Introduction
Early weaning in flatfish species remains one of the main bottlenecks in larval production, due to the high dependence on live feed and the limited efficiency of conventional microdiets. During early developmental stages, larvae possess an immature digestive system, which constrains the digestion of complex ingredients and limits the transition to inert diets. Therefore, the formulation of highly digestible microdiets tailored to larval physiological requirements is critical to improve weaning success (Hamre et al., 2013; Engrola et al., 2018). Extrusion technologies are widely used in commercial microdiet production due to their robustness and scalability. However, feed for early larval stages requires precise control of physical properties such as particle size, water stability, and nutrient leaching, as well as the ability to incorporate sensitive functional ingredients. Fluid bed agglomeration emerges as a promising alternative, enabling enhanced control over particle structure and functionality. This technology facilitates the inclusion of sensitive compounds such as protein hydrolysates, bioactive ingredients, and thermolabile nutrients while preserving their integrity and bioavailability. Protein hydrolysates have been shown to improve digestibility, feed acceptance, and digestive development in marine fish larvae (Kolkovski, 2001; Poonnual et al., 2025). Additionally, fluid bed technology allows optimization of key physical properties such as particle cohesion, water stability, and reduced nutrient leaching, which are essential for maximizing effective ingestion during first feeding and weaning stages. These characteristics position fluid bed agglomeration as a highly suitable technology for the development of species- and stage-specific microdiets. The aim of this study was to evaluate the potential of fluid bed agglomerated microdiets incorporating different protein hydrolysates, compared to commercial extruded diets, using two key aquaculture species: Senegalese sole (Solea senegalensis) and turbot (Scophthalmus maximus).
Materials and Methods
Two independent trials were conducted under controlled experimental conditions. In the Senegalese sole trial, larvae were obtained from natural spawning and the experiment lasted 21 days. Treatments were performed in triplicate: (1) commercial micro-extruded diet (control); (2) diet D1 produced by fluid bed agglomeration, including low molecular weight hydrolysate "A"; (3) diet D2 produced using the same technology, including higher molecular weight hydrolysate "B"; (4) diet D3 including a hydrolysate with a broad molecular weight range. Fish fed diets D1, D2, and D3 started co-feeding three days earlier than those fed the commercial diet. Dry weight, total length, and weight gain were evaluated. In the turbot trial, larvae were fed from mouth opening for 26 days with different microdiets: one commercial reference diet and two experimental diets (X1 and X2) with different protein hydrolysate profiles. Growth parameters (weight and length), relative growth rate (RGR), survival, feed intake, and feed conversion ratio (FCR) were recorded. In both trials, data were statistically analyzed after testing normality and homogeneity of variances, using parametric or non-parametric tests as appropriate (α = 0.05).
Results and Discussion
In Senegalese sole, improved growth was observed with fluid bed agglomerated diets, with lower molecular weight hydrolysates further enhancing performance. In turbot, key parameters such as weight, RGR, survival, intake, and FCR did not differ significantly among treatments at the end of the pelagic phase. However, improved growth was observed during the first 10 days in larvae fed fluid bed agglomerated diets (X1 and X2). Previous studies have shown that protein hydrolysates can enhance palatability and digestibility in marine fish larvae (Kolkovski, 2001; Gisbert et al., 2012). Overall, results from both trials demonstrate that fluid bed agglomeration enables the development of microdiets with performance comparable or superior to conventional commercial diets. This is particularly relevant for larval nutrition, where ingredient composition and physical properties are critical for feed intake and utilization. The ability to design microdiets tailored to specific species and developmental stages positions this technology as a promising tool to improve early weaning efficiency in flatfish aquaculture.
Acknowledgment
This work is part of project E!4876 FlatFIRST (No. 17254, operation No. MPr-2023-4-17254; COMPETE2030-FEDER-01171000; ALGARVE-FEDER-01171000), funded by the EUROSTARS-3 programme and co-funded by the European Union through Algarve 2030 and COMPETE 2030, under Portugal 2030.
References
Gisbert, E., Skalli, M., Fern��ndez-D��az, D., Est��vez, A., Andree, M. (2012). Protein hydrolysates from fish by-products as feed ingredients for gilthead sea bream (Sparus aurata) larvae. Aquaculture, 338–341, 96–104.
Kolkovski, S. (2001). Digestive enzymes in fish larvae and juveniles—implications and applications to formulated diets. Aquaculture, 200, 181–201.
Hamre, K., Yufera, M., R��nnestad, I., Boglione, C., Concei����o, L., Izquierdo, M. (2013). Fish larval nutrition and feed formulation: knowledge gaps and bottlenecks. Reviews in Aquaculture, 5, S26–S58.
Engrola, S., Arag��o, C., Valente, L.M.P., Concei����o, L.E.C. (2018). Nutritional modulation of marine fish larvae performance. In: Y��fera, M. (Ed.), Emerging Issues in Fish Larvae Research. Springer.
Poonnual, D., Tola, S., Yuangsoi, B. (2025). Evaluation of functional marine protein hydrolysates as fish meal replacements in low-fish-meal diets: effects on Asian seabass (Lates calcarifer). Animals, 15, 3285.