## 2 New Trends in Aquaculture Feed Production
There are several limitations in using plant sources in rainbow trout diets, including: antinutritional factors (ANF), amino acid, fatty acid, and mineral contents, and palatability. Some additives can be used to reduce the negative effects of plant proteins. Alternative oil sources, including canola, palm oil, and chicken fat, are commonly used to replace fish oil. Replacing fish oil with alternative oils reduces the omega-3 content of trout fillets. The use of finishing diets, rich in fish oil, reduces the level of HUVECs (highly unsaturated fatty acids), fatty acids that are accumulated during the growing phase [3]. Some alternative ingredients, such as canola oil, palm oil, and chicken fat, are used to replace fish oil. In fish feed, oil extracted from algae, which has high omega-3 content, can partially replace fish oil [23]. Replacement of fishmeal and fish oil in feeds with plant-based products may affect the health of rainbow trout through changes in gut morphology, immune function, and interferences with the normal endocrine system function. In the future, more research is needed on the effects of new protein sources on trout growth and health. The use of plant-derived ingredients in diets may also change the microbial populations living in the fish gut [24]. Metagenomic analysis, which is performed by next-generation sequencing (NGS) techniques, allows the identification of the main bacterial species of the fish gut microbiome [25]. In France, rainbow trout populations have been selected to efficiently digest diets containing some plant-based ingredients [26]. The environmental impact of raw materials throughout the production chain and feeds used during the production phase is determined by life cycle assessment (LCA) methods [27]. In fish farming, the nutrition phase accounts for more than 90% of carbon dioxide equivalent emissions [28]. The substitution of fishmeal with plant-based ingredients (e.g., soy) results in increased pollution [29] and ecotoxicity related to the use of fertilisers and pesticides during the plant production phase, reduction in natural resources, and loss of biodiversity, especially due to deforestation (particularly in Brazil) [30]. Between 2000 and 2020, the amount of marine-derived product used in salmonid feed manufacturing decreased from 33.5% to 14.5%, while lipids decreased from 31.1% to 10.4% [31]. Despite success in replacing fishmeal and fish oil with plant-based products, the feed industry is looking for new ingredients to reduce environmental impacts. Proteins obtained from insect and microalgae meal can be used to partially replace fishmeal and fish oil in fish feed [32]. [PERSON] et al. [33] studied the replacement of fishmeal and fish oil with plant-based ingredients. The results of the experiment showed that the environmental impact of rainbow trout feeds varied with respect to the geographic origin of the fishmeal and fish oil (e.g., Peru or Norway) and the type of plant used (e.g., canola or palm oil). Feeds are the most important contributors to the environmental impact in fish farming [33; 34]. In LCA, the allocation method used strongly influences the environmental impact results [35]. The allocation of resources and the calculation of the environmental impact of coproducts (e.g., fishmeal and fish oil) are two very important activities in LCA [36; 37; 38]. Economic allocation has been frequently used in LCA studies applied to the aquaculture sector [39; 40]. According to [PERSON] et al. [41], economic product allocation allows a more accurate estimation of the environmental impact. This method is also suggested in the European Commission's guidelines for the animal feed sector [42].