An application of artificial reefs to reduce organic enrichment caused by net-cage fish farming: Preliminary results

Two artificial reefs made of porous recycled high-density polyethylene fence material were moored on the sea floor (20-m depth) off the North Beach of Eilat (Gulf of Aqaba, Red Sea) in April 1999. One reef was situated below a commercial net-cage fish farm, and a control reef was deployed outside the area of direct influence of the farm. The objective was to determine if a reef below farm cages could alleviate organic loading of the environment by serving as a biological filter. To examine this question in both qualitative and quantitative terms, the two reefs were compared with regard to organic matter in the sediment close by, abundance of fish and invertebrates, particle filtration (retention) efficiency, and change in biomass of fouling organisms. In addition, control sites situated near the two reels were also monitored to compare the status of the sediment and of associated biota with those of the reefs. The differences in organic matter content of the sediment between each reef and its respective control site were not significant. Also, fish community metrics did not differ significantly, although the total number of fish was 30% higher at the farm site. Biomass of fouling organisms (particularly tunicates and bryozoans) varied inconsistently between sites over time. Particle retention efficiency of the reefs was estimated at 240 g C d^-1, based on the reduction in particulate Chl a in water samples taken downstream of the reefs compared with samples taken upstream.