A theoretical approach concerning nutrient emissions to inform monitoring and management of mariculture farms

As demand for production requires stakeholders to consider national marine space to keep up with product demand, it is crucial that the question of environmental impact is prioritized. A multitude of biotic and abiotic challenges face the expansion of the mariculture industry in the ultraoligotrophic waters of the Levant Basin of the eastern Mediterranean Sea (EMS). Several local monitoring efforts have addressed sediment enrichment and water quality, however, estimating the loading of bioavailable nitrogen as a result of farming has not been attempted. In this study, baseline dissolved nutrients and sediment samples were collected in situ at a monoculture farm 3 km offshore (300 t caged biomass of Sparus aurata), and from a control site. Analysis of all samples indicated no significant impact of the fish farms when compared to the control. The ratio of in situ nitrogen (N) to phosphorus (P) was similar to the N:P ratio from a published mesocosm. Thus, the nitrogen excretion rates of gilthead sea bream (Sparus aurata) from this study were utilised to produce a theoretical calculation and waste budget. The calculation was configured to the input of artificial fish feed pellets fed to the mariculture system per day for 700 t caged fish (the governmental limit of production). The input of artificial food pellets will yield 197,252 kg of bioavailable N per year into the water column (540 kg/day). Given this local point source of nutrients, the in situ results and calculation suggest that farming at 700 t should not have significant measurable effects on the marine system and are rapidly assimilated by primary producers. Mixed methods studies combining field collection and published case studies may further strengthen science-based management of the marine space and provide justification in developing long-term monitoring protocols.