Aquaculture Research. 2019;00:1–9. wileyonlinelibrary.com/journal/are | 1 © 2019 John Wiley & Sons Ltd 1 | INTRODUCTION Arapaima gigas is a species endemic to the Amazon region, known locally as ‘pirarucu’, and with a production of 8.6 tons in 2016. Its production has expanded because it presents rapid growth, reaching 10 kg in just one year, breaths air and accepts artificial feed despite being a carnivorous fish (Cavero et al., 2003; IBGE, 2016; Oliveira et al., 2012). The production of native species has grown by 43.7% Received: 17 July 2019 | Revised: 4 September 2019 | Accepted: 8 September 2019 DOI: 10.1111/are.14332 ORIGINAL ARTICLE Effects of an Enterococcus faecium‐based probiotic on growth performance and health of Pirarucu, Arapaima gigas Natalino da Costa Sousa 1 | Márcia Valéria Silva do Couto 1 | Higo Andrade Abe 1 | Peterson Emmanuel Guimarães Paixão 2 | Carlos Alberto Martins Cordeiro 3 | Emilly Monteiro Lopes 3 | Jonathan Stuart Ready 4 | Gabriel Fernandes Alves Jesus 5 | Mauricio Laterça Martins 5 | José Luiz Pereira Mouriño 5 | Paulo César Falanghe Carneiro 6 | Alexandre Nizio Maria 6 | Rodrigo Yudi Fujimoto 6 1 Graduate Program in Animal Science, Federal University of Pará (UFPA), Belém, Brazil 2 Graduate Program in Health and Environmental, Tiradentes University (UNIT), Aracaju, Brazil 3 Federal University of Pará (UFPA), Bragança, Brazil 4 Laboratory for Integrated Ichthyology, Centre for Advanced Studies of Biodiversity, Federal University of Pará (UFPA), Belém, Brazil 5 Aquaculture Department, AQUOS – Aquatic Organisms Health Laboratory, Federal University of Santa Catarina (UFSC), Florianópolis, Brazil 6 Brazilian Agricultural Research Corporation (EMBRAPA), Aracaju, Brazil Correspondence Rodrigo Yudi Fujimoto, EMBRAPA‐Brazilian Agricultural Research Corporation Tabuleiros Costeiros, Av Beira Mar 3250, 49025‐040 Aracaju, SE, Brazil. Email: ryfujim@hotmail.com Funding information FAPESPA‐Amazon Foundation for Supporting Studies and Research Abstract Intensive fish farming has resulted in an increased concern for disease outbreaks. Probiotic use is one of the strategies being developed to improve fish health and productivity. Measures of probiotic colonization, growth performance, haemato‐ logical characteristics and parasite load were used to evaluate the effect of diets supplemented with Enterococcus faecium on growth and health of Arapaima gigas ju‐ veniles. A completely randomized design with four treatments (diet with E. faecium at 1 × 10 6 CFU/g and 1 × 10 8 CFU/g, control diet and diet with the culture medium MRS) and three replicates was used. Ninety‐six Arapaima juveniles were distributed in 12 cages fed with the specified diet for 68 days. Colonization of the intestinal tract by lactic acid bacteria reduced the total number of heterotrophic bacteria in fish fed with probiotics compared to controls. Fish fed a supplemented diet contain‐ ing 1 × 10 8 CFU/g presented higher values of weight gain, survival and fish growth uniformity, and lower values of feed conversion ratio. The prevalence of Trichodina sp. could have affected the survival of fish in the control group. Reduction in para‐ site load and an increase in haematocrit, the number of erythrocytes, thrombo‐ cytes, neutrophils and monocytes were also observed in fish fed the diet containing 1 × 10 8 CFU/g. Enterococcus faecium presented a probiotic effect in A. gigas juveniles and can be recommended for use at a concentration of 1 × 10 8 CFU/g to modify the gut microbiota, improve growth performance and haematology and reduce parasitic load. KEYWORDS Amazonian fish, pirarucu, probiotic, supplementation