Product acceptance was positive and comparable to other unconventional products, such as cowpeas and the meal from the oil extraction of sacha-inchi, an Amazonian plant. The finding is part of a search for plant sources to replace corn and soybeans in the composition of tambaqui feed, reducing costs for fish farmers. Such unconventional products often do not meet the needs of the production sector, especially due to the small amounts available. The residue from the guava pulp agroindustry meets the requirements of quantity, production scale, and especially nutritional quality. The use of such residue fits a model of circular economy in tambaqui farming. The residue from the guava pulp agroindustry can partially replace corn in the feed formula for young tambaqui (Colossoma macropomum) by up to 75%. At this level, researchers from Embrapa Western Amazon consider that acceptance of the guava residue was positive and comparable to other unconventional products assessed for the species, such as low and high-tannin sorghum, residual meal from sacha-inchi oil extraction, and cowpeas. Unconventional plant sources for the composition of tambaqui feed have already been investigated by several researchers to find alternatives to traditional ingredients: corn, soy bran and soybean oil, which are expensive for Amazon feed producers. While tambaqui has the ability to make use of food sources of plant origin, including unconventional ones, many of such investigations stumble on situations in which the production of such alternative ingredients does not meet the needs of local feed factories, especially in terms of quantity, which are often insufficient for commercial production; others have seasonality in their production, not supplying the factories throughout the year; and, in other cases, they do not have the suitable nutritional quality to replace commodities (corn, soy bran and soybean oil). The study results are published in Embrapa's Research and Development Bulletin, August 2023, entitled “Uso do Resíduo da Agroindústria de Polpa de Goiaba na Alimentação de Juvenis de Tambaqui” [Use of Guava Pulp Agroindustry Residue in Young Tambaqui Diets], authored by researchers Jony Dairiki and Cheila Boijink, from Embrapa Western Amazon, in partnership with Amanda Silva (undergraduate scholarship holder), Francisco Simão (master's student in aquaculture), Lorena Lanka Silva (master's student in aquaculture), and Leonardo Takahashi, professor at Júlio de Mesquita Filho São Paulo State University (Unesp). The researcher Jony Dairiki explains that the test with the guava pulp agroindustry residue found that this unconventional ingredient can meet all the abovementioned requirements, as it comes from an agroindustry that has quantity, production scale and special nutritional quality. One of the few deficiencies found in the product concerns the high level of crude fiber, which may have caused lower weight gain in fish fed with 100% replacement. In such scenario, a better balance of the feed, combined with a reduction in fiber from other plant sources in the formula (e.g. soy bran) could enhance the use of the residue from the guava pulp agroindustry. Circular economy and lower cost for fish farmers It is worth noting that dietary fibers are resistant to intestinal enzymatic hydrolysis, with complete or partial microbial fermentation in the fishes' large intestine. In some cases, if the fiber provided is insoluble, it may increase the speed of gastrointestinal transit, and thus reduce nutrient digestion time and subsequent absorption. According to the researcher, the acceptability of feeds with increasing levels of guava pulp agroindustry residue was positive. Except for weight gain, there was no significant difference in the other variables analyzed, proving it is possible to partially replace up to 75% of the corn in the feed with the guava pulp agroindustry residue. The use of this residue as a circular economy model in tambaqui feed can be recommended. The state of Amazonas relies on conventional ingredients from other locations, whose cost is topped by the price of their freight, which is passed on to fish farmers when they purchase the feed. Therefore the circular economy proposal of using by-products generated by local agroindustries is an interesting alternative to reduce the problem. The aim of this study was to assess the replacement of corn, by 0% (control), 25%, 50%, 75% and 100%, with the guava pulp agroindustry residue in the nutrition of young tambaqui. The animals were fed for 60 days with experimental feeds in two daily meals until apparent satiation. Tambaqui is an omnivorous species, that is, in nature it consumes both foods from plant sources, such as fruits and seeds, and foods of animal origin, such as insects, zooplankton, and snails, among others. This characteristic makes tambaqui an easily adaptable species to fully feed-based diets, especially with the use of plant ingredients. Use of unconventional ingredients One of the lines of research developed by Embrapa Western Amazon for over a decade evaluates the use of unconventional ingredients to reduce dependence on conventional ingredients: soybean bran and oil, in addition to corn, in the composition of feed for commercial fish in Amazonas state, such as tambaqui itself and matrinxã (Brycon amazonicus), another species of Amazonian fish. The main results obtained in those studies indicate the possibility of including and replacing conventional ingredients with cowpeas (Vigna unguiculata); sacha inchi (Plukenetia volubilis) and its byproducts (leaf bran and residual meal from the oil extraction), an Amazonian plant that is rich in omega 3 fatty acids, whose inclusion led to the incorporation of those acids in the carcass and fillet, proving the addition of nutritional value to the fish; and the residues obtained in banana farming (leaf bran, heart and stem), which can provide zootechnical performance related to the control of parasites. Alternative sources for feed Other studies using conventional ingredients with a smaller scale of use in the Amazon, such as sugarcane yeast, low tannin sorghum and high tannin sorghum have shown that these products can compose feeds for tambaqui and matrinxã, not only reducing the cost of feed but also providing control of parasites like acanthocephalus and gill monogeneans. All these results stimulate the mass production of alternative sources for the composition of feeds aiming at the recovery of the state's protagonism in the production of native commercial fish. Data from the Brazilian Fish Farming Association (Peixe BR) indicate that Amazonas currently takes the fifth place among the top native fish producing states in Brazil. In the study, the concept of circular economy was proposed aiming at the rational use of resources, so that residues from the guava pulp agroindustry can be used as inputs to manufacture new products, in this case, tambaqui feed. Partners in research To conduct the research, Embrapa partnered with the companies Amazônia Polpas and Nalto Polpas, which supplied the guava residue; the National Institute of Amazon Research (Inpa), “Júlio de Mesquita Filho” São Paulo State University (Unesp) - Jaboticabal Campus, Nilton Lins University (Uninilton Lins), Federal Institute of Education, Science and Technology of Amazonas (Ifam),and the Amazonas State Research Support Foundation (Fapeam), which granted undergraduate and graduate scholarships; and, for funding, with the São Paulo State Research Support Foundation (Fapesp) through the project “Nutrição para a Produção Sustentável de Tambaqui: Tolerância aos Carboidratos, Efeito Poupador de proteína e Uso de Alimentos Alternativos” [Nutrition for the Sustainable Production of Tambaqui: Carbohydrate Tolerance, Protein-Saving Effect and Use of Alternative Foods], led by researcher Jony Dairiki.