Assessing the Role of Incubation Temperature as a Barrier to Successful Establishment of Coho Salmon (<scp><i>Oncorhynchus kisutch</i></scp>) in a Rapidly Warming Arctic

Warming associated with climate change is driving poleward shifts in the marine habitat of anadromous Pacific salmon (<i>Oncorhynchus</i> spp.). Yet the spawning locations for salmon to establish self-sustaining populations and the consequences for the ecosystem if they should do so are unclear. Here, we explore the role of temperature-dependent incubation survival and developmental phenology of coho salmon (<i>Oncorhynchus kisutch</i>) as a potential early life history barrier to establishment in an Arctic stream. We exposed embryos to temperatures previously recorded in the substrate of an Arctic groundwater spring-fed spawning environment. Using a common garden experimental design, coho salmon embryos were exposed to treatments that thermally mimicked four spawning dates from August 1 to October 1 (AUG1, SEPT1, SEPT15, and OCT1). Spawning temperatures were 6°C at the warmest (AUG1) and 1.25°C at the coldest (OCT1). We observed low survival rates in SEPT1 (41%) and OCT1 (34%) and near complete mortality in the other treatments. While far below what is considered normal in benign hatchery-like conditions, these rates suggest that temperatures experienced at these spawning dates are survivable. We detected differences in developmental rates across treatments; embryos developed 1.9 times faster in the warmest treatment (AUG1, 120 days) compared to the coldest (OCT1, 231 days). Differences in accumulated thermal units (ATUs) needed for hatching ranged from 392 ATUs in AUG1 to 270 ATUs in OCT1, revealing compensation in developmental requirements. Given these findings, the most thermally suitable spawning dates within our study are between September 15 and October 1, which facilitates hatching and projected nest emergence to occur in spring warming conditions (March-September). Broadly, our findings suggest that spawning sites within thermal tolerances that can support the survival and development of coho salmon exist in the North American Arctic. Whether the habitat is otherwise suitable for transitions through other life stages remains unknown.