A natural additive in the diet to improve growth and reduce energy expenditure of gilthead seabream (Sparus aurata L.): Attenuation of high stocking density stress responses

Fish welfare is gaining momentum due to consumers' trends and legal regulations. Therefore, aquaculture industry is in the way of mitigating stress to the animals and, thus, improving the physiological status, with the consequent increase in economical profits. As a consequence, feed additives have been proposed as potential stress relievers through the diet. However, the effectiveness of these extracts on stress responses and growth regulation is still to be defined for different species and stressors. Here we show the effects of the commercial natural-additive "Relaquax" on growth performance, endocrine regulation of hypothalamic-pituitary-interrenal (HPI) and somatotropic (GH/IGF-I) axes, as well as intermediary metabolism, of gilthead seabream (Sparus aurata L.) reared in low and high stocking density conditions. For this purpose, S. aurata juveniles were maintained for 3 months at different combined conditions of feeding (control diet or supplemented with the additive) and initial stocking density (low or high; 4.0 or 11.5 kg m(-3), respectively). At the end of the experimental period, brain, pituitary, head kidney and liver gene expression of endocrine factors involved in stress and growth regulation were determined. In addition, liver and muscle enzyme activities of the intermediary metabolism of carbohydrates, lipids and amino acids were assessed. Our results showed an increased growth performance in fish fed with the additive, mediated through hepatic igf1 enhanced expression at both stocking densities. In the same way, higher body mass was associated to inhibition of lipid catabolism in muscle (decreased GPDH and HADH activities), as a consequence of diet supplementation. The additive has also reduced amino acid catabolism in liver (decreased AST activity), as well as enhanced glycolysis in muscle (increased PK activity). In conclusion, this additive was confirmed as a natural alternative to improve aquaculture and welfare of seabream, especially under high stocking density conditions.