How much waste can the amphipod Gammarus insensibilis remove from aquaculture effluents? A first step toward IMTA

In recent years marine amphipods have been highlighted as an alternative live feed resource for Integrated Multi-Trophic Aquaculture (IMTA). Gammarus insensibilis Stock, 1966 is a native amphipod from the Mediterranean Sea and the north Atlantic Ocean and it is highly abundant in southern Spain marsh ponds. Its high potential for being intensively reared has been demonstrated in previous studies, as this species can feed on detritus and presents an interesting nutritional profile. In the present work, G. insensibilis specimens were maintained in closed batches at three different densities (100, 500 and 1000 ind L−1) and fed on dry or wet detritus (uneaten food and faeces from a fish RAS effluent) in order to assess for the first time the amount of wastes that they were able to remove. In parallel, a trial was performed to study the effect of dissolved inorganic nitrogen compounds (highly abundant in aquaculture effluents) on their survival, and the median lethal concentrations (LC50) in 96 h were calculated. Amphipods in all experiments and conditions showed promising survival rates higher than 80%. No advantages were observed when gammarids were maintained at high densities due to the significantly higher ammonium concentrations in these treatments, having an effect on intake but allowing sub-lethal conditions. Indeed, Gammarus insensibilis showed the highest tolerance for nitrate (1308 mg N-NO3− L−1), followed by nitrite (39.77 mg N-NO2− L−1) and ammonium (33.23 mg N-NH4+ L−1). Individuals at low densities removed significantly higher amounts of detritus, between 154.98 and 169.78 mg (dry weight detritus) per g (wet weight amphipods) and day. No differences were observed between removal rate of dry or wet detritus. Thus, authors recommend the use of wet detritus for a better handling and up-scaled trials in open or RAS systems equipped with a biofilter in order to avoid high ammonium concentrations impacting on intake.