The non-lethal threat of hypoxia: ecological effects and physiological responses of estuarine species

Abstract

Hypoxia [dissolved oxygen (DO) < 2 mg/L] is one of the key threats to some of the most productive regions of the marine environment (e.g., estuaries). Although mortality can occur, mobile organisms have the potential to avoid the most severe low oxygen conditions, but suffer ecologically significant indirect and sublethal impacts as a result. In Washington State, USA, a fjord estuary of the Puget Sound marine ecosystem, known as Hood Canal (110 km), regularly experiences seasonal hypoxia. My dissertation addresses several important gaps in the current knowledge pertaining to the non-lethal biological effects of hypoxia on the mobile benthic and pelagic species of Hood Canal. Using acoustic telemetry, Chapter 1 quantified movement patterns and distributional shifts of Dungeness crab (Metacarcinus magister) and English sole (Parophrys vetulus), two abundant demersal species. Although highly mobile, both species displayed more localized, rather than large-scale, directional movement relative to hypoxia. Dungeness crab in particular showed significant distributional shifts towards shallower waters. Chapter 2 expanded the investigation of hypoxia’s influence in the estuary by assessing spatiotemporal patterns of nearshore macrofaunal community composition. Using underwater video monitoring, the hypoxic region of Hood Canal was found to be primarily composed of hypoxia tolerant invertebrates and fewer fish species compared to the more oxygenated region. Additionally, tolerant and sensitive species displayed distinct DO-tolerance thresholds and responses to reduced oxygen levels. In Chapter 3, we employed experiments and field sampling to measure the hepatic mRNA expression of hypoxia-inducible factor-1α (HIF-1α) as a biomarker for hypoxia exposure of Pacific herring (Clupea pallasii). In the lab, HIF-1α mRNA increased in a threshold and a dose-like response under various hypoxia treatments. In the field, the spatial pattern of gene expression was counter to the hypoxia gradient in the estuary, yet the more severe hypoxic time period appeared to correspond with higher mRNA levels. Chapter 4 investigated the generalized relationship between hypoxia and Dungeness crab harvest (3-S) management strategy, an important fishery in Puget Sound. Inferred by the shoaling behavior from the Chapter 1 study, an age-structured population model was constructed to test several hypoxia-scenarios with other stressors, including harvest, illegal crab fishing, and incidental capture mortality. We found the 3-S management strategy most sensitive to the influence of hypoxia when other sources of demographic restrictions were considered, underscoring the uncertainty associated with a data-poor species under multiple anthropogenic and environmental stressors.