The use of essential oils (EO's) as feed additive has been explored as tool to improve fish growth and health. The aim of this study was to evaluate the oregano essential oil (OEO) (Origanum vulgare) dietary supplementation for the catfish Lophiosilurus alexandri. Four diets supplemented with different OEO levels (0.0, 1.0, 2.0 and 4.0 g kg−1) were evaluated in a feed trial conducted with fish of 15.7 ± 1.8 g and 9.5 ± 0.45 cm for 13 weeks. Were evaluated growth performance, biochemical parameters, intestinal histomorphometry, whole‐body composition and muscle growth. Polynomial analysis showed that weight gain, length gain and specific growth rate were improved in fish fed with OEO supplemented diets, being 2.37, 2.56 and 2.33 g kg−1 the respective levels that maximize these variables. Plasma glucose, cholesterol, triglycerides, LDL and HDL increased with OEO levels between 1.80–2.47 g kg−1. Intestinal histomorphometry was not affected by OEO. Body protein content and hyperplasic muscle fibres were maximized with 2.10 and 2.25 g kg−1 OEO, respectively. Body moisture, total lipids, ash and hypertrophic muscle fibres were not affected by OEO. In conclusion, dietary supplementation with 2.10–2.56 g kg−1 OEO improves growth performance, body protein retention and muscle growth of L. alexandri.

Given the ecological unsustainability of using fish oil (FO) as the main source of longchain polyunsaturated fatty acids (LC-PUFA), the investigation of alternative dietary LC-PUFA sources is crucial for aquaculture. Tambaqui (Colossoma macropomum) is a valuable economic aquaculture resource in Brazil, capable of endogenously elongating and desaturating linoleic (LA; 18:2n-6) and α-linolenic (ALA; 18:3n-3) acids to longer and physiologically vital LC-PUFA. Yet, it is unclear how this pathway is regulated by different oil sources in tambaqui. Thus, we designed an experiment with 2 different oil diets (fish oil-FO and vegetable oil-VO) at 2 different concentrations (5% or 10%) to investigate the molecular regulation of fatty acid biosynthesis in this species. We observed no differences in survival, body weight gain (BWG) and feed conversion ratio between the treatments. Gene expression analysis shows that fads2 and elovl5 are upregulated in liver, while fads2 and elovl2 are up-regulated in brain of tambaqui fed with VO. The transcription factors pparβb and pparγ are also up-regulated in the brain by VO diet, when compared to FO diet. The VO diet also contributed to the biosynthesis of LC-PUFA in liver and specifically DHA in the brain. Overall, our approach shows that lipid metabolism-relevant genes are regulated by different dietary lipid sources.