Broad whitefish (Coregonus nasus) ecology and habitat use in Arctic Alaska: spawning habitat suitability, isotopic niches, life-history variations, and climate change risks to subsistence fisheries

Abstract

Broad Whitefish (Coregonus nasus) is a critically important subsistence species for Alaska's Indigenous communities, yet little is known about the basic ecology of this species at the individual level. Understanding habitat use by Broad Whitefish is challenging due to their mobility and our limited ability to track fish throughout their lives as they move among a suite of habitats that are spatially dispersed, change over time, and are often temporary. The Arctic is undergoing major landscape and ecosystem transformation from climate change and oil and gas development, which may threaten Arctic ecosystems used by Broad Whitefish. This dissertation presents new information on the ecology of Broad Whitefish captured in the Colville River, Alaska. In Chapter 1, an intrinsic potential (IP) model for Broad Whitefish was used to estimate the potential of streams across the watershed to provide spawning habitat. Results were compared with movement patterns of radio-tagged prespawn Broad Whitefish. In Chapter 2, ecological niches utilized by Broad Whitefish were investigated via stable isotope analyses of muscle and liver tissue and otoliths from mature fish. In Chapter 3, strontium isotope (⁸⁷Sr/⁸⁶Sr, ⁸⁸Sr) otolith chronologies across individuals' lives were used to quantify life-history attributes and reconstruct migration patterns of fish. Finally, in Chapter 4, the current understanding of ongoing and future changes to the habitat, productivity, and behavior of Broad Whitefish were summarized to assess risks facing Arctic freshwater ecosystems and fishes more broadly. IP model results showed the majority of habitat with high IP (≥ 0.6) was located within the braided sections of the main channel, which encompassed > 1,548 km, and starting in mid-July, prespawn fish used habitats in the middle and lower watershed. Stable isotope analysis revealed a range of [delt]¹³C (-31.8- -21.9‰) and [delta]¹⁵N (6.6- 13.1‰) across tissue types and among individuals. Cluster analysis of muscle tissue δ¹³Cˈ, δ¹⁵N, δ¹⁸O, and δD indicated that Broad Whitefish occupied four different foraging niches that relied on marine-and land-based (i.e., freshwater and terrestrial) food sources to varying degrees across the summer period. Strontium isotopes revealed six main life histories, including three anadromous types (59%), one semi-anadromous type (28%), and two nonanadromous types (13%), suggesting greater complexity in life-history types than previously documented. Climate change is expected to continue to alter Arctic hydrology and, therefore, suitability, connectivity, and availability of habitats critical for Broad Whitefish population persistence. Warming and lengthening of the growing season will likely increase fish growth rates; however, the exceedance of threshold stream temperatures will likely increase physiological stress and alter life histories, which is likely to have mixed effects on Arctic subsistence fishes and fisheries. This information on Broad Whitefish spawning intrinsic potential, foraging niches, and life histories provides crucial knowledge to understand critical habitats used across time and space, which will help managers and conservation planners better understand the risks of anthropogenic impacts, such as climate change and oil and gas development, and help conserve this vital subsistence resource.