Artificial reefs: from ecological processes to fishing enhancement tools

Abstract

Polovina published the article “ Artificial reefs: nothing more than benthic fish attractors” in 1990 and in it debated the potential for artificial reefs to substantially increase standing stock of marine resources. Artificial Reef (AR) technology was strictly oriented towards improving commercial fishing (STONE et al., 1991) from the 1930s, when the Japanese government invested in large scale artificial reefs, until the 1990´s. The first three International Conferences on Artificial Reefs and Related Aquatic Habitats (CARAH) focused on the improvement of fisheries around the world (SEAMAN; SPRAGUE, 1991; GROVE; WILSON, 1994). However, over the last twenty years, the expansion and diversification of AR use has resulted in a shift in the focus of AR-based research towards a more ecosystematic approach (largely due to advances in SCUBA), focusing on a better understanding of the AR's ecological function and its effect on marine benthic and fish communities (JENSEN, 2002; BORTONE et al., 2011). Questions concerning the AR's function address the interactions between the artificial and natural environments, and understanding how benthic and fish species (mostly commercial species) benefit from the presence of artificial habitats. Production at higher trophic levels (usually of commercial species) normally depends on production at lower levels (bottom-up control). Production levels can be modulated by physical forcing and the structure of the marine food webs (top-down control), with environmental constraints determining the community structure of the fauna and flora (SNELGROVE; BUTMAN, 1994). Physical factors (e.g. currents) and chemical bottom sediment components (in particular phosphorous) are therefore vital for reef production. An alternative approach for assessing the ecological implications of reef structures is the use of a bottom-up approach, i.e. assessing the role of ARs in enhancing primary production and energy transfer to the lower trophic levels of the benthic food web. ARs are normally deployed in areas where sandy habitats predominate. The presence of these man-made reef structures will affect: i) nearby sandy areas and also ii) water column processes in the vicinity of ARs. Habitat linkages among distinct reef compartments are driven by hydrological processes (changes in water flow patterns), sediment type, geomorphological, chemical-physical and biological processes (FABI et al., 2002; KIRKE, 2003; DEDIEU et al., 2007; FALCAO et al., 2007, 2009) and also biotic processes (YANAGI; NAKAJIMA, 1991; LINDQUIST et al., 1994; PEPE et al., 1998; SHENG, 2000; DALE; PREGO, 2002; WILLIAMS; POLUNIN, 2001; FABI et al. 2006; EINBINDER et al., 2006). AR structures can affect biological processes differently within different reef compartments. The understanding of the different mechanisms behind these processes is essential for evaluating community responses to man-made perturbations, including fishing. ARs are known to affect nearby sandy benthic sediments and their communities (BULLERI, 2005). When an AR obstructs current flow, a lee wave or stationary wave is formed, which can trap drifting larvae and seaweeds (SHENG, 2000). Moreover, fish attracted to ARs can significantly increase nutrient production in the water column - by excreting ammonium, urea and faeces - which is then incorporated into the reef food web. This process contributes to the organic enrichment of sediments due to entrapped drift algae and other organic materials driven from reef biological activities and deaths of reef associated organisms. Deposition of biomass in the lee of the reefs favours benthic remineralization, promoting nutrient regeneration in pore water (FABI et al., 2002; DEDIEU et al., 2007). Sediment nutrient enhancement from deposited organic material can be incorporated into the reef food web, enhancing sandy benthic production which contributes to the rehabilitation of sandy coastal areas via sediment chemical trophic chain pull-out (FALCAO et al., 2007, 2009). The biochemical role of ARs in this respect is particularly important in shallow coastal