Partial replacement of fish meal with Chlorella meal improved flesh quality without compromising growth and health condition in largemouth bass (Micropterus salmoides)

Chlorella meal (CM) was evaluated in largemouth bass ( Micropterus salmoides ) as an alternative protein source to fish meal (FM). For this purpose, six isoproteic (50% crude protein, CP), isolipidic (9% crude lipid; CL), and isoenergetic (21 MJ kg −1 ) diets containing graded levels of CM (0, 105, 210, 315, 525 and 700 g kg −1 ) were tested in triplicate for 90 days. The replacement of FM by CM ranged from 0% to 100%. Results showed that largemouth bass may tolerate moderate levels of FM replacement by CM (up to 45%, equivalent to 315 g kg⁻ 1 ) without compromising growth performance, feed efficiency, or fillet quality. However, higher substitution levels (75% and 100%, corresponding to 525 and 700 g kg⁻ 1 ) significantly impaired growth performance and feed conversion, effects that were likely related to reduced digestibility of CM compared to FM, and lower levels of dietary n-3 PUFAs. Despite the growth reduction at high inclusion levels, CM supplementation provided notable benefits, including enhanced antioxidant capacity (reduced MDA levels), reduced serum triglyceride levels, and modulation of amino acid metabolism and mTOR signalling pathways, the latter maintaining normal muscle fibre structure and texture properties. Fillet and skin pigmentation were also improved through natural carotenoid deposition, following a hormetic dose-response pattern dependent on the skin region considered and the CM inclusion level. However, complete FM replacement altered the fatty acid profile by decreasing n-3 PUFAs while increasing n-6 PUFAs, potentially compromising the nutritional quality of fillets, and modified sensory properties of the flesh. These findings suggest that CM represents a promising sustainable alternative to FM in largemouth bass diets at moderate inclusion levels, though optimization through dietary supplementation with fish oil or other n-3 PUFA sources is necessary to maintain optimal fillet nutritional quality and sensory attributes when higher replacement levels are targeted. • Chlorella meal (CM) can replace fishmeal (FM) up to 45% in diets (315 g kg⁻¹) for largemouth bass. • CM inclusion at 525 and 700 g kg⁻¹ (75 and 100% FM replacement) reduced growth and feed intake. • CM reduced the levels of oxidative stress in serum, and increased skin and flesh pigmentation. • FM replacement by CM did not affect the textural quality of the flesh. • Metabolomics revealed that CM modulated amino acid metabolism and mTOR signalling pathway.