Effect of Euphausia superba Powder Enrichment on Fatty Acid, and Amino Acid Composition and Astaxanthin Content of Moina mongolica Daddy

To evaluate the nutritional enrichment effects of Euphausia superba meal on Moina mongolica Daday, different concentrations of krill meal (26, 39, 52, and 65 mg·L−1) were applied as enrichment supplements, with non-enriched M. mongolica serving as the control. After a 6 h enrichment period, the compositions and contents of amino acids and fatty acids in M. mongolica were analyzed, and the astaxanthin content was simultaneously determined. The results showed that, compared with the control group cultured on a mixed diet of photosynthetic bacteria, yeast, and Nannochloropsis, enrichment with Antarctic krill meal at appropriate concentrations (26–39 mg·L−1) markedly altered the fatty acid profile of M. mongolica. In particular, the levels of highly unsaturated fatty acids (HUFAs), including ∑ω6-PUFA and ∑ω3-PUFA, were significantly enhanced, with a pronounced increase in eicosapentaenoic acid (EPA). Furthermore, the total amino acid content, as well as the contents of six flavor-enhancing amino acids and ten essential amino acids for fish, were significantly higher in the enriched groups than in the control (p < 0.05). However, excessive supplementation of krill meal inhibited the growth of M. mongolica, suggesting a concentration-dependent effect. In addition, enrichment with whole Antarctic krill meal for 6 h did not significantly increase the astaxanthin content in M. mongolica, which may be related to the limited assimilation efficiency within the short enrichment duration. Overall, enrichment of M. mongolica with Euphausia superba meal at appropriate concentrations (26–39 mg·L−1) effectively improves its fatty acid and amino acid composition, thereby enhancing its nutritional value as live feed for aquaculture larvae.