The combined production of three varieties of lettuce (romaine, iceberg, and red leaf) with flathead grey mullet (Mugil cephalus) was tested in triplicate in three independent coupled aquaponic units with no thermal control. For this purpose, a total of 114 fish (2.5 kg/m³) were stocked in each fish tank (2 m³), and 92 lettuces were planted in the hydroponic unit (6 m²). As no thermal control was included in the design of the aquaponic system, water temperatures declined from maximum values of 20.4 °C to minimum values of 5.0 °C, which directly affected fish growth. However, the conditions imposed by the aquaponic system were suitable for promoting lettuce’s growth and external appearance, as no pests or leaf discoloration were noticed. Lettuce survival was similar among the three tested varieties (98.5 ± 1.7%). The yields for the romaine and iceberg varieties were 384 ± 100 g/lettuce and 316 ± 70 g/lettuce, respectively, and that for the red leaf variety was lower, at 176 ± 75 g/lettuce. Yield values ranged between 3.6 and 4.4 kg/m² depending on the replicate considered (4.0 ± 0.4 kg/m²). According to present results, each aquaponic unit required ca. 2.6–2.7 L of water per unit of lettuce produced.

European eel (Anguilla anguilla) elvers (initial body weight (BW) = 3 g) were raised in triplicate for 60 days in a biofloc system (BFT) at 21 °C. Data from the current first study evaluating this farming technology indicated that European eel elvers adapted well to BFT systems as data on growth performance (specific growth rate = 1.48% ± 0.13 BW/day and FCR = 1.05 ± 0.09) indicated, with production costs using BFT being lower than conventional RAS units. The most critical issues associated with this aquaculture system were the maintenance of the biofloc in tanks by the regular addition of refined sugar (46% C) to keep a relationship for C:N of 20:1, and the prevention of emergence of opportunistic pathogens like the monogenean Pseudodactylogyrus sp. The overall results of this study in terms of elvers’ performance and quality and the composition of the biofloc material and its microbial composition indicated that BFT, which is considered to be one of the most cost-effective, sustainable, and environmentally friendly farming systems due to its zero water exchange and improvement of feed conversion ratio by the dietary contribution of bioflocs, may be satisfactorily used for farming European eels elvers at a density of 2 kg/m³. However, further studies are needed to test this technology with older eel stages.

The early intestinal response of Mugil cephalus to fishmeal replacement with alternative protein sources under biofloc technology (BFT) was assessed. Juvenile mullet (82.68 ± 0.05 g) were distributed among three dietary treatments with decreasing fishmeal inclusion levels of 15%, 5%, and 0%, corresponding to FM15, FM5, and FM0, respectively. After 27 days of feeding, intestinal samples were analyzed for microbiota composition, transcriptomic responses and enzymatic activity. The FM0 group exhibited reduced intestinal enzyme activity, whereas no significant differences were observed in terms of gut microbiota among the diets. Furthermore, transcriptomic analysis revealed differentially expressed genes associated with enriched GO terms and KEGG pathways related to lipid metabolism. This response was consistent with the lower mesenteric fat indices and whole-body lipid content observed in fish fed the FM0 diet, suggesting an early metabolic adjustment associated with complete fishmeal replacement and the higher dietary protein content. All diets exhibited high protein efficiency ratios (9.98 ± 2.42) and low feed conversion ratios (0.40 ± 0.17), with no significant differences observed among them. However, complete fishmeal replacement negatively affected intestinal physiology and lipid storage, indicating that these alterations were not fully compensated for by the biofloc system and may not be physiologically sustainable over longer periods. Further studies over an extended period could be carried out to evaluate the long-term effects of these diets.