Adult chinook salmon heat stress and reproductive consequences in Southcentral Alaska

Abstract

This thesis explores the causes and potential fitness consequences of heat stress on spawning Chinook salmon (Oncorhynchus tshawytscha) in an era of rapid warming in Alaska. Cook Inlet, Southcentral Alaska, accounts for nearly half of the state's total sport, personal use, and subsistence harvest of hatchery-produced Chinook salmon. Southcentral Alaska provides an excellent opportunity to understand the potential for hatcheries to interact with climate warming and increasing habitat alteration with human development. The first objective of this thesis is to improve understanding of the factors associated with heat stress responses among and within Chinook salmon populations in relation to water temperatures. Extensive field surveys were conducted on prespawn adult Chinook salmon in two adjacent rivers in Southcentral Alaska to collect a known biomarker heat shock protein 70 (HSP70) as a non-lethal tool for thermal stress quantification. The research team for this study quantified HSP70 abundance in 39 female Chinook salmon of both hatchery and wild origin in the relatively cool, glacial-fed Crooked Creek in 2020 and in 144 wild and hatchery Chinook salmon, majority female, in the warmer groundwater-fed Ninilchik River in 2020 and 2021. We also sampled an additional 130 broodstock Chinook salmon immediately after gamete collection and extended holding in raceways and handling next to the Ninilchik River. To compare HSP70 expression across river populations, rearing conditions, and hatchery broodstock holding, we used generalized additive models that allow for non-linear responses between heat stress and temperature. I, with help of a larger research team, found that Ninilchik River Chinook salmon exhibited lower HSP70 levels at a given temperature than Crooked Creek Chinook salmon, in part challenging the notion that the relatively warm system would result in greater heat stress. We also found a hatchery effect, with wild individuals expressing higher HSP70 than those with hatchery origins from both rivers. Our most prominent and consistent result was a significant difference in HSP70 between the Ninilchik River Chinook salmon that were handled for gamete collection and those that were not, showing that the additional handling for associated hatchery programs in these rivers appears to elevate the HSP70 response without heat stress-inducing temperatures. My second goal is to quantify the impact of heat stress on aspects of reproductive performance. I was able to quantify a prespawn mortality rate in female Chinook salmon as well as calculate an average number of days that wild Chinook salmon spent on the Ninilchik River spawning grounds. I also analyzed reproductive success in the hatchery by associating Ninilchik River Chinook salmon HSP70 levels to their progeny's egg survival to the eyed stage under controlled conditions. We found that there was no significant relationship between HSP70 concentration and egg survival to the eyed stage. Managers have consistently observed low egg survival of Ninilchik River hatchery fish, and this work suggests temperature effects and heat stress are unlikely to be the primary cause of that mortality. Applied aspects of the work suggest managers should focus efforts on limiting handling and holding stressors while collecting broodstock in the Ninilchik River rather than focusing on temperature per se. While the ultimate consequences of heat stress for the reproductive success of Chinook salmon remain unclear, taken as a whole, this work sheds light on some of the factors that may facilitate or impede adaptive responses to warming conditions by adults of this species.