Changes in ocean water temperature associated with global climate change are bound to enormously affect fish populations, with potential major economic consequences in the aquaculture and fisheries industries. A link between temperature fluctuations and changes in fish stress response is well established. In this study, we aimed to assess the effects of a short- (4 days) or a long-term (4 months) exposure to warm temperature in the stress physiology of European sea bass (Dicentrarchus labrax) larvae and juveniles. First, cortisol (i.e. the main stress hormone in fishes) analysis was used to confirm that a steady and short-term elevation of temperature acts as a physiological stressful event in these fish, and cortisol release is indeed above a metabolic increase linked to temperature. Moreover, our results verified that measurement of cortisol released into the water can be reliably employed as a non-invasive indicator of acute thermal stress in experimental conditions. Secondly, the different effects on the genetic cascade underlying the stress response between long-term low or high thermal treatments were evaluated at two larval development stages via candidate-gene and whole-transcriptome approaches. Interestingly, opposite expression for some key stress genes (nr3c1, nr3c2 and hsd11b2) were observed between developmental stages, highlighting the distinct adaptive mechanisms controlling the primary and secondary responses to a stressor. Surprising expression patterns for some understudied genes involved in the stress axis were also revealed, including crhr1, mc2r, mc5r, trh or trhr, which should be further explored. Finally, evaluation of cortisol content in scales was successfully used as a biomarker of chronic thermal stress, with 10x more cortisol in fish kept at 21 °C vs 16 °C after 4 months, supporting the gene expression results observed. The use of such a method as a proxy of long-term stress, unprecedented in the literature, holds a vast array of applications in further research, in particular, in the investigation of the impact of global warming on wild fish populations.

与全球气候变化有关的海水温度变化势必会极大地影响鱼类种群，对水产养殖和渔业行业产生潜在的重大经济影响。温度波动与鱼类应激反应变化之间的联系已得到很好的建立。在这项研究中，我们旨在评估短期（4天）或长期（4个月）暴露于温暖温度对欧洲鲈鱼（Dicentrarchus labrax）幼虫和少年的应激生理的影响。首先，皮质醇（即鱼类中的主要应激激素）分析用于确认温度的稳定和短期升高是这些鱼类的生理应激事件，而皮质醇的释放确实高于与温度有关的代谢增加。此外，我们的结果证实，释放到水中的皮质醇的测量可以可靠地用作实验条件下急性热应激的非侵入性指标。其次，通过候选基因和全转录组方法，在两个幼体发育阶段，评估了长期低热处理或高热处理对应激反应的遗传级联的不同影响。有趣的是，某些关键应激基因（nr3c1，nr3c2和hsd11b2）的表达相反）在发育阶段之间观察到，突出了控制对应激源的主要和次要反应的独特适应机制。还揭示了涉及应力轴的一些未充分研究的基因的令人惊讶的表达模式，包括crhr1，mc2r，mc5r，trh或trhr，应进一步探讨。最终，鳞片中皮质醇含量的评估成功地用作了慢性热应激的生物标记，四个月后，将鱼类中的皮质醇保持在21°C和16°C相比，增加了10倍，支持了观察到的基因表达结果。这种方法作为长期压力的替代手段，在文献中是前所未有的，在进一步研究中，尤其是在研究全球变暖对野生鱼类种群的影响方面，具有广泛的应用。