Evaluation of a bacterial protein meal as a novel aquafeed ingredient for warmwater species: Barramundi (Lates calcarifer) and Mozambique tilapia (Oreochromis mossambicus)

Bacterial protein meals (BPMs) hold strong potential as sustainable alternative protein sources for aquafeeds, yet their ingredient apparent digestibility coefficients (IADCs) remain poorly characterized, limiting broader adoption. This study evaluated the digestibility of a bacterial protein meal (BPM) derived from brewery by-products using a diet-substitution method, together with the associated digestive enzymatic responses, in barramundi ( Lates calcarifer ) and Mozambique tilapia ( Oreochromis mossambicus ). Two cohorts of 150 fish per species (48.9 ± 4.2 g barramundi; 48.7 ± 11.4 g tilapia) were reared in recirculating brackish-water systems (28 °C; 5 ppt) across two independent trials. IADCs were determined for 42 fatty acids and 17 amino acids, along with Michaelis–Menten parameters (Km, Vmax) for trypsin, chymotrypsin, and leucine aminopeptidase extracted from fish gut. Protein and energy IADCs of BPM were 71.8 ± 5.3% and 64.0 ± 4.3% in tilapia, and 88.1 ± 0.9% and 75.1 ± 4.5% in barramundi. In tilapia, essential amino acid digestibility exceeded 85% except for alanine, histidine, threonine, and valine. BPM inclusion had negligible effects on lipid and fatty-acid digestibility. With control diet, Km values for trypsin, chymotrypsin, and leucine aminopeptidase were 0.01, 0.24, and 0.13 mM in tilapia, and 0.02, 0.43, and 0.07 mM in barramundi respectively. In both species, concurrent shifts in Vmax and Km were observed between dietary treatments, indicating a degree of digestive proteome plasticity that allows compensatory regulation of enzyme activity—an effect more pronounced in barramundi than in Mozambique tilapia. Overall, the tested BPM was highly digestible in both species and had minimal impact on gut enzyme functionality, supporting its suitability for inclusion in commercial aquafeed formulations.