Introduction
Aquaculture's reliance on fishmeal (FM) as a primary protein source poses significant sustainability and economic challenges due to limited availability, fluctuating prices and pressure on wild pelagic stocks (FAO, 2020). Therefore, it is necessary to identify high-quality alternatives source of proteins that maintain growth and nutritional value, especially for carnivorous species like European seabass (Dicentrarchus labrax). Cyanobacteria and insects, such as spirulina (SPI, Arthrospira platensis) and black soldier fly (BSF, Hermetia illucens) have emerged as promising candidates due to their interesting amino acid profiles and rapid production cycles. Nevertheless, the synergistic effects of combining these two distinct alternative proteins in diets remain insufficiently explored. The goal of this study was to replace completely FM with different combinations of Spirulina and BSF to evaluate their impact on the growth performance, health and product quality of European seabass, providing insights into the feasibility of integrated sustainable feed formulations.
Materials and Method
The study comprised two consecutive experiments conducted in recirculating aquaculture systems. In the first experiment, juvenile European seabass (17.33 ± 0.05 g) were fed, in triplicate, seven isoproteic and isolipidic diets for 56 days to evaluate fishmeal (FM) replacement. Diets included a control (CTRL, 15% FM), and four additional diets (MIX1–4) in which FM was fully replaced by BSF and SPI blends at HI:SPI ratios of 80:20, 60:40, 40:60, and 20:80, respectively. Growth performance and somatic indices were recorded. Subsequently, a bacterial challenge was performed by intraperitoneally injecting Vibrio anguillarum into fish from the four best-performing groups (CTRL, HI100, MIX1, and MIX2) to assess disease resistance. In the second experiment, these four selected diets were tested over 127 days on larger fish (102.05 ± 0.41g) until the control group tripled in body weight. In addition to growth performance, product quality of fillets was evaluated by measuring muscle pH, colorimetric parameters (L*, a*, b*), and cumulative water loss (drip, thawing, and cooking loss) to determine the impact of alternative proteins on the final harvestable product.
Results
In the first experiment, the complete replacement of fishmeal with a moderate inclusion of spirulina MIX1 (80/20) and MIX2 (60/40) did not significantly affect growth, feed conversion ratio (FCR) and biosomatic indexes. Following the Vibrio anguillarum challenge, a significant linear trend was observed and survival improved in the fish fed the MIX2 diet. Formulations with spirulina content exceeding 60% significantly reduced feed intake, weight gain, and specific growth rate (SGR). Moreover, fish fed SPI100 exhibited a marked increase in FCR and a drastic reduction in hepatosomatic index values compared to the other diet groups.
In the second experiment, results indicated that total or partial replacement of FM led to a significant reduction in growth performance. The CTRL group achieved the highest final body weight and SGR, followed by HI100 and MIX1, while MIX2 showed the lowest growth. FCR and survival was similar across all treatments. Additionally, the fish fed the alternative proteins exhibited a lower condition factors and viscerosomatic index compared to the CTRL group. Regarding product quality, dietary treatments did not significantly affect either muscle pH or the water retention properties of fillets. On the other hand, significant differences were observed in colour parameters; fish from the MIX2 group exhibited significantly higher redness (a*) in the tail section compared to MIX1, while MIX1 showed significantly lower yellowness (b*) in the head and mid-sections compared to CTRL."
Discussion
The results suggest a a moderate SPI inclusion could provide additional protection to the fish when exposed to some pathogens. On the other hand, the significant decline in growth performance and feed intake in diets exceeding 60% SPI suggests that, despite being nutritionally dense, high-level inclusion may impact palatability or metabolic efficiency in European seabass. Nevertheless, a moderate inclusion of Spirulina (MIX2) seemed to offer a functional advantage, progressively enhancing survival against Vibrio anguillarum. The second experiment suggests that the complete replacement of fishmeal with these two alternative ingredients causes a deterioration of growth parameters when adopted for the entire production cycle. The results indicate that while replacing FM with combinations of BSF and SPI may provide benefits in the short term, additional research is required to understand the limiting factors affecting long term use in European sea bass.
Acknowledgment
Financial support for this research has been provided by PRIMA, a program supported by the European Union, under grant agreement No 2231, project CIPROMED (PRIMA Call 2022 Section 1 Agri-food IA).
References
FAO. 2020. The State of World Fisheries and Aquaculture 2020. Sustainability in action. Rome. https://doi.org/10.4060/ca9229en