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The relationship between thermal tolerance and hypoxia tolerance in Amazonian fishes Jung, Hyewon Ellen


The Amazon contains 20% of the world’s freshwater fish species that are predicted to experience an increase in temperature by up to 2.2 to 7˚C within the next century. An increase in temperature will likely be associated with an increase in the frequency, duration, and magnitude of hypoxic bouts, creating an even greater challenge. Thermal tolerance may be limited by the ability to supply and deliver enough oxygen to tissues at critical temperatures, as is the case for hypoxia tolerance, thus both may be associated with similar mechanisms in fish. A direct relationship between thermal and hypoxia tolerance however, has not yet been investigated in a wide range of fish species. To address this, I conducted acute thermal tolerance (CTMax) and hypoxia tolerance (% air saturation at loss of equilibrium) assays in 20 species that spanned a broad phylogenetic range. In fish acclimated to the temperatures within the current temperature range of the Amazon River (28 or 31˚C), I found a positive relationship between CTMax and hypoxia tolerance. In fish acutely transferred to higher temperatures of 33 or 35˚C, there was a reduction in hypoxia tolerance relative to that at 28 or 31˚C. Acclimation (10 days or 4 weeks) to 33 or 35˚C did not increase hypoxia tolerance, and in some species there was a further reduction in hypoxia tolerance. Acclimation to 33 or 35˚C (10 days or 4 weeks) and exposure to hyperoxia (>200% air saturation) increased CTMax, although in most species only moderately. One of the most significant findings of my thesis was that most species failed to acclimate to the higher temperatures: of the 13 species investigated, 2 species did not survive 10 days or 4 weeks (chronic lethal maximum) at 31˚C, 9 species did not survive at 33˚C, and only 2 species survived 35˚C. Overall, acclimation to higher temperatures that are predicted to occur within the next century had little or no effect on thermal tolerance and reduced hypoxia tolerance indicating that the high fish biodiversity of the Amazon may be at risk given the predicted changes in temperature and hypoxia associated with climate change.

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