Introduction
Recirculating aquaculture systems (RAS) have been increasingly recognised for their properties allowing a tightly controlled environmental discharge and improving resource-use efficiency, positioning them as a highly competitive alternative to traditional systems (Ahmed and Turchini, 2021; Timmons et al., 2018). However, the complex water circuit in RAS generates significant turbulence and shear forces that promote and accelerate the fragmentation of organic particulate matter. This progressive breakdown of particles reduces the effectiveness of the conventional solid removal methods, consequently increasing the possibility of solids remaining in suspension. The accumulation of these suspended solids (SS) can therefore compromise water quality and animal welfare, interfere with system performance and increase the risk of operational failure (McMillan et al., 2002; Schumann and Brinker, 2020). Consequently, effective SS management remains a critical key challenge in the design and operation of RAS, requiring the continuous optimisation of the already costly treatment and filtration strategies. To address the gap of effective, energy-efficient solutions, novel strategies are needed to secure an early-stage mitigation for the accumulation of SS in RAS, while being compatible with already existing operational systems. This study was based on the "floating faeces" approach (Unger and Brinker, 2013) and investigates the effects of cork (Quercus suber L.) as a functional feed ingredient to reduce faecal density and facilitate a direct solid removal from a full-RAS operation husbanding the freshwater phase of Atlantic salmon (Salmo salar L.).
Materials and Methods
A 21-week experiment evaluated the inclusion of 3% cork granules (���: 0.7–1 mm) in a commercial RAS diet during the freshwater phase of Atlantic salmon. Two replicate semi-technical RAS were used. Pre-smolts (74.8 ± 11.4 g) were stocked at density of ≈20 kg m-3 and fed either a control diet or the same basal diet supplemented with 3% indigestible cork granules (+Cork). After 12 weeks, diets were switched between systems to control for system effects. Fish were hand-fed to apparent satiation three times daily and feed intake was recorded. Solids in the control were managed via bottom drains and continuous surface skimming, while the +Cork tanks were equipped with an overflow pipe for buoyant faeces removal. Faecal removal efficiency was quantified weekly over 24 hours using specialised equipment. Total suspended solids (TSS) were measured weekly before morning drainage from three locations: before the drum filter (BDF), after the drum filter (ADF), and in the backwash water (BWW). Faeces were sampled for digestibility and faecal density analyses. Water samples were taken weekly from three system points for total phosphorus (TP), total bound nitrogen (TNb) and routine water quality analyses.
Results
The +Cork treatment produced significant amounts of floating faecal material, which was rapidly aspirated by surface pipes and directed to the drum filter. No adverse effects on feeding behaviour were observed. Growth performance and feed utilisation were consistent with commercial standards in both groups, and cork inclusion did not adversely affect growth. Feed intake (TFI) tended to be higher in Control, while the +Cork treatment significantly enhanced SGR, TGC, and PER (p<0.05). In line with this improved performance, FCR was nominally lower in the +Cork group, although this difference did not reach statistical significance. Reflecting the production of floating casts, faecal density was significantly lower than that of water in +Cork (p<0.05). The +Cork treatment markedly improved solids and nutrient removal: overflow removed 60.27% vs. 4.27% in Control group; drum filter removal efficiency reached 86.5% in +Cork vs. 42.4% in Control (p<0.001). Pre-mechanical treatment TSS was more than 3-fold higher in +Cork (p<0.05), but post-filtration TSS was significantly lower (p=0.034). TP and TNb were significantly higher before filtration in +Cork (p<0.05) yet lower or similar after filtration, with greater particulate nutrient removal and reduced leaching potential (p<0.05).
Discussion
This study demonstrates, for the first time in Atlantic salmon, that incorporating 3% cork granules into a commercial RAS diet reduces faecal density below that of water (<1.00 g cm-3). This physical shift in faeces enables rapid surface removal via overflow pipes without requiring additional energy costs, and without compromising growth, feed utilisation, or welfare—despite a slight reduction in feed digestibility induced by the cork ingredient. By facilitating immediate surface to drum filter transport, this strategy minimised faecal exposure to tank conditions, i.e., flow rates, turbulence and shear forces, thereby minimising disintegration, fragmentation and nutrient leaching. Consequently, solid and nutrient management is remarkably improved; rather than accumulating within the system, TSS and critical nutrients are partitioned into a concentrated waste stream. This is reflected in significantly enhanced water quality (reduced TSS, TAN, and CO2). This approach can be easily applied to existing RAS and other aquaculture configurations (e.g. net pens). Overall, incorporating cork into commercial salmon diets represents a practical and sustainable strategy for superior waste capture and nutrient recovery, supporting cleaner production and the principles of circular economy in aquaculture.
Acknowledgement
This work was funded by the European Union��s Horizon 2020 research and innovation programme under the Marie Sk��odowska-Curie grant agreement No. 956481.
References
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