Addition of dissolved nitrogen and dissolved organic carbon from wild fish faeces and food around Mediterranean fish farms: Implications for waste-dispersal models

Abstract

Leaching of ammonium (NH4+) and dissolved organic carbon (DOC) from food pellets used at three fish farms in the Mediterranean Sea and the faeces of four different species of farm-associated wild fish (Trachurus mediterraneus, Mugil cephalus, Trachinotus ovatus and Boops boops) were determined. They were placed in seawater and agitated slowly (5 cm s− 1) to reflect natural conditions during their fall to the sediment. Two temperatures were tested, 25 °C and 15 °C, to assess the influence of seasons on leaching rates. Leaching from fish faeces was generally higher compared to food pellets. T. mediterraneus faeces leached more NH4+ and DOC than M. cephalus, T. ovatus and B. boops. The results showed that there is an important addition of NH4+ and DOC to the water column during sinking of the faeces and that this is species-dependent. Water turbulence and faeces composition seemed to have a higher influence than temperature on the leaching process. Due to the high abundance and biomass of farm-associated fish in the Mediterranean and their capacity to remove waste, they appear to be an important component for models that predict the impact of aquaculture. Large biomasses of wild fish at fish farms may reduce the impact on benthic systems but increase the nitrogen and carbon loads into the water column, affecting the pelagic system and modifying the spatial dispersion of wastes.