Circular economy in an inland aquaculture farm in Marche Region, (Italy): new sustainable by-products feeds tested in brown trout (Salmo trutta fario)

ABSTRACT: In recent years, the rapid expansion of aquaculture has heightened the need for sustainable farming practices, particularly in the development of feeds that do not compete with human food resources. This study evaluated the use of agricultural by-products—specifically legumes and grape residues—as alternative high-value feed ingredients for brown trout (Salmo trutta fario) in inland aquaculture. The trial was conducted on 900 specimens at a local aquaculture facility in Visso (Macerata, Marche Region, Italy), as a part of the European project "AgrI-fiSh”, which also seeks to support the socio-economic recovery of mountain territories affected by the year 2016 earthquake. Preliminary results are encouraging, indicating the potential of these by-products as sustainable feed alternatives. However, further research is required to optimize diet formulations and assess their effectiveness in more productive fish species. The study supports the potential of circular feed practices in promoting environmental sustainability, economic resilience, and local resource valorisation. Keywords: brown trout, circular economy, grape by-products, food wastes, inland aquaculture 1. INTRODUCTION In recent years, the principles of the circular economy have gained prominence in efforts to improve sustainability in both human food production and animal feed systems. A central challenge is minimizing competition between feed and food resources, while simultaneously reducing waste. According to the European Union (EU) Commission Council Directive 2008/98/EC (Directive 2008/98/EC), “waste” is defined as “any substance or object which the holder discards or intends or is required to discard”. The use of processing by-products in aquaculture feed may reduce pressure on fish stocks as well as diminish the waste and negative environmental impact. Aquaculture is among the fastest-growing food production sectors worldwide, playing a key role in global food security by providing, particularly as wild fishery resources are overexploited or nearing their maximum sustainable yield. In aquaculture feed, the use of high-quality food-competing components, like fishmeal, fish oil and soybean meal, should be reduced in order to prevent the worsening shortage of raw materials used to produce human food (van Riel et al., 2023). In fact, food production and a variety of livestock and aquaculture feeds compete for resources, which would be lessened by using more food system by-products and wastes as feed ingredients (Sandström et al., 2022). Moreover, promoting fisheries activities aimed at capturing wild fish to be used as feed for farmed fish species is no longer sustainable (FAO, 2024), as the research of cheaper and less impactful alternative diets can contribute to support aquaculture farms of all the economic dimension. Small-scale trout farms in central Italy, particularly those rearing Salmo trutta fario, face unique challenges. Regulatory changes banning the release of non-native fish into natural waterways have halted public restocking programs, historically a key income stream. Furthermore, the 2016 earthquake in the central Apennines severely impacted the region’s rural economy, and the market for trout destined for recreational fishing has significantly contracted. As a result, many farms are downsizing, diversifying, or closing altogether, threatening the socio-economic fabric of inland mountain communities (Aleffi & Cavicchi, 2020; Bedini & Bronzini, 2018; Dottori et al., 2024). Based on this scenario, a preliminary study was performed to assay sustainable and innovative aquaculture feeds for salmonids. The aim was to investigate the possibility to employ agricultural wastes, as alternative sources of proteins and oils, to replace conventional feedstuffs, such as fish meal and fish oil (Borsetta et al., 2024). This approach aims to promote sustainable aquaculture practices and support the revitalization of earthquake-affected rural areas in the Marche region. 2. MATERIALS AND METHODS For this trail, agricultural by-products were used as alternative protein and lipid sources in aquafeeds. These included chickpea, fabe bean, lentil, grape seed oil and grape pomace. The legume and cereal discards were supplied by a local pasta manufacture company, while grape by-products were sourced from a nearby vineyard. Feed pellet (2-3 mm) were produced at a local mill – Palmieri, Matelica, Italy. Two experimental diets (A and B) were formulated to be isoproteic (40%) and isolipidic (21%) and developed using these by-products. A commercially standard diet (Diet C), taken from the market, primarily composed of soybean meal, served as control. The experimental diet formulations are under patenting license by the University of Camerino. The experimental trial was carried out and lasted from December 2023 to April 2024 (132 days) at a trout farm located in Visso (Macerata, Italy), within the Sibillini National Mountain Park. A total of 900 brown trout (Salmo trutta fario), with an initial average weight of 53±15.5 g and length of 15.8±1.6 cm, were randomly distributed across 9 concrete tanks (212 x 45 x 19 cm), with 3 replicates per diet and a stocking density of 29 kg/m3. Water temperature and dissolved oxygen were monitored using both a fixed multiparameter probe (EXO2, YSI, Yellow Springs, USA) and a portable probe (YSI - ProSolo ODO T, RT ENVIRONMENT srl – Veggiano, PD, Italy). Dissolved nutrients, such as nitrite (NO2), nitrate (NO3) and ammonium (NH4) were analyzed chemically applying a modification of procedures developed by Greenberg et al. (1992). At the end of the trial, water quality and growth performances were evaluated. The Condition Factor (K) was calculated as K=fish weight/(fish length3)*100. Data were submitted to statistics analysis, by means of GraphPad 10.4.0 (621), comparing the means among the groups using the one-way ANOVA test, followed by Tukey’s multiple comparison test (p value <0.05, significant). 