Investigating Coral Disease Spread across the Hawaiian Archipelago.

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2017-05

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Coral diseases negatively impact reef ecosystems and they are increasing worldwide; yet, we have a limited understanding of the factors that influence disease risk and transmission. My dissertation research investigated coral disease spread for several common coral diseases in the Hawaiian archipelago to understand how host-pathogenenvironment interactions vary across different spatial scales and how we can use that information to improve management strategies. At broad spatial scales, I developed forecasting models to predict outbreak risk based on depth, coral density and temperature anomalies from remotely sensed data (chapter 1). In this chapter, I determined that host density, total coral density, depth and winter temperature variation were important predictors of disease prevalence for several coral diseases. Expanding on the predictive models, I also found that colony size, wave energy, water quality, fish abundance and nearby human population size altered disease risk (chapter 2). Most of the model variation occurred at the scale of sites and coastline, indicating that local coral composition and water quality were key determinants of disease risk. At the reef scale, I investigated factors that influence disease transmission among individuals using a tissue loss disease outbreak in Kāne‘ohe Bay, O‘ahu, Hawai‘i as a case study (chapter 3). I determined that host size, proximity to infected neighbors and numbers of infected neighbors were associated with disease risk. Disease transmission events were very localized (within 15 m) and rates changed dramatically over the course of the outbreak: the transmission rate initially increased quickly during the outbreak and then decreased steadily until the outbreak ended. At the colony scale, I investigated disease progression between polyps within individual coral colonies using confocal microscopy (chapter 4). Here, I determined that fragmented florescent pigment distributions appeared adjacent to the disease front of infected coral and had fewer intact polyps than in healthy coral fragments. These results suggested that disease progression within colonies affected with chronic and acute Montipora white syndromes are highly localized rather than systemic and their bacterial pathogens directly attack the coral tissue rather than zooxanthellae. Overall, my dissertation research indicates that watershed condition and coral community configuration can facilitate and/or inhibit coral disease spread, and that disease transmission may be more spatially constrained than previously thought.

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