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Circular economy driven feed ingredients and emerging protein sources, such as insects and microbial meals, has the potential to partially replace fishmeal in diets of high-trophic fish. Even though growth and feed performance are often unaffected at low inclusion levels, the metabolic effects are unknown. This study examined the metabolic response of juvenile turbot (Scophthalmus maximus) to diets with graded fishmeal replacement with plant, animal, and emerging protein sources (PLANT, PAP, and MIX) in comparison to a commercial-like diet (CTRL). A 1H-nuclear magnetic resonance (NMR) spectroscopy was used to assess the metabolic profiles of muscle and liver tissue after feeding the fish the experimental diets for 16 weeks. The comparative approach revealed a decrease in metabolites that are associated with energy deficiency in both tissues of fish fed with fishmeal-reduced diets compared to the commercial-like diet (CTRL). Since growth and feeding performance were unaffected, the observed metabolic response suggests that the balanced feed formulations, especially at lower fishmeal replacement levels, have the potential for industry application.

Direct measurements of temperature-dependent weight gains are experimentally challenging and time-consuming in long-lived/slow-growing organisms such as Antarctic fish. Here, we reassess methodology to quantify the in vivo protein synthesis rate from amino acids, as a key component of growth. We tested whether it is possible to avoid hazardous radioactive materials and whether the analytical pathway chosen is robust against analytical errors. In the eelpout, Pachycara brachycephalum, 13C9H1115N1O2 phenylalanine was injected intraperitoneally and muscle tissue was sampled before injection and at 1.5 h time intervals up to 6 h thereafter. The incorporation of 13C15N-labeled-phenylalanine into muscle was monitored by quantification of bound and free phenylalanine through liquid chromatography–mass spectrometry. We found an increase in the pool of labeled, free phenylalanine in the cytosolic fraction that leveled off after 4.5 h. The labeled phenylalanine bound in the proteins increased linearly over time. The resulting protein synthesis rate (Ks) for P. brachycephalum was as low as 0.049 ± 0.021% day−1. This value and its variability were in good agreement with literature data obtained from studies using radioactive labels, indicating that this methodology is well suited for characterizing growth in polar fish under in situ conditions in remote areas or on research vessels.