Hypoosmotic challenge induces an acute stress response in European seabass with limited modulation by diets containing insect meal

More frequent and severe torrential rainfall and flooding are expected in Northern Mediterranean countries, which may exacerbate salinity drops in European seabass (Dicentrarchus labrax) farms. Based on the hypothesis that diet could reduce stress caused by climate-related events, this study evaluated: 1) the physiological response of seabass to hypoosmotic stress, and 2) its modulation by diets with increasing levels of insect meal (IM; a mixture of Hermetia illucens and Tenebrio molitor). A control diet (15% fishmeal) and three other diets, in which the IM mixture replaced 3–50% fishmeal protein, were provided for 75 days at 35 ppt. Then, fish were exposed to water at 5 ppt for four hours. The hypoosmotic challenge reduced plasma Na⁺, Cl⁻, and Mg²⁺ levels and induced an acute stress response, evidenced by increased plasma cortisol, glucose, and lactate. This was accompanied by downregulation of hypothalamic corticosteroid receptors and genes associated with neurogenesis. Plasma lysozyme levels and the activity of hepatic antioxidant enzymes, namely superoxide dismutase and glutathione peroxidase (GPx), also increased after the challenge, while lipid peroxidation remained unaltered. Overall, the IM diets had little capacity to modulate the stress response, although fish fed higher IM levels tended to show a smaller increase in cortisol after the challenge. GPx activity was the only parameter showing a significant diet × stress interaction, with a larger increase after the hypoosmotic challenge in fish fed higher IM levels. In conclusion, the hypoosmotic challenge induced a stress response in European seabass, but modulation by the IM diets was limited.