Effects of replacing fish meal and fish oil with microalgal meals (Arthrospira platensis and Schizochytrium limacinum) in diets for juvenile Totoaba macdonaldi

We evaluated graded replacement of fishmeal and fish oil with a blended microalgal meal (AM) composed of protein-rich Arthrospira platensis and DHA-rich Schizochytrium limacinum (2.8:1) in juvenile Totoaba macdonaldi over nine weeks. Five isonitrogenous and isolipidic diets replaced 0, 25, 50, 75, or 100% of fishmeal plus fish oil with AM. Growth performance, feed utilization, apparent digestibility, whole-body composition, muscle fatty acids, and digestive enzyme activities were assessed. Weight gain (WG) and protein efficiency ratio declined with increasing AM inclusion; AM25–AM50 showed reduced growth relative to AM0, and AM75–AM100 exhibited marked growth depression. Broken-line analyses indicated threshold responses in feed utilization, with estimated thresholds at 36.7% AM for feed intake (FI) and 44.3% AM for feed conversion ratio (FCR); accordingly, FI and FCR were generally stable through AM0–AM50, but FI decreased, and FCR worsened at AM75–AM100. Apparent digestibility also showed threshold responses: apparent digestibility coefficients of dry matter (ADCd) and lipid (ADCl) exhibited thresholds at 25% AM, with values declining from AM50 onward and reaching the lowest levels at AM75–AM100, whereas apparent protein digestibility (ADCp) remained stable up to a higher threshold (70.8% AM) and decreased only at AM100. Intestinal trypsin activity increased with AM inclusion, whereas pyloric caeca lipase activity decreased, consistent with impaired lipid digestion and utilization at higher AM levels. Muscle fatty acid profiles mirrored dietary composition: EPA decreased, and DHA increased with higher AM inclusion, increasing the muscle DHA/EPA ratio. Under the tested conditions, 25% AM inclusion did not alter FI, FCR, or PPV relative to AM0, but reduced WG and PER. Thus, no tested replacement level can be considered fully equivalent to the control for juvenile T. macdonaldi . • Weight gain was significantly higher at 0% AM replacement. • FCR remained stable up to 44.3% AM replacement and worsened thereafter. • ADCd and ADCl declined with increasing AM replacement, showing early breakpoints at 25% AM. • Nonetheless, ADCp remained stable up to a breakpoint of 70.8% AM replacement. • Trypsin activity increased with increasing AM replacement, whereas lipase activity decreased. • Fillet fatty acid profiles mirrored the diets; DHA and n-6 PUFA increased with AM replacement.