2.1. Experimental Design and Fish Maintenance

The present study was conducted in a pilot recirculating aquaculture system (RAS) equipped with automatic water treatment facilities (Supplementary Figure S1), located at the Romanian Center for Modelling Recirculating Aquaculture Systems from “Dunărea de Jos” University of Galați. The conceptual installation framework of the RAS was designed to ensure proper technological treatment for water quality and fish biomass welfare. The flux scheme includes a well-thought-out sequence of components displayed on two levels (basement, and ground floor). There are main components (1), including growth units (24 tanks, each of 1 m³, 109 cm in diameter), mechanical filters, biological filters, equipment for transfer of dissolved gases (degassing carbon dioxide, oxygen injection, and contactors), disinfection systems with UV radiation, pumps, and water quality monitoring and control equipment; as well as auxiliary components (2), such as automatic feeders, an ozone generator, and an independent electric generator.

The hybrid sturgeons were obtained at the hatchery of a commercial farm belonging to Danube Research Consulting (DRC), via artificial reproduction using a stock of 9 aquaculture F1 breeders (4 bester females, and 5 males, of which were 3 besters and 2 belugas). Before fertilization, the eggs from each female were divided into two groups, each group being fertilized with the pooled milt of males of the same hybrid/species. The obtained larvae were reared in a flow-through system until they reached the age of 3 months. Prior to the experiments, the fish were reared in a pilot RAS station until the age of 5 months, when they were used for experiments. The fish were sorted to narrow the size within each group, and to allow for small variations at the start of the experiment. The experiment was set as two parallel trials, called T1 and T2. For the first trial (T1), 90 B hybrids with an average initial weight of 181.15 ± 21.21 g and 90 BB hybrids with an average initial weight of 181.98 ± 28.65 g were each randomly distributed into 6 fiberglass tanks (1 m³) in 2 different densities (2 kg/m² (LD) and 4 kg/m² (HD)) for 6 weeks (3 replicates for each hybrid; 12 tanks in total). For the second trial (T2), 196 BB hybrids (335.24 ± 39.30 g/specimen) were distributed in 8 tanks (1 m³) at 4 stocking densities (in duplicate): 5 kg/m²—SD1; 8 kg/m²—SD2; 10 kg/m²—SD3; and 12 kg/m²—SD4. The second trial was performed in parallel with the first trial.

Fish were fed three times daily (2% of tank biomass per day) with commercial pellets for sturgeons (54% protein, 15% fat, 0.5% fiber, ash, and 21.1 kJ·g⁻¹; Alltech Coppens, Leende, the Netherlands). The feed was administered manually to avoid feed competition and fighting amongst fish. When uneaten pellets were observed, the feed administration was stopped, and the remained pellets were quantified and dried after removal from the tank using a small net. Body weight and standard length were recorded weekly for 10 fish/tank (chosen at random), and the feeding rates were adjusted to the new biomass of each rearing unit.