Gastrulation and hatch as critical thermal windows for salmonid embryo development

Climate change and impoundment increase river temperatures, shifting the bioclimatic envelope in which freshwater biota have evolved and increasing salmonid egg mortality. To mitigate this, conservation flows from reservoirs are often implemented to maintain favourable water temperatures downstream from impoundments throughout salmonid embryo development. However, as water to maintain conservation flows becomes scarcer, there is a need to understand the requirements of salmonid embryos and balance these with anthropogenic demands. This study combines a laboratory‐based and a modelling approach to test the effect of different temperatures on the survival from fertilisation to hatch of a model salmonid species. Further, the effect of dropping temperatures from high to optimal conditions at hatch—a perceived period of greater sensitivity to high temperatures—is tested. The study shows embryo mortality increases with temperature and is greatest during gastrulation and hatch. Also, embryos that experienced high temperatures during gastrulation had high mortality rates at hatch, even when hatch conditions were optimal. This indicates sublethal developmental abnormalities caused by high temperatures during gastrulation increase mortality at hatch. Therefore, to maintain high rates of salmonid embryo survival, cold water resources from reservoirs ideally will target both gastrulation and hatch developmental stages.