Exposure to exogenous egg cortisol does not rescue juvenile Chinook salmon body size, condition, or survival from the effects of elevated water temperatures

Climate change is leading to altered temperature regimes which are impacting aquatic life, particularly for ectothermic fish. The impacts of environmental stress can be translated across generations through maternally-derived glucocorticoids, leading to altered offspring phenotypes. Although these maternal stress effects are often considered negative, recent studies suggest this maternal stress signal may prepare offspring for a similarly stressful environment (environmental match). We applied the environmental match hypothesis to examine whether a prenatal stress signal can dampen the effects of elevated water temperatures on body size, condition, and survival during early development in Chinook salmon Oncorhynchus tshawytscha from Lake Ontario, Canada. We exposed fertilized eggs to prenatal exogenous egg cortisol (1000ng*mL-1 cortisol or 0ng*mL-1 control) and then reared these dosed groups at temperatures indicative of current (+0°C) and future (+3°C) temperature conditions. Offspring reared in elevated temperatures were smaller and had a lower survival at the hatchling developmental stage. Overall, we found that our exogenous cortisol dose did not dampen effects of elevated rearing temperatures (environmental match) on body size or early survival. Instead, our eyed stage survival indicates that our prenatal cortisol dose may be detrimental, as cortisol-dosed offspring raised in elevated temperatures had lower survival than cortisol-dosed and control reared in current temperatures. Our results suggest that a maternal stress signal may not be able to ameliorate the effects of thermal stress during early development. However, we highlight the importance of interpreting the fitness impacts of maternal stress within an environmentally relevant context.气候变化正导致温度格局发生改变，进而威胁水生生物的生存，其中尤以变温鱼类（ectothermic fish）受到的影响最为显著。环境胁迫的影响可通过母体源性糖皮质激素（maternally-derived glucocorticoids）跨代传递，最终导致子代表型发生改变。尽管这类母体胁迫效应常被视为有害，但近期研究显示，此类母体胁迫信号或可使子代提前适应相似的胁迫环境，即环境匹配假说（environmental match）。本研究应用环境匹配假说，旨在探究产前胁迫信号能否缓解水温升高对加拿大安大略湖奇努克鲑（Oncorhynchus tshawytscha）早期发育阶段的体长、身体状况及存活率的影响。我们将受精卵暴露于产前外源卵皮质醇（exogenous egg cortisol）处理中：处理组施加1000ng·mL⁻¹皮质醇，对照组施加0ng·mL⁻¹皮质醇；随后将各组幼鱼分别饲养在代表当前（+0℃）与未来（+3℃）水温条件的环境中。饲养于高温环境中的子代个体体型更小，且孵化阶段的存活率更低。总体而言，本研究发现外源皮质醇处理并未缓解高温饲养对体长及早期存活率的影响，未达到预期的环境匹配效应。相反，眼点期（eyed stage）存活率数据表明，产前皮质醇处理或存在不利影响：高温环境中饲养的皮质醇处理组子代，其存活率低于当前水温下饲养的皮质醇处理组与对照组子代。本研究结果提示，母体胁迫信号或无法缓解早期发育阶段的热胁迫影响。但本研究同时强调，需在符合环境相关性的背景下解读母体胁迫对适合度（fitness）的影响。