Assessment of the removal efficiency of fish farm effluents by grey mullets: a nutritional approach

Abstract

A ¢eld study was carried out to assess the use of the bottom-feeding grey mullet (Mugil cephalus) within benthic enclosures as a means to reduce the benthic impacts of a net cage ¢sh farm in the Gulf of Aqaba, Red Sea. Five experimental 1-m 3 net-cage enclosures were stocked with 12 grey mullets each (¢sh weights 50, 70 or 144 g). The enclosures had no bottom and the mullets had access to the enclosed organically enriched sediments and to particulate matter (PM) falling from the overlying cages. Sediment traps were used to quantify and qualify the PM falling from the ¢sh farm to the sea £oor. Simultaneously, a feeding trial was performed with mullets (50 and 70 g) in experimental tanks. In order to estimate the potential growth rate and to quantify energy and protein requirements, the ¢sh were fed a formulated diet with known composition. After approximately 70 days at sea, mullets in the enclosures had used up all the available food in the sediment and gained up to 0.78 g day � 1 ¢sh � 1 . Applying the values for energy and protein requirements for maintenance and growth derived from the experimental trial, estimates indicated that the grey mullets eiectively removed 4.2 g organic carbon, 0.70 g nitrogen and 7.5 mg phosphorus kg � 1 mullet m � 2 day � 1 from the organically enriched sediment. Thus deployment of grey mullets may be an e⁄cient means to improve the quality of sediments below intensive net-cage ¢sh farms.