et al., 2000). Throughout history, it is likely that similar practices have been employed by artisanal fishers across the world (Simard, 1995).
The modern concept of artificial reef appeared in Japan during the 20th century, after World War II, and has been adopted in the Mediterranean in the second half of the 1900s. To date, around 300 artificial reefs are deployed in the Mediterranean and the Black Sea and their main purpose is to enhance fisheries and improve fisheries management.

The increasing interest for artificial reefs has given rise to concerns regarding possible negative impacts caused by the use of unsuitable materials and the dumping of waste. Consequently, during the past fifteen years, the need has emerged to develop guidelines to support managers and scientists in the placement of artificial reefs in the European seas (OSPAR Commission, 1999 and 2009; UNEP?MAP, 2005; London Convention and Protocol/UNEP, 2009).
In 2009, the General Fisheries Commission for the Mediterranean (GFCM) of the Food and Agriculture Organization of the United Nations (FAO) initiated a debate on the use of artificial reefs in the Mediterranean and the Black Sea, in particular to enhance and manage fisheries and fishing resources (GFCM, 2010). This issue was addressed during the annual sessions of the GFCM Subcommittee on Marine Environment and Ecosystems (SCMEE) leading to an ad hoc workshop in January 2011 (GFCM, 2011 and 2012). Acknowledging the increasing interest shown by several Mediterranean countries towards artificial reefs, the workshop concluded, among others, that updated guidelines to support potential artificial reef developers would be needed to establish and monitor artificial reefs in the coastal waters of the Mediterranean and the Black Sea.
This document aims at providing the best and most generally accepted guidelines regarding management practices for artificial reef planning, siting, construction, anchoring and monitoring in the Mediterranean and the Black Sea. These guidelines will provide users, managers and planners with essential information and guidance on the most effective methods for enhancing and protecting natural resources as well as for improving fisheries and aquaculture opportunities.
The objectives of these guidelines are to:
• update the information reported in previous guidelines;
• assist countries in the planning and deployment of artificial reefs on the basis of scientific criteria;
• avoid the pollution or degradation of aquatic ecosystems due to the deployment of unsuitable materials and waste dumping;
• prevent possible negative impacts caused by the deployment of artificial reefs;
• provide information on the different scopes and types of artificial reefs, as well as on their potential effects;
• provide technical information on the deployment, monitoring, management and socio? economic effects of artificial reefs.
The chapters below provide a definition of artificial reefs as well as a list of the technical terms that are used in the document and should be employed when referring to artificial reefs. The main reasons for which artificial reefs are usually constructed are also explained. The subsequent sections address aspects related to international legislation, planning, siting, materials, design and placement, including several examples of artificial reef construction in the Mediterranean Sea. Finally, possible negative impacts as well as suggestions to facilitate the standardization of monitoring methodologies and appropriate management are illustrated.

1.1. Definition of artificial reefs

For the purposes of these guidelines, the following has been adopted as a standard definition to promote a common understanding of the term. This definition derives from the Guidelines for the placement at sea of matter for purpose other than the mere disposal (construction of artificial reefs) (UNEP?MAP, 2005), the Guidelines for the Placement of Artificial Reefs (London Convention and Protocol/UNEP, 2009), the Assessment of construction or placement of artificial reefs (OSPAR Commission, 2009), and the Guidelines and management practices for artificial reef siting, use, construction, and anchoring in Southeast Florida (Lindberg and Seaman, 2011).
An artificial reef is a submerged (or partly exposed to tides) structure deliberately placed on the seabed to mimic some functions of a natural reef, such as protecting, regenerating, concentrating and/or enhancing populations of living marine resources. This includes the protection and regeneration of habitats. It will serve as habitat that functions as part of the natural ecosystem while doing ‘no harm’.
The term excludes artificial islands, cables, pipelines, platforms, mooring, and structures for coastal defence (e.g. breakwaters, dikes, etc.) which are primarily constructed for other purposes, as well as the fish aggregation devices (FADs) employed to merely attract fish in certain fishing areas.

1.2. Objectives of artificial reefs

Artificial reefs can be considered as interventions of engineering technology to recover and/or improve natural habitats, increase productivity and manage aquatic resources.
In this context, artificial reefs are used in coastal waters worldwide for many purposes, e.g.:
• protecting sensitive habitats from fishing activities;
• restoring depleted habitats;
• mitigating habitat loss;
• enhancing biodiversity;
• improving populations of aquatic organisms by providing shelter for juvenile and mature individuals as well as for adults during delicate life stages (e.g. moulting season for crustaceans);
• providing new substrates for algae and mollusc culture;
• enhancing professional and recreational fisheries;
• creating suitable areas for diving;
• providing a mean to manage coastal activities and reduce conflicts; • research and educational activities;
• creating potential networks of marine protected areas (MPAs) to manage the life cycles of fish and connectivity.
The objectives of artificial reefs deployment are not mutually exclusive as artificial reefs are often created for more than one purpose (e.g. protection from fishing and finfish enhancement). In this case, they are defined as “multipurpose artificial reefs”.

1.3. Terminology

The use of a standard terminology for the different components of an artificial reef helps artificial reef developers to avoid confusion. In this document the following hierarchy based on that used for Japanese reefs (Grove et al., 1991) has been adopted (Fig. 1):
• Reef unit or module: the smallest element constituting an artificial reef. The modules can be placed singly on the seabed or assembled.
• Reef set: structure formed by the assemblage of reef units.
• Reef group: area constituted by more modules and/or reef sets.
• Reef complex: formed by more than one reef group.
In this document the term “structure” refers to a module or a reef set.

Figure 1. Hierarchy of the different components of an artificial reef (from Grove and Sonu, 1983).