Preliminary assessment of grey mullet (Mugil cephalus) as a forager of organically enriched sediments below marine fish farms

Grey mullet (Mugil cephalus) were stocked in small cages at 23 m depth below a commercial net cage fish farm in the northern Gulf of Aqaba (Eilat) to test their ability to reduce organic matter levels in the enriched sediments. The cages were open to the seabed and 100 g fish were stocked at 0.4 and 1 kg per m² in March 1992. Sediment cores were taken from the enriched seafloor adjacent to and below the mullet cages, and at a nearby undisturbed reference station. Sediment samples were used to determine loss on ignition (LOI), redox potential and macrofauna. After only 7 weeks, both LOI and redox potential levels in the sediments under the 1 kg per m² cage had dropped by more than 50% in comparison to the station without fish and were comparable to the reference station. Conversely, the sediments below the 0.4 kg per m² cage did not show substantial change in either LOI or redox potential in comparison to the unstocked control site. The macrofauna community in the sediments below both mullet cages changes from an almost mono-specific population of small nematodes to an assemblage of variable-size nematodes and polychaetes. This preliminary study indicates that it is possible to reduce organic matter and oxidize sediments by placing grey mullets on the seafloor in cages stocked at the proper biomass to deal with the flux of particular organic matter.