Mercury concentrations and feeding ecology of fishes in Alaska

Abstract

Mercury (Hg) is a ubiquitous contaminant found in nearly every fish species analyzed. Certain forms of Hg accumulate efficiently in fish tissues, sometimes reaching concentrations of concern for human and wildlife health when consumed. This has motivated considerable research and interventions surrounding fish consumption with Hg concentrations as the underlying cause of over 80% of fish consumption advisories in the United States and Canada. The ecological and physiological drivers that influence the concentrations of Hg in fishes are complex and vary among taxa spatially and temporally. Studying these drivers and their respective influences on Hg concentrations can help elucidate observed differences in Hg concentrations across space and time, which can then be used to improve management and consumption strategies. Here I present a series of studies focused on the chemical feeding ecology of Hg by measuring total Hg (THg) concentrations and ratios of nitrogen and carbon stable isotopes in multiple fish species from three regions in Alaska. In Chapter 2 I described foundational field collection efforts to characterize the fish communities from West Creek and the Taiya River in Klondike Gold Rush National Historical Park, and the Indian River in Sitka Historical National Park, Alaska. This chapter and agency report presents a survey of the fish species assemblage of the rivers and laid the framework for the regional analyses I conducted in the study presented in Chapter 3. In Chapter 3 I report inter- and intra-river comparisons of THg concentrations and associated feeding ecology of riparian Dolly Varden, separated by anadromous barriers in each system. I concluded that resident Dolly Varden that co-habit riverine locations with spawning salmon consume more salmon eggs than resident Dolly Varden from other locations of the same river that do not co-habit with spawning salmon. This is reflected in the isotopic composition of their tissues, and subsequently the THg concentrations of these fish are lower relative to Dolly Varden from parts of the same river above anadromous barriers. In Chapter 4, I describe regional patterns of THg concentrations and stable isotope values of carbon and nitrogen in nine species of fish and invertebrates from the Bering Sea and North Pacific Ocean along the Aleutian Islands, using Steller sea lion management zones as a spatial framework. I determine that most species from the Western Aleutian Islands have greater THg concentrations, and more negative δ¹³C values than those from the Central Aleutian Islands, indicating ecosystem-wide differences in THg concentrations and fish feeding ecology. I also determined that Amchitka Pass, a well-documented oceanographic and ecological divide along the Aleutian Island chain, aligns better with differences in THg concentrations than the boundary between Steller sea lion management zones. In Chapter 5, I report THg and methylmercury concentrations in fishes of Kotzebue Sound, including seven species that are important for subsistence users. I determined that fork length influences Hg concentrations within individual species, and that trophic relationships within a food web, a factor associated with biomagnification, influences Hg concentrations across the entire food web. I also observed that muscle tissues from virtually every individual fish had Hg loads below the State of Alaska's criteria for unlimited consumption. Taken together, the work conducted in this dissertation helps us better understand the ecological dynamics of Hg in aquatic food webs and has contributed to Hg monitoring of fish resources across parts of Alaska.