Introduction
The aquaculture sector increasingly prioritises resource efficiency and sustainability, particularly within recirculating aquaculture systems (RAS), which can play a pivotal role in mitigating environmental impact by facilitating water reuse and reducing waste accumulation. Optimising feed formulations in RAS is essential for enhancing nutrient efficiency, with macroalgae by-products, including those obtained from agar extraction industries, representing a sustainable alternative that does not compete with terrestrial resources [1]. However, these by-products’ high non-starch polysaccharide (NSP) content may hinder digestibility in carnivorous fish species, limiting their dietary inclusion. To address this challenge, nutritional strategies such as pre-treatments (e.g., chemical hydrolysis) and exogenous enzymes have been proposed to improve NSP breakdown and enhance nutrient bioavailability [2]. Despite this drawback, NSP-rich ingredients may improve RAS performance by enhancing faecal binding, resulting in larger and more compact faeces that are efficiently removed by mechanical filtration [3], ultimately lowering management costs. Thus, this study explores the potential of high inclusion of an alkaline pre-treated Gelidium corneum solid by-product (GBP) obtained from the agar extraction industry, both alone and in combination with exogenous enzymes, on digestibility and fish faecal physical properties, aimed at tailoring RAS aquafeed for European seabass.
Materials and methods
Four isonitrogenous, isolipidic, and isofibrous experimental diets (CP 44%, EE 16%, crude fibre 1.7%) were formulated: one control diet and three test diets with 15% alkaline pre-treated Gelidium corneum by-product (GBP; NaOH 1N, 15 min autoclaving), replacing soybean and whole wheat meals, supplemented or not with 1% or 2% of a commercial thermostable NSP-degrading enzyme complex (Natugrain® TS-Feed Enzyme, BASF, Germany). All diets included 0.5% DM chromic oxide as a digestibility marker. The trial was conducted at CIIMAR (Matosinhos, Portugal) using 108 European seabass (79.9±2.4 g) randomly distributed into 12 thermo-regulated RAS tanks equipped with faecal sedimentation columns. Diets were randomly assigned to triplicate groups, fish were hand-fed to satiation twice daily, and faeces were collected before the first meal, centrifuged, and stored frozen per tank over 14 days. Tanks were cleaned 30 minutes post-feeding to remove residual waste. Apparent digestibility coefficients (ADCs) for dry matter, protein, lipid, gross energy, inorganic and organic acid detergent fibre (ADF), neutral detergent fibre (NDF), and amino acids were determined following standard protocols [4]. Fresh faecal samples from each tank were analysed for particle size distribution (fines ≤0.5 mm; medium 0.6–1.2 mm; large ≥1.2 mm) using volumetric and photometric methods [5]. Data related to ADCs and faecal physical traits were analysed by ANOVA using the PROC GLM of SAS, with diet as a fixed effect. Least square means were compared using Bonferroni’s test, with a significance level set at p<0.05.
Results and discussion
The ADCs of energy (84.4%, on average), dry matter (67.6%), protein (92.6%), and lipid (97.5%) were similar among control and the three test diets (p>0.05), aligning with reference values reported for most carnivorous fish species [6]. Inorganic and organic ADF fractions were undigestible in all dietary treatments. In contrast, the digestibility of the NDF fraction was higher in the GBP diet supplemented with 2% Natugrain® compared to the control and non-supplemented or 1% Natugrain® supplemented GBP diets (35.5% vs. 21.8% on average, p<0.05). These results confirm the reported beneficial effect of dietary supplementation with exoenzymes (i.e., carbohydrases) to complex polysaccharides, releasing carbohydrate oligomers and monomers, which reduces polymerisation and viscosity and increases bioavailability to the animal [7]. The ADCs of essential amino acids remained high across all diets, with those of lysine and methionine, key for growth and metabolism, exceeding 90%, indicating that 15% GBP inclusion did not affect their bioavailability. In addition, dietary inclusion of GBP, alone or enzyme-supplemented, enhanced the physical properties of faeces, increasing the volume (+12%) and length (+13%) of large-sized faeces (≥1.2 mm) compared to the control diet (p<0.05). This effect is likely attributed to fibre components of GBP, which may enhance water retention and contribute to a more compact faecal matrix, increasing gastrointestinal motility and promoting faecal aggregation [5].
Conclusions
Overall, 15% dietary inclusion of GBP did not affect energy, macronutrients, and amino acids digestibility, whereas supplementing the GBP-based diets with 2% of a carbohydrase enzyme complex enhanced the NDF digestibility. In addition, diets containing GBP improved faecal characteristics. Given the importance of water quality management in RAS, these findings suggest that incorporating low-cost by-products, such as GBP, into aquafeeds may optimise waste management and increase the economic sustainability of closed systems. Further studies should assess the impact of dietary GBP inclusion on fish growth, health, and water quality, while optimising aquafeed formulations for different farming conditions to enhance resource efficiency.
References
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Acknowledgments
This study was supported by the Portuguese Foundation for Science and Technology (FCT) through the project MB4AQUA (reference 2022.06587.PTDC, https:doi.org/10.54499/CEECINST/00064/2021/CP2812/CT0001).