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Abstract
Temperature is a critical factor influencing both hosts and parasites in aquatic systems. As climates continue to warm, changing temperature regimes are likely to alter the relationship between temperature and host-parasite interactions. However, the direction and magnitude of climate effects on diverse host-parasite systems is unclear, with some systems experiencing increases in parasite infection prevalence and disease-induced mortality while others are predicted to show declines in parasite diversity and abundance. Fisheries species are essential to ecosystem function and human livelihoods, and many are imperiled due to overfishing and habitat degradation. Therefore, it is critical to understand more about the environmental drivers of parasitism in economically and culturally important fisheries hosts. Using a combination of experiments, statistical modeling, and mechanistic modeling, I investigate how temperature influences host-parasite relationships in fisheries systems. My empirical work centers around shrimp black gill disease (sBG), which is caused by the apostome ciliate Hyalophysa lynni infecting commercial shrimp (both White Litopeneus setiferus and Brown Farfantepeneus aztecus shrimp) in the South Atlantic Bight and Gulf of Mexico. First, I investigated how seasonally changing temperatures influenced the rate of transmission, host recovery, and parasite-induced mortality in a series of laboratory and field experiments. These experiments suggested that seasonal changes in transmission were responsible for producing the patterns of prevalence in wild shrimp but provided mixed evidence regarding the role of seasonally varying host recovery. I then used this information to develop a mechanistic model that tested hypotheses regarding the transmission mode of H. lynni and the role of seasonally varying transmission and recovery on sBG prevalence. These models indicated that variable transmission was required to reproduce patterns of field prevalence. Next, I investigated how the relationship between sBG prevalence and environmental variables may have shifted over longer multi-annual timescales through a time-series analysis of monthly field prevalence data spanning nearly 20 years. This analysis highlighted strong associations of temperature with shifting patterns of sBG prevalence and indicated that over the course of the time series, the timing of peak prevalence has shifted to earlier during the summer. Finally, I examined the role of temperature on host-parasite interactions across multiple fisheries systems through a meta-analysis that investigated the relationship between temperature and parasite-induced mortality, and how this relationship was altered by host and parasite traits. I found that across diverse host-parasite systems, parasite-induced mortality increased with increasing temperature, and that hosts infected by opportunistic parasites, and hosts from the Order Salmoniformes, were more sensitive to increases in mortality with temperature than average. Collectively, my work exemplifies the importance of temperature in aquatic host-parasite systems and highlights the utility of addressing questions relevant to both fisheries managers and the scientific community.