Interaction of disease and temperature on the aerobic scope of freshwater fish and implications for changing climates

Climate change is a major threat to both freshwater and marine ecosystems on a global scale. There is evidence that for aquatic pathogens and parasites, increasing water temperatures will favour increasing transmission rates and virulence. Increasing water temperature may stress fish and transiently compromise the immune system, exacerbating the effects of infection. The high energy costs of an upregulated immune response will have consequences on other physiological processes such as growth and reproduction. In order to study the effects of temperature on pathogenicity, a bioassay to challenge the Australian native freshwater pygmy perch, Nannoperca vittata with a bacterium Photobacterium damselae damselae was carried out. The bioassay was carried out at two temperatures; 17°C, thought to be the optimum for fish survival and growth, and 28°C, presumed to be approaching the upper critical limit for the species. Nannoperca vittata was found to be susceptible to infection by P. damselae damselae, the first time that infection has been demonstrated in native Australian freshwater fishes. The effect concentration that caused mortalities in 50% of the population (EC50) was lower at 28°C (4.82x105 CFU ml-1) than at 17°C (2.81x106 CFU ml-1). Fish mortalities were significantly greater and times to death were significantly shorter at 28°C than at 17°C and this is the first time that this has been demonstrated experimentally for P. damselae damselae. Following the pathogenicity trial, the aerobic scope of exposed versus non-exposed pygmy perch of both 17°C and 28°C experimental groups was determined in a respirometer. Aerobic scope is defined as the difference between maximum metabolic rate and standard metabolic rate. The aerobic scope was greater in exposed than in unexposed fish at 17°C and conversely, greater in unexposed than in exposed fish at 28°C.. This difference occurred because standard metabolic rate increased with exposure at both temperatures, whereas maximum metabolic rate increased at 17°C, but not at 28°C. The increase in standard metabolic rate at both temperatures was expected as a consequence of an upregulated immune response following exposure to the bacterium. The difference in maximum metabolic rate is hypothesised to be a consequence of a compensatory increase in oxygen carrying capacity, which is counteracted by a persistent immune response at 28 but not at 17°C. This hypothesis requires further testing. The results from this study suggest, firstly, that fish are less tolerant of infection at higher water temperatures and secondly, that a combination of higher water temperature and increased exposure to pathogens may decrease aerobic scope and therefore fitness. This study showed that higher temperatures decrease aerobic scope in fish, thus suggesting that rising temperatures with global warming may have the same effect. This is the first study to demonstrate such a response in an Australian native freshwater fish. Further research in this field is urgently required to enable conservation management plans that address the threats posed to native freshwater fish species through climate change