Sludge from sea-based Atlantic salmon (Salmo salar L.) production: quantification, composition, and potential application in integrated multi-trophic aquaculture

Future growth of Atlantic salmon (Salmo salar L.) in Norway is tied to finding solutions for major ecological challenges connected to salmon lice, escapees, and nutrient emissions from sea cages. At the same time, nutrient-rich sludge from salmon production comprises a valuable resource for the cultivation of lower trophic species using an integrated multi-trophic aquaculture (IMTA) approach. This study aimed to quantify the sedimentation of aquaculture sludge under sea cages of an Atlantic salmon aquaculture site and to qualify the composition of this sludge. Additionally, the study evaluated the hypothetical use of sludge from sea-based aquaculture as a feed source for polychaetes Hediste diversicolor. Using sediment traps, sludge samples were collected under two different Atlantic salmon sea cages, at two different depths, and three different sampling dates. Subsequently, they were quantified, and their composition was assessed with regards to carbon (C), nitrogen (N), phosphorus (P), lipid, fatty acid (FA), protein, amino acid (AA), and ash content as well as elemental ratios and composition of FAs and AAs. The quantity of collected sludge was significantly different between sea cages, with a strong positive correlation between feed input and collected sludge (R (2) = 0.98, p < 0.05). Sampling depth did not affect the quantity of collected sludge in the sediment traps (2215 +/- 480 mg DW day(-1)), and no significant difference in sedimented sludge as a proportion of theoretically produced sludge (12.94 +/- 2.16%) was found when comparing the different cages and sampling depths. Furthermore, the composition of collected sludge was similar at all sampling points. The overall nutritional value was lower compared to sludge from land-based aquaculture; regardless, sludge from sea-based salmon production can in theory be considered a potential feed resource to be used for the production of polychaetes H. diversicolor.