Supplementary material from "Wild-caught Pacific salmon mothers enhance offspring performance via egg cortisol provisioning "

Maternal stress hormones can profoundly influence offspring development, but their adaptive significance in wild fish remains unclear. Evidence in free-living birds and mammals suggests that this maternal cue can be beneficial. Meanwhile, laboratory studies elevating cortisol levels in fish eggs reveal negative outcomes for offspring, including reduced survival, smaller body size, and a blunted stress response. Therefore, we leveraged the natural variation in egg cortisol levels among wild-caught Pacific salmon females to examine how cortisol influences female reproductive investment and offspring phenotypes. We collected over 100 female coho salmon (Oncorhynchus kisutch) from three rivers in British Columbia, Canada, and spawned them at an enhancement hatchery. We found substantial individual variation in circulating cortisol levels, which positively correlated with egg cortisol content but not investment in eggs. While higher egg cortisol was associated with increased embryo mortality, the surviving offspring exhibited enhanced metabolic capacity and stress responsiveness. The corresponding molecular responses suggest that maternal cortisol acts as a programming signal preparing offspring for stressful environments. These findings reveal a complex trade-off between early-life survival and fitness-related benefits mediated by maternal transfer of stress hormones, offering insights into how stressed mothers may prepare offspring for independence post-utilization of maternal yolk stores.