Introduction
Alien species, also referred to as non-indigenous species, have rapidly colonised the Eastern Mediterranean basin. Those exhibiting invasive potential have caused significant impacts on biodiversity, ecosystem services, and human well-being. Among them, Lagocephalus sceleratus (Gmelin, 1789) is a highly opportunistic species responsible for substantial damage to fishing gear, catches, and local biodiversity, while also posing serious risks to human health. This species is highly toxic and currently holds no economic value in the Mediterranean market, as it is prohibited from commercialisation under EU legislation. Its continued spread is facilitated by the absence of natural predators, combined with high growth and reproductive rates. This situation highlights the urgent need for alternative management strategies for invasive species. In parallel, the rapid expansion of aquaculture has increased the demand for aquafeeds, while the overexploitation of marine resources for fishmeal production necessitates the identification of sustainable alternative protein sources. The present study aimed to evaluate the potential use of fishmeal derived from Lagocephalus sceleratus as a partial or total replacement for conventional fishmeal in diets for European sea bass (Dicentrarchus labrax).
Materials and methods
Specimens of Lagocephalus sceleratus were processed into fishmeal through cooking, pressing, drying, and grinding. The resulting meal was subjected to two thermal treatments (160°C and 210°C) to reduce tetrodotoxin (TTX) levels, producing lower temperature (T1) and higher temperature (T2) variants. Five experimental diets were formulated, in which conventional fishmeal was replaced totally with unprocessed Lagocephalus sceleratus meal (UnP) or thermally treated variants (T1 and T2), including one diet with partial substitution by T1 (Combo). The UnP diet served as a negative control to evaluate the tolerance of European sea bass to TTX. European sea bass with an initial body weight of 39 g were randomly distributed into 15 tanks, with each dietary treatment tested in triplicate. Fish were reared in an open-flow system under controlled water quality conditions for 106 days (20°C, >90% O2 saturation) and were fed the experimental diets to apparent satiation twice daily. Faeces were collected daily for the last 15 d of the trial to determine apparent digestibility coefficients (ADC). Digestibility, whole-body composition, intestinal trypsin activity, gut microbiota, immune status, and histological analyses were performed to comprehensively evaluate the effects of the experimental diets. TTX levels were assessed in liver, muscle, and whole-body samples using LC–MS/MS (LOD: 0.0005 mg/kg). Microbial community analysis was conducted on gut DNA using 16S rRNA gene amplicon sequencing (Illumina MiSeq, 2 × 300 bp, V3–V4 region). Sequences were processed in Mothur (v1.48.0), with OTUs defined at 97% similarity and taxonomically assigned against the SILVA database (release 138.1).
Results and discussion
Most production parameters differed significantly among treatments. Specific growth rate (SGR) and feed conversion ratio (FCR) were significantly improved in fish fed the UnP diet. The reduced performance observed in fish fed T1 and T2 diets were likely associated with a decline in the nutritional quality of the meals, as high processing temperatures may promote lipid oxidation, degradation of highly unsaturated fatty acids, and overall nutrient loss (Phung et al., 2020). The highest protein digestibility was observed in fish fed the UnP diet, followed by the Control group. Intermediate values were recorded in the Combo group, while fish fed thermally processed fishmeals (T1 and T2) exhibited markedly reduced protein digestibility, consistent with their lower final body weight. Similarly high lipid digestibility above 95% was observed in the UnP and Control diets. The lowest lipid digestibility was recorded in fish fed the high-temperature processed fish meal (T2). Significant differences were observed in whole-body lipid content. Fish fed the T2 diet exhibited the highest lipid levels compared to the Control and UnP groups. This outcome is likely associated with the markedly reduced protein digestibility observed in this group, which may have led to increased feed intake to meet energy demands, resulting in greater lipid deposition. Previous studies have associated reduced digestive enzyme activity, particularly trypsin, with impaired growth performance and feed utilisation (Murashita et al., 2018). In the present study, intestinal trypsin activity was highest in fish fed the Combo, UnP and Control diets. In contrast, the lowest trypsin activity was observed in fish fed the T2 diet, supporting the reduced performance observed in this group. Moreover, no significant differences were observed in gut bacterial communities among dietary groups. The dominant bacterial phylla – Pseudomonadota, Actinomycetota, Bacillota and Bacteroidota – commonly reported in the fish gut microbiomes, were consistently present across all dietary treatments, with only minor variations in relative abundance. Serum trypsin inhibitor activity, an indicator of immune resistance to the pathogens evasion process, was highest in fish fed the Unp, Combo, and T2 diets, with no significant differences compared to the Control group. The lowest values were observed in the T1 group. Lysozyme activity, reflecting antibacterial capacity against Gram-positive bacteria, was significantly higher in fish fed the UnP diet and the Combo diet compared to the Control and T2 groups. Concerning the antibacterial activity against Gram-negative bacteria, T1 and Combo diets showed significantly higher values than the Control diet. No histological differences were observed in the liver among experimental groups. Moderate hepatocellular vacuolation, indicative of lipid accumulation (steatosis), was consistently observed. Similarly, no morphological differences were detected in either the anterior or posterior intestine. All groups exhibited intact epithelial structure, with no signs of hypertrophy or hyperplasia. Complete replacement of conventional fishmeal with unprocessed Lagocephalus sceleratus meal (UnP) resulted in superior growth, nutrient digestibility, immune response, and digestive enzyme activity. Partial substitution with lower temperature treated meal T1 (15%) achieved comparable performance. Thermal processing reduced the nutritional quality of the meals, as evidenced in vivo by decreased growth, protein digestibility, and enzyme activity in fish fed exclusively treated diets. Importantly, no TTX accumulation was detected in any tissue. These findings support the potential of valorisation of L. sceleratus as a sustainable alternative protein source, contributing to both invasive species management and aquaculture sustainability. However, further validation under long-term and large-scale production conditions is required.
Acknowledgment
The project was co-founded by Greece and the European Union under the Fisheries and Maritime Operational Program 2014-2020 (MIS 5067491). https://lagomeal.gr/
References
Murashita, K., Matsunari, H., Furuita, H., Rønnestad, I., Oku, H., & Yamamoto, T. (2018). Effects of dietary soybean meal on the digestive physiology of red seabream Pagrus major. Aquaculture, 493, 219-228.
Phung, A. S., Bannenberg, G., Vigor, C., Reversat, G., Oger, C., Roumain, M., ... & Wang, S. C. (2020). Chemical compositional changes in over-oxidized fish oils. Foods, 9(10), 1501.