3. RESULTS AND DISCUSSION In this preliminary trial, up to 70% of the conventional aquaculture feedstuffs were replaced in the two experimental diets with new agricultural by-products, well known for their beneficial antioxidant properties and high content of polyunsaturated fatty acids (Quagliardi et al., 2024). The residual skin, seeds and stalks represent the 25% of total grape weight discarded during the wine making process and they still have high nutraceutical potential. Grape pomace provides dietary fiber and phenolic compounds, while grape seeds are rich in unsaturated fatty acids (n-6) (Beres et al., 2017). 3.1 Water quality parameters Water parameters such as nutrients, temperature and oxygen of the tanks were recorded throughout the entire duration of the experiment. Table 1 reports the mean values and standard deviations of the main water quality parameters monitored during the experimental trial for each experimental diet and for control diet. No statistically significant differences were found among the groups, highlighting that the good quality of the water was maintained even in the tanks where the experimental diets where administered. Table 1 Water parameters of the tank replicates differentiated by diet (mean ± s.d.). Water parameter Diet A Diet B Diet C Temperature (°C) 9.53 ±0.28 9.53±0.29 9.52±0.29 Oxygen saturation (%) 80.75±1.75 81.3±2.32 80.78±1.83 Ammonium (mg/l) 0.02459±0.0074 0.02644±0.0091 0.02923±0.0089 Nitrites (mg/l) 0.00163±0.0008 0.00208±0.001 0.00198±0.001 Nitrates (mg/l) 0.17212±0.0725 0.1708±0.0767 0.15798±0.064 Temperature and oxygen measured in all the experimental tanks with the portable probe were compared with the measurements done with fixed probe in order to identify any variations not recorded manually. However, no significant differences were found among them (Fig.1). The differences between the manual and continuous sensors are likely due to several factors, such as the manual sensor not being fully stabilized in terms of temperature and oxygen (O₂), but these differences are negligible. 3.2 Growth performances All groups showed a significant increase in total mean length (final mean = 17±1.8 cm): However, the final mean body weight was lower in fish fed the experimental diets (Diet A e B) compared to the Control group (Diet C). Statistically significant differences in weight were noted among all three groups (p < 0.05), with the two experimental diets also significantly differing from each other. The condition factor (K), an indicator of fish health and body robustness, followed the same trend, reflecting the biometric data and indicating slightly reduced growth efficiency in the experimental groups compared to the commercial feed. Although the experimental feeds resulted in slightly reduced growth performance compared to the commercial control, fish health remained unaffected, and overall growth was satisfactory. 4. CONCLUSIONS This preliminary trial demonstrated that brown trout showed lower growth performance compared to more genetically selected aquaculture species like rainbow trout (Oncorhynchus mykiss). However, it successfully established the feasibility of using agro-industrial by-products—such as grape pomace and legume processing waste— that follow strict organic certification protocols, adding traceability and value to the overall production process, as sustainable, cost-effective alternatives to conventional feed ingredients. These materials support the valorisation of local organic agricultural chains, in line with the EU target goal of 25% of organic productions, adding traceability to local agrifood value chains. The trial confirmed the technological viability of these alternative feeds, with positive results for pellet stability, integrity, and palatability. Despite limitations in growth, the feeds were found to be nutritionally adequate and environmentally responsible. This study provides a foundation for further research, including ongoing trials on rainbow trout under the same conditions and upcoming replications using a third diet formulation. The work is particularly relevant for small-scale farms in the Marche region, especially post-earthquake areas like Visso, offering a path to reduce environmental impact and stimulate local economies and enhance market competitiveness In conclusion, the study is a promising step toward sustainable aquafeeds and circular aquaculture systems, with further research needed to optimize formulations and broaden applicability. ACKNOWLEDGEMENTS This project is part of the PRIMA Programme supported by the European Union. 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