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In this study, the effect of adding chia oil and avocado extract to the diet of Nile tilapias (Oreochromis niloticus) was evaluated after 0, 15, 30 and 45 treatment days. The inclusion of chia oil and avocado extract aims to nutritional enrichment of fish fillet by the use of a new lipid source and a by‐product of low cost and high antioxidant capacity. Thus, experimental diets were formulated by the addition of 0.14% of ethanolic extract of avocado peel (Treatment I), 1.90% of chia (Salvia hispanica L.) oil (Treatment II) or both (0.14 and 1.90%, respectively) for Treatment III. There was an increase in the total omega‐3 (n‐3) fatty acid content of fish muscle tissue (29.01–78.81 mg g−1 TL) due to Treatment II (TII). Among n‐3 fatty acids, the amount of alpha‐linolenic acid (LNA, 18:3n‐3) increased sixfold after 45 days of feeding with the TII diet. The largest increases in antioxidant capacity were observed in the lipophilic fraction (L‐ORAC(FL)), from 3.40 to 19.50 μmol of ET g−1 from fish treated with chia oil (TII), while differences between treatments and periods were not detected in the hydrophilic fractions (H‐ORAC(FL)), indicating that excessive amounts of antioxidant compounds are simply excreted by the animals. Thus, chia oil contributed to an enhancement in the nutritional quality of Nile tilapia, both in terms of fatty acid content and total antioxidant capacity. Practical applications: Multiple recent studies have emphasised the benefits to human health of including omega‐3 fatty acids and antioxidants in the diet. For this reason, the inclusion of chia (S. hispanica L.) oil and avocado (peel) residue was proposed in order to increase the amount of omega‐3 fatty acids in tilapia fillets and to improve muscle tissue antioxidant capacity. It is notable that these parameters increased significantly, especially the quantity of omega‐3 fatty acids, after a 45‐day trial, providing a differentiated product with a high nutritional quality. From the addition of chia seed oil in rations, it was observed a significant increase of alpha‐linolenic content in tilapia fillets. The effects of supplemented feeding were more pronounced until 30 days. For antioxidant compound analysis, in most of the cases, better results were observed from L‐ORAC(FL) assay. Thus, the fish which received rations supplemented with chia seed oil showed a superior nutritional quality, especially in terms of fatty acid composition.

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Recently, great attention has been directed towards the use of essential oils from aromatic plants as antimicrobials and antioxidant in food matrix. Fish is well known to be a high perishable food. Indeed, fish muscle is susceptible to suffer protein and lipid oxidation during frozen storage, which can lead to the development of softening and undesirable volatile molecules. However, the possible inclusion of essential oils in fish feed for preserving fish flesh quality during storage is still unclear. For this reason, the potential protective effects of the incorporation of a dietary essential oil constituted by eucalyptol, carvacrol and thymol, to rainbow trout’s (Oncorhynchus mykiss) feed were here investigated. Frozen fish fillets resulting from trout fed the essential oil showed a significant protection of specific muscle proteins against the oxidation produced during frozen storage at –10ºC for 6 months. Essential oil-enriched feed decreased carbonylation of specific myofibrillar (α-actinins-1 and -3, myosin heavy chain, myomesin-1, pyruvate kinase, tropomyosin, troponin-T and actin) and sarcoplasmic proteins (glycogen phosphorylase, creatine kinase, fructose-bisphosphate aldolase A and phosphoglycerate mutase 2). Essential oils also increased actin stability and preserved muscle protein solubility and water holding capacity. In addition, essential oils inhibited the onset of lipid oxidation and rancidity, resulting in frozen fish with superior textural quality and sensory scores. As a final conclusion, the inclusion of essential oils in farmed rainbow trout feed is largely efficient for increasing fish quality and shelf life during frozen storage, mainly through a selective-antioxidant effect on muscle proteins.