Effects of dietary tryptophan and chronic stress in gilthead seabream (Sparus aurata) juveniles fed corn distillers dried grains with solubles (DDGS) based diets

Abstract

Distillers' dried grains with solubles (DDGS) has low tryptophan (Trp) relatively to the branched-chain amino acids (BCAA) levels, and this may reduce transport of Trp through the blood-brain barrier due to competition for the same transport carrier. This may affect synthesis and release of serotonin, with negative consequences in stress tolerance. In the present study, it is hypothesized that a Trp/BCAA unbalance in high DDGS diets may impair the capacity of gilthead seabream (Sparus aurata) juveniles to cope with chronic stress induced by high stocking density. Three DDGS-based diets (30%DDGS+13%FM) were formulated and supplemented with Trp at 0, 0.13, and 0.25% of the diet and tested in triplicate, at two initial stocking densities (5 and 16 kg m−3), in a 2 × 3 total randomized factorial design. The growth trial was performed with 12 g fish and lasted 64 days. Irrespective of the diet, high stocking density reduced growth performance and feed intake, but not feed efficiency. Plasma protein, triglycerides, and cholesterol levels; whole-body lipid, hepatosomatic index, and liver glycogen; hepatic activity of key-enzymes of glycolysis and lipogenesis were also reduced. Moreover, plasma glucose level and hepatic activity of key-enzymes gluconeogenesis were increased.Irrespective of stocking density, diets supplementation with Trp did not affect growth and feed efficiency, but increased hepatic lipase activity and reduced liver lipids, plasma triglycerides and cholesterol levels, and hepatic activity of key-enzymes of amino acid catabolism. Moreover, dietary Trp supplementation restored plasma glucose levels of fish kept at high stocking density to levels similar to that of fish kept at low stocking density.Overall, present results indicate that high stocking density reduced growth performance without affecting feed efficiency of gilthead seabream. Dietary Trp supplementation did not counteract the negative effect of stocking density on growth performance but seemed to mitigate stress response of gilthead seabream juveniles kept at high stocking density.