Comparative life cycle assessment of rainbow trout (Oncorhynchus mykiss) farming at two stocking densities in a low-tech aquaponic system

The present study assessed the effect of two fish stocking densities (Low, 3.81 kg/m3 vs High, 7.26 kg/m3) on the environmental footprint associated with the production of rainbow trout (Oncorhynchus mykiss) and lettuce (Lactuca sativa) in an experimental low-tech aquaponic system. A gate-to-gate and a cradle-to-gate Life Cycle Assessment models were used. The functional unit was 1 kg increase of table-size rainbow trout (about 330 g body weight). Mass allocation, economic allocation, and system expansion were applied to resolve the multifunctionality of the tested system. The impact categories assessed were global warming (GWP, kg CO2-eq), acidification (AP, g SO2-eq) and eutrophication (EP, g PO4-eq) potentials, cumulative energy demand (CED, MJ), freshwater ecotoxicity (ECO, CTUe), water depletion (WD, m3 water equivalent). In the gate-to-gate model, considering mass allocation, the production of 1 kg increase of rainbow trout emitted on average 8.8 kg CO2-eq (GWP), 56 g of SO2-eq (AP) and 64 g of PO4-eq (EP), while the CED was 161 MJ, the ECO was 186 CTUe, WD was 0.061 m3. Global warming, cumulative energy demand and freshwater ecotoxicity were the impact categories mostly affected by the changes in fish stocking density. A high density was associated with a lower environmental impact per kg of fish produced both considering the gate-to-gate and cradle-to-gate approaches. Electricity was the dominant contributor in all the impact categories, ranging from 64% of EP to 93% of ECO while feed production accounted for 19% of WD and 10% of GWP. The change of the energy source from a common grid mix to a photovoltaic system substantially reduced global warming whereas the improvement of feed conversion ratio decreased eutrophication potential. Based on life cycle assessment, the farming of rainbow trout in aquaponics is a promising alternative to common flow-through systems, particularly in view of reducing water use.