Using strontium isotopes to track Pacific salmon migrations in Alaska

Abstract

Pacific salmon (Oncorhynchus spp.) are an important cultural, ecological, and economic natural resource in Alaska. Not only do salmon maintain an important mechanism of nutrient transport between marine, aquatic, and terrestrial ecosystems, but they also provide a sustainable food and economic resource for human communities. A challenging issue in the management, conservation, and research of Pacific salmon is tracking their responses to perturbations across the multiple scales of population structure that characterize these species. Research has shown how the inherent biodiversity of Pacific salmon imparts resiliency to environmental change, and temporal stability to their overall productivity and the human systems dependent upon such productivity (e.g., fisheries). The vast biodiversity of salmon arises primarily via precise natal homing of adults to their rivers of origin, resulting in locally adapted populations. Thus, there have been considerable efforts to develop methods to effectively manage and monitor Pacific salmon biodiversity. One important example is using genetic differentiation among populations to discern the relative contributions of genetically distinct stocks in mixed stock fishery harvests. In the Bristol Bay region, sockeye salmon (O. nerka) harvests can be discerned at the watershed level (i.e., the nine major watersheds contributing to the fishery). However, tens to hundreds of locally adapted populations exist within each of these watersheds and methods to apportion fishery harvests to this finer scale population structure are lacking. This dissertation presents a new method in Alaska to discern fine-scale population structure (i.e., within watersheds) of Chinook salmon (O. tshawytscha) harvests using a naturally occurring geochemical tracer in rivers, strontium (Sr) isotopes (⁸⁷Sr/⁸⁶Sr). To this end, in Chapter 1, I characterize the statewide geographic variation on multiple spatial scales in ⁸⁷Sr/⁸⁶Sr ratios of Alaska's rivers and discuss the geochemical and geological controls of observed ⁸⁷Sr/⁸⁶Sr ratios. In Chapter 2, I approach the persistent problem of evaluating site-specific temporal variation, especially in remote Subarctic and Arctic regions, by employing the non-migratory behavioral ecology of slimy sculpin (Cottus cognatus). Finally, in Chapter 3, I demonstrate how the development of temporally and spatially robust ⁸⁷Sr/⁸⁶Sr baseline datasets within the Nushagak River was able to apportion a mixed stock fishery harvest of Chinook salmon conducted in Nushagak Bay back to natal sources at the sub-basin watershed level. Because of the conservative nature of the ⁸⁷Sr/⁸⁶Sr ratio during physical and biological processes, the development of this method is applicable not only to Chinook salmon, but also to other salmon species (e.g., sockeye and coho salmon, O. kisutch). Additionally, the development of baseline ⁸⁷Sr/⁸⁶Sr information (e.g., waters) and an overall research framework to employ this tracer in provenance studies, have statewide implications for the research and management of other migratory animals.