Gray mullets ameliorate organically enriched sediments below a fish farm in the oligotrophic Gulf of Aqaba (Red Sea)

Net cage mariculture has an impact on the local benthos due to discharge of large amounts of particulate organic matter that alters the underlying sediment geochemistry. A field study was carried out to assess the use of the bottom-feeding gray mullet Mugil cephalus within benthic enclosures as a means to reduce the benthic impacts of fish farms. Four experimental 1 m³ enclosures were stocked with 12 gray mullets each (mean ± standard error [SE] stocking density 1720 ± 25 g enclosure^-1), while 3 identical enclosures without mullets served as controls. The enclosures had no bottom so that mullets could feed from and bioturbate the sediment. Sediment cores were taken from the 7 enclosures and from 3 bare (unendosed sediment) sampling stations. Cores were analyzed for dissolved oxygen, total dissolved sulfides (TDS), water content and organic matter, as well as for the presence of macrofauna. Seventy days after the beginning of the field trial, a comparison between sediments inside the mullet enclosures and those in bare and enclosed control sites indicated that they had only 14 and 19% of the TDS and 69 and 44% of the sediment oxygen demand, respectively. These changes in the sediment geochemistry were accompanied by a steep increase in abundance of opportunistic mud snails Nassarius sinusigerus in the mullet enclosures. Essentially no snails or other macrofauna were found in the other treatments. One of the most dramatic changes observed in the mullet enclosures, as compared with the bare and enclosed controls, was the reduction in thickness of the organically enriched sediment layer by 5 cm. The 'missing' sediment was equivalent to 2.6 kg organic carbon and corresponded to a mean carbon removal rate of 20.6 g m^-2 d^-1. While foraging for food and swimming near the sediment surface, mullets within the benthic enclosures resuspended detritus, increased the oxygen supply to the benthos and oxygenated buried organic matter, thereby dramatically improving the status of organically enriched, reduced sediments within a relatively short period of time. Therefore, deployment of detritivores in enclosures under active net cage fish farms may be a viable means to alleviate some of the environmental impacts of intensive mariculture.