Nutrient recovery and sludge management in seabream and grey mullet co-culture in Integrated Multi-Trophic Aquaculture (IMTA)

Co-culture of the detritivorous flathead grey mullet (Mugil cephalus) and gilthead seabream (Sparus aurata) in land-based Integrated Multi-Trophic Aquaculture (IMTA) system was evaluated with respect to yield, Specific Growth Rate (SGR), Food Conversion Ratio (FCR), survival, nitrogen assimilation rate and efficiency. Water from the Gulf of Aqaba was pumped into three cylindrical m³ tanks. Each tank was stocked with seventy 11.37 ± 3.7 g mullets. In each tank three floating rectangular cages were placed, each stocked with 100, 42.9 ± 7.5 g seabream. Seabream were fed a pelleted diet (46% protein) throughout the day. Sludge of the seabream sank to the bottom where the mullets fed on them. Final yields were 170.7 kg of seabream (100% survival) and 9.68 kg of grey mullet (82% survival), i.e., a biomass yield ratio of 1 kg mullet vs.17 kg seabream. Co-culture of seabream and mullet increased the overall biomass production by 4–8%, increased nitrogen assimilation by the fish by 6–8%, reduced FCR by 10–15%, reduced amount of the sludge by 98%, and increased dissolved nitrogen emissions by 7%. Since the income from the mullet yield is relatively marginal, mullet co-culture in an IMTA system should be evaluated considering the specific site, the local market price of the mullet, the local water treatment costs and relevant environmental factors. Statement of relevant While several studies have considered the mullet as a potential candidate for co-culture with feed-fed fish underneath sea cages very little quantitative data on the ability of mullet to remove sludge has been provided. The present study evaluated grey mullet (Mugil cephalus) performance as a detritivore and potential by-product in co-culture with seabream (Sparus aurata) in a land-based IMTA. This study gives, for the first time quantitative data, including nitrogen budget of the seabream/mullet co-culture and evaluation of the criteria for using mullet as a biofilter in land based facilities.