Introduction
The increasing global demand for fish and seafood has led to significant growth in the aquafeed industry. However, this expansion has raised sustainability concerns due to the limited availability of marine-derived ingredients, such as fishmeal (FM) and fish oil (FO), which are essential in conventional aquafeeds. As the global production of these ingredients nears its maximum capacity, it is crucial to find sustainable alternatives to ensure the continued growth of the aquaculture sector. Over the past few decades, terrestrial plant-based ingredients such as soy, rapeseed, and maize have been used to partially replace FM and FO. While these ingredients show promise, they have limitations, such as an unbalanced amino acid and fatty acid profile, which can impact fish growth and health (Cardinaletti et. al., 2022). In recent years, protein alternatives from terrestrial animal by-products, such as poultry by-product meal (PBM), have gained attention as viable substitutes for FM (Psofakis et. al., 2020). PBM contains a balanced amino acid profile but may require supplementation with other protein sources like feather meal hydrolysate (FMH) to address deficiencies in lysine and methionine. In addition, alternative oils such as algae oil have been proposed as substitutes for FO, providing essential omega-3 fatty acids. This study aims to investigate the effects of a diet completely replacing FM with PBM and FMH, alongside the complete substitution of FO with algae oil and rapeseed oil, on the growth performance, gut health, and liver morphology of rainbow trout (Oncorhynchus mykiss ). The results from the experiment were compared with the nutrient-based model FEEDNETICS™ (FEEDNETICS Software, SPAROS ). The model was used to do a follow-up of the trial and assess whether the observed fish performance aligned with theoretical expectations.
Materials and methods
Two experimental diets were formulated to evaluate the effects of poultry by-product meal (PBM) and fish oil (FO) replacement in rainbow trout (Oncorhynchus mykiss ) diets. The control diet (FM) contained 15% fishmeal, 5% poultry meal (PM), and FO, while the PBM diet fully replaced FM with 15% PM and 5% feather meal hydrolysate (FMH). FO was substituted with 5.35% VeraMaris® algae oil and rapeseed oil. The diets were extruded at SPAROS facilities, Portugal. The 90-day feeding trial was conducted at “Fattoria del Pesce” in Cerano, Italy. Juvenile rainbow trout (n = 2,908; initial weight = 27.5 ± 0.3 g) were randomly distributed in four 10 m³ outdoor tanks (727 fish/tank, initial density : 20 kg/m³). Following a one-week acclimation, fish were fed once daily at 1.5–2.0% biomass. Water quality parameters were monitored regularly. Growth performance was assessed by monthly weighing, and daily mortality was recorded. The data were entered into FEEDNETICS™ to model fish growth based on dietary inputs.
At the end of the trial, 32 fish per diet were sampled for histological evaluation (liver and intestinal morphology), short-chain fatty acid (SCFA) analysis, and fillet fatty acid composition. Histological samples were stained with hematoxylin-eosin and analyzed using microscopy. SCFAs were quantified using HPLC, while fatty acid profiles were determined via gas chromatography. Statistical analyses were conducted at the individual fish level using Student’s t-test or Mann-Whitney test (p < 0.05).
Results and Discussion
Growth Performance and Feed Utilization
Rainbow trout readily accepted both diets, and the 90-day feeding trial showed no significant differences in survival rates (>97%), final body weight (FM: 170.80 ± 7.61 g, PBM: 166.91 ± 6.62 g), or feed conversion ratio (FCR: FM = 1.33 ± 0.05, PBM = 1.35 ± 0.04) between dietary groups. The specific growth rate (SGR) was similar (p > 0.05) between fish fed FM (2.03 ± 0.04 % day⁻¹) and PBM (2.00 ± 0.08 % day⁻¹). No external or internal lesions were observed in either group.
Intestinal and Liver Histomorphology
Histological examination revealed well-organized intestinal tissue in both groups, with significantly longer intestinal folds (p < 0.05) in the anterior intestine of PBM-fed fish. However, the PBM group exhibited a significant increase in submucosa thickness in the posterior intestine (p < 0.05), possibly indicating a mild inflammatory response. Liver histology showed normal parenchymal structure in both feeding groups, with PBM-fed fish exhibiting reduced hepatic lipid accumulation compared to FM-fed fish.
Fecal Short-Chain Fatty Acids (SCFAs) and Fillet Fatty Acid Composition
SCFA analysis indicated significantly higher acetate (18.73 mmol/L) and butyrate (0.91 mmol/L) levels in PBM-fed fish. Fatty acid composition analysis revealed lower saturated fatty acids (SFAs) and significantly higher polyunsaturated fatty acids (PUFAs), including EPA+DHA, in PBM-fed fish, with improved nutritional indices (atherogenic and thrombogenic indices).
Benchmarking of the trial results with FEEDNETICS ™
The FEEDNETICS™ application model, served as a reference point and supported the conclusion of this study, in terms of the performance of fish fed diets with PBM and natural algal oil.
Conclusions
Replacing 20% of fishmeal with PBM and FMH, along with fully substituting FO with VeraMaris ® algae oil, sustained growth performance, gut morphology, and liver health in rainbow trout. Although fillet PUFA levels decreased, improved nutritional indices suggest potential human health benefits. Additionally , the FEEDNETICS™ model provided valuable validation, reinforcing the observed performance of fish fed PBM- and algal oil-based diets. These findings underscore the potential of PBM and algal oil as sustainable, nutritionally balanced alternatives for environmentally friendly trout diets.
Acknowledgements
Research was supported by two projects: (1) Project “BIO = C = O”, PON-MISE I&C 2014– 2020 FESR, ASSE 6 REACT Priorita di investimento 13. i Azione I, “Finanziato nell’ambito della risposta dell’Unione alla pandemia di COVID-19” . Project codex N. F/200078/03/X45, CUP B41B20000280005; (2) I-FISH. Protocollo nr: 414352—del 07/12/2023—AOO_IAI—AOO_Incentivi Fondo per la Crescita Sostenibile-Accordi per l’innovazione di cui al D.M. 31 Dicembre 2021 e D.D. 14 Novembre 2022.
References
Cardinaletti, G., Di Marco, P., Daniso, E., Messina, M., Donadelli, V., Finoia, M. G., ... & Tibaldi, E. (2022). Growth and welfare of rainbow trout (Oncorhynchus mykiss) in response to graded levels of insect and poultry by-product meals in fishmeal-free diets. Animals, 12(13), 1698.
Psofakis , P., Karapanagiotidis , I. T., Malandrakis , E. E., Golomazou , E., Exadactylos , A., & Mente, E. (2020). Effect of fishmeal replacement by hydrolyzed feather meal on growth performance, proximate composition, digestive enzyme activity, haematological parameters and growth-related gene expression of gilthead seabream (Sparus aurata). Aquaculture, 521, 735006.