Determination of tryptophan requirements for juvenile silver catfish (Rhamdia quelen) and its effects on growth performance, plasma and hepatic metabolites and digestive enzymes activity

•Tryptophan is an essential requirement for silver catfish nutrition.•Fish were fed diets with 1.0, 1.8, 2.6, 3.4, 4.2 g/kg tryptophan.•Higher WG and PRC were obtained with diets containing 2.5 and 3.4 g Trp/kg.•Low-tryptophan diets increased gluconeogenic pathways.•Acid protease was higher in fish fed diets with 3.1 g Trp/kg.

The knowledge about nutritional requirements of fish is essential to improve utilization of nutrients. This study was conducted to estimate tryptophan requirements for silver catfish. Groups of 30 fish (4.65 ± 0.68 g) were stored in tanks (125 L) and fed mixed diets containing different tryptophan concentrations (1.0, 1.8, 2.6, 3.4 and 4.2 g/kg of diet) until apparent satiation. After 60 days, growth performance, body composition, nutrient retention, plasma and liver metabolites and digestive enzymes were determined. The data were evaluated with nonlinear and polynomial regressions, and a highly significant effect of tryptophan (P < 0.001) was observed for growth, protein retention coefficient and acidic protease activity. The lowest tested concentration (1.0 g Trp/kg) resulted in less growth and nutrient retention in the body. Fish fed inadequate tryptophan levels exhibited reduced growth, higher hepato-somatic indices and altered enzymatic and metabolic processes, indicating gluconeogenic processes. The estimated tryptophan amino acid requirements for maximum weight gain and protein retention coefficients ranged from 2.5 to 3.4 g/kg depending on the statistical model used for the estimation.