Introduction
The rainbow trout (Oncorhynchus mykiss) is one of the most widely farmed freshwater fish species globally, known for its rapid growth, high feed efficiency, and valuable nutritional profile. Its adaptability to various farming conditions makes it an ideal species for evaluating alternative and sustainable feed ingredients. One such ingredient is barley (Hordeum vulgare L.), which is widely used as a raw material in the starch industry and generates protein-rich by-products that can be valorised in both the food and feed sectors (Yalcin et al., 2007). In feed production, barley is considered an excellent source of nutrition and energy. Additionally, it plays an important agronomic role by contributing to nitrogen accumulation in the soil, improving soil fertility, and reducing the need for nitrogen-based fertilizers. Moreover, barley’s ability to stimulate soil microbial activity makes it a valuable precursor crop in sustainable farming systems (Lammas and Shitikova, 2024).
Beyond its agronomic value, barley also offers substantial environmental benefits, particularly in terms of reducing the carbon footprint of aquafeeds. Preliminary evaluations conducted within the framework of the present study indicate that replacing fishmeal with increasing levels of Barley Protein Concentrate (BPC) can lower greenhouse gas emissions associated with feed production: from 4887.3 kg CO₂ eq./ton in the control diet to 4133.5 kg CO₂ eq./ton in the diet containing 30% BPC, representing a 15.4% reduction. These findings reinforce the potential of BPC as a climate-friendly alternative ingredient.
In this study, we specifically assessed the effects of increasing dietary inclusions of BPC on growth performance, digestive enzyme activities, intestinal and hepatic histology, and fillet quality traits in rainbow trout.
Materials and Methods
Three experimental diets containing increasing levels (10%, 20%, and 30%) of BPC and a corresponding progressive reduction in fish meal content (5%, 2.5% and 0%) were tested. A fourth diet, without the inclusion of BPC, was used as a control. Additionally, astaxanthin was included in all experimental diets to allow flesh pigmentation (salmon coloration) in the trout. Diets were tested on triplicate fish groups of 50 individuals (initial weight: 123 g) over a period of 90 days. During the trial, feed was provided to apparent satiation, temperature was kept at 15.0 ± 1.0 °C and oxygen above 10.5 mg L-1. At the end of the trial, final body weight (FBW), specific growth rate (SGR), feed intake (FI) and feed conversion rate (FCR) were calculated. Total alkaline protease, trypsin, chymotrypsin, pepsin, α-amylase, and bile salt-activated lipase were tested to evaluate the enzymatic activity. Histological samples of liver and intestine were taken to evaluate the tissue response. Samples of the fillet were taken to perform instrumental analyses and a sensory panel test to investigate potential organoleptic differences between the diets.
Differences among treatments is considered significant at p < 0.05.
Results and Discussion
Fish fed diets with up to 20% BPC showed growth performance, feed intake, and feed conversion ratio comparable to the control, consistent with previous findings in salmonids fed with BPC (Bell 2016). At the highest inclusion (30%), slight reductions in growth and nutrient efficiency indices occurred, due to reduced protein digestibility and fiber content.
Digestive enzyme activities were moderately influenced by dietary BPC. Literature on trout suggests proteases and lipases activity can decrease slightly at lower inclusion levels of fishmeal and higher inclusion of BPC, due to anti-nutritional factors (Zaretabar 2021). Nonetheless, enzyme activities remained within physiological ranges, suggesting preserved digestive function.
Histological examination of intestinal tissue revealed well-preserved mucosal structure in fish fed up with 20% BPC, with normal villi height and enterocyte integrity. At 30%, minor enteritis or mucosal thinning appeared, as noted in similar studies with high inclusion of plant proteins (Romarheim 2008). Hepatic histology remained within normal parameters, though slight hepatocyte vacuolization or lipid accumulation was observed at the highest BPC level, possibly indicating early metabolic adaptation. Texture and color parameters showed minor variation across diets. Sensory evaluation confirmed acceptable texture and flavor across all groups, though minor differences in some organoleptic properties of flesh were observed, particularly at higher BPC inclusion levels, as noted by de Francesco (2008) in a study with inclusion of a plant protein mixture.
Conclusion
Overall, the trial confirmed that moderate levels (10–20%) of BPC can replace fishmeal without adverse effects on growth, health, or fillet quality in rainbow trout, supporting its application in sustainable aquafeeds. Given the global significance of rainbow trout in freshwater aquaculture, it is essential to investigate feed ingredients that can contribute to reducing the carbon footprint of its farming.
Acknowledgements
This work is part of the research activities carried out within the framework of the ASTRO supply chain agreement, funded under the initiative “Contratti di Filiera nel settore della pesca e dell’acquacoltura” promoted by the Italian Ministry of Agriculture, Food Sovereignty and Forests (Ministero dell’Agricoltura, della Sovranità Alimentare e delle Foreste – MASAF).
References
Bell, J. Gordon, et al. "Evaluation of barley protein concentrate and fish protein concentrate, made from trimmings, as sustainable ingredients in A tlantic salmon (Salmo salar L.) feeds." Aquaculture Nutrition 22.2 (2016): 326-334.
Lammas, M. E., and A. V. Shitikova. "Spring barley is a promising crop in agricultural production." BIO Web of Conferences. Vol. 139. EDP Sciences, 2024.
Romarheim, Odd Helge, et al. "Lipid digestibility, bile drainage and development of morphological intestinal changes in rainbow trout (Oncorhynchus mykiss) fed diets containing defatted soybean meal." Aquaculture 274.2-4 (2008): 329-338.
Yalçın, Erkan, Süeda Çelik, and Esra İbanoğlu. "Foaming properties of barley protein isolates and hydrolysates." European Food Research and Technology 226.5 (2008): 967-974.
Zaretabar, Amine, et al. "One step toward aquaculture sustainability of a carnivorous species: Fish meal replacement with barley protein concentrate plus wheat gluten meal in Caspian brown trout (Salmo trutta caspius)." Aquaculture Reports 20 (2021): 100714.
de Francesco, Matilde, et al. "Effect of long-term feeding with a plant protein mixture based diet on growth and body/fillet quality traits of large rainbow trout (Oncorhynchus mykiss)." Aquaculture 236.1-4 (2004): 413-429.