Sara Cartan

Phytogenic compounds like carvacrol and thymol are gaining attention as functional additives to improve welfare and stress resilience in aquaculture; however, their underlying physiological mechanisms remain unclear. This study evaluated the effects of the dietary inclusion of carvacrol and thymol on the HPI axis and Gh/Igf system under chronic stress in European seabass (Dicentrarchus labrax). For this purpose, fish were fed either a control diet or a carvacrol- and thymol-supplemented diet for 71 days at both low and high stocking densities. At the end of the feeding period, the expression of several endocrine factors of the HPI axis and Gh/Igf system in the brain, pituitary, head kidney, liver and skeletal muscle was assessed. In addition, growth performance, as well as plasma and mucosal cortisol and secondary stress biomarkers, were evaluated. High stocking density impaired growth performance, increased brain crh and crhbp expression, and reduced plasma TAG, demonstrating HPI axis activation and higher energy demand under this stressful condition. Chronic stress also upregulated brain and pituitary ghra expression, suggesting a Gh-associated increased catabolism. Conversely, dietary carvacrol and thymol attenuated HPI axis activation, as well as brain and pituitary Gh receptors expression. Furthermore, regardless of stocking density, this dietary supplementation upregulated ghra, ghrb, and nr3c1, suggesting a stimulated liver metabolism, and improved mucosal health status (decreased cortisol levels and enhanced total immunoglobulin content). Overall, carvacrol and thymol exert a regulatory effect on stress physiology, attenuating the stress response to high stocking density and enhancing resilience in European seabass.