Is it possible to cut down fishmeal and soybean meal use in aquafeed limiting the negative effects on rainbow trout (Oncorhynchus mykiss) fillet quality and consumer acceptance?

Poultry by-products (PBM) and Hermetia illucens prepupae meal (HM) are promising alternative protein sources for aquafeeds, being palatable, nutritious, and/or cost-effective, in addition to fostering circular economy. Their inclusion in rainbow trout (Oncorhynchus mykiss) feeds, although not detrimental for growth and wellbeing, leads to a decreased fatty acid (FA) quality, specifically, a decreased polyunsaturated FA content. The present study assessed the effect of five different diets on rainbow trout's biometric, marketable, and physical traits, oxidative status, fillet nutritional quality (of fresh and of 60-day stored fillets) and consumers' liking. The diets tested were: i) a positive-control diet, based on fishmeal (CF); ii) a plant protein-based diet (CV) resembling current commercial diets, used as additional control; 60% of the vegetable proteins were substituted with either iii) PBM (P60), iv) HM (H60) or v) both PBM and HM (H10P50). The principal component analysis considering all parameters showed two neat clusters, one grouping CF samples, the other grouping the rest of the samples. Specifically, hepatosomatic index and colour were significantly altered by the different administered diets. The most evident differences between dietary treatments were found in the FA profile, with CF fish showing a distinct FA profile in comparison to the other groups. Although the highest eicosapentaenoic and docosahexaenoic acid contents were found in CF group, the total polyunsaturated FA content was not significantly different between the dietary treatments, as linoleic and linolenic acids were the lowest in the CF fillets (p < 0.001). Storage seemed to exacerbate the differences found immediately after slaughter. As a consequence of the FA profile, conjugated dienes were significantly (p < 0.05) higher in the CF group compared to all the others. MDA-eq. content, although unaffected by diet at day 0, was higher (p < 0.01) after 60 days of storage in the CF group compared to H60, P60, and H10P50 fillets. Consumer likings were favourable and not statistically different between the dietary groups. Only 20% of the consumers criticised the gustative attributes of P60 and H10P50, while almost 25% of them criticised CF, CV, and H60. In summary, the P60, H60 and H10P50 diets were not less than the commercial-like CV diet. Combining PBM and HM showed positive results and needs further studies. Improving the n-3/n-6 ratio remains a priority to be addressed when formulating aquafeeds with innovative ingredients