Introduction
Due to the high cost of fishmeal and concerns about its limited availability and sustainability as the global aquaculture sector develops, research into suitable substitutes has been a longstanding priority. One of the most commonly used plant-based alternatives is soybean meal, which is generally regarded as the most nutritious protein feed for aquatic animals. Soybean meal has a relatively high protein content and a complementary blend of amino acids, making it suitable for inclusion in commercial fish feeds at levels of approximately 10-12%. The nutritional value of soybean meal (SBM) is often limited for cultured species due to deficiencies in methionine and the presence of antinutritional factors (ANFs).Various methods have been explored to reduce the adverse effects caused by these ANFs and specific oligosaccharides, with fermentation being a prominent approach of interest (Kumar et al., 2020). This study aimed to evaluate the potential of fermented soybean meal (FSBM) as an alternative protein source to fishmeal for the diets of European seabass (Dicentrarchus labrax) and to examine its effects on feed efficiency, growth performance, and immune response in a field trial under intensive aquaculture conditions.
Materials and Methods
A 12-week trial was conducted to examine the effects of varying levels of fermented soybean meal (FSBM) (PERMEX VET Kft., Hungary) as a replacement for fishmeal (FM). Six diets were formulated to be isonitrogenous (46% crude protein) and isolipidic (16% crude lipid). These included a control diet containing 30% FM and three experimental diets that replaced FM with 30%, 50%, and 70% FSBM. All diets were produced as 3.5 mm pellets through a cooking extrusion process by SPAROS Lda (Olhão, Portugal). At the end of the feeding trial, the fish were collectively anesthetized, and 16 fish per cage were randomly sampled. Ten of these fish were used to assess proximate composition and fillet quality. From three additional fish, tissues from the anterior and mid intestines, along with the liver, were collected and immediately fixed in 10% buffered formalin for histological analysis. Finally, blood samples were collected from the caudal vein of three other fish per cage, and the serum was used for immunological assessments. All animal handling and sampling procedures adhered to the EU directive 2010/63.
Results and Discussion
Overall, there were no statistically significant differences in growth performance and feed utilization indices (P > 0.05) among the experimental feeds. However, it is notable that the fish fed the 50% fishmeal (FSBM) diet exhibited the highest numerical growth parameters, including fish weight gain, specific growth rate (SGR), daily growth index (DGI), protein efficiency ratio (PER), and daily feed intake. In terms of feed conversion ratio (FCR), the lowest values were observed in cages fed diets containing 30% and 50% FSBM (1.32 ± 0.06 and 1.33 ± 0.03, respectively). Similar to a previous lab trial (Roussos et al., 2023), the diet with 70% fishmeal substitution performed worse than the control, indicating limitations associated with higher inclusion rates of FSBM in the diets of European seabass. Furthermore, no significant differences were observed in the fish somatometric indices. However, the fish fed a diet containing 30% fermented soybean meal (FSBM) exhibited the lowest perivisceral fat index (2.87 ± 0.64) compared to the control group (3.42 ± 0.59). The overall body composition and fillet composition of the fish were not significantly affected by the substitution of FM with FSBM at any level of its inclusion. Nevertheless, a decrease in crude protein level (18.24 ± 1.57, 18.93 ± 1.32, and 18.29 ± 1.02) was noted in the fillets of fish fed diets containing 30%, 50%, and 70% FSBM, respectively, compared to the control group (19.42 ± 1.63). Additionally, the inclusion of FSBM did not impact the retention of n-3 long-chain polyunsaturated fatty acids (LC-PUFAs), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in the fish muscle, as no significant differences were found. Moreover, no significant differences were found in serum total proteins, myeloperoxidase, and anti-proteases (% trypsin inhibition) among the different dietary treatments. However, protease activity was significantly lower in the diet containing 70% FSBM compared to both the control and the diet with 50% inclusion of FSBM. Also, a non-significant decreasing trend was observed in the serum lysozyme levels of fish that were fed diets containing increasing levels of FSBM. Lysozyme activity has been linked to bile acids (BA) concentrations, which play a role in regulating immune functions (Zhao et al., 2025). Additionally, BA have been associated with the presence of dietary taurine in feed. However, the substitution of fishmeal (FM) with plant proteins can lead to a decrease in taurine levels. It will be very interesting to further investigate the supplementation of fishmeal substitute-based feeds with taurine. Previous studies conducted by Kotzamanis et al. (2020a, 2020b) demonstrated that European seabass showed improved growth and fillet quality when fed a diet that included a mixture of soybean products and other plant meals, supplemented with 1% or 2% taurine as a replacement for fishmeal. These findings suggest that taurine is an essential nutrient that is often lacking in diets with high levels of soybean meal and other plant meal inclusions.
The results clearly demonstrated the high nutritional value of Fermented Soybean Meal (FSBM) for European seabass. In line with our previous trial, FSBM enabled the successful replacement of up to 50% of fishmeal without negatively affecting growth performance and feed utilization. However, it is important to emphasize that further research is needed to fully understand the effects of high inclusion rates of FSBM on the immune status of cultured fish.
References
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