Hot and already bothered: exploring effects of warming waters on an imperiled freshwater fish, Pugnose Shiner «Notropis anogenus»

Climate change has emerged as an increasingly critical threat to freshwater systems that may affect freshwater fishes through direct effects on their thermal physiology, but also indirectly by altering the magnitude of biotic interactions and/or the relative abundance of co-habiting species. Understanding the direct and indirect effects of temperature represents a significant knowledge gap that may be vital to effective conservation strategies for freshwater fishes. This is especially true for already imperiled species because these taxa tend to be more sensitive to changes in their environment and are already contending with other, and potentially interactive, stressors. In this thesis, I used a combination of ecophysiology and field-based techniques to examine effects of elevated water temperature on Pugnose Shiner, Notropis anogenus, an endangered fish species in Canada. In Chapter 1, I investigated effects of temperature on critical thermal maximum (CTmax) and standard metabolic rate (SMR) in juvenile Pugnose Shiner reared for 4 months across five different temperatures. CTmax was measured under normoxia and acute exposure to hypoxia to test for oxygen sensitivity of upper thermal limits in this species. CTmax increased with elevated water temperature. Agitation temperature (temperature at which fish show behavioural signs of thermal stress) also increased with rearing temperature and occurred on average 4.32°C above acclimation temperatures. CTmax was lower when fish were acutely exposed to hypoxia during CTmax trials and showed no increase between 19 and 25°C. Surprisingly, CTmax under acute hypoxia increased sharply at the highest temperature. SMR increased between 16 and 25°C, and then declined significantly at 28°C, indicating that the ability of Pugnose Shiner to reverse thermodynamic effects on SMR is limited and there was no evidence of long-term thermal compensation. Interestingly, gill size (e.g. total gill filament length, hemibranch area) increased with long-term exposure to high temperature, which may increase oxygen uptake capacity and help to fuel increased metabolic demands. In Chapter 2, I explored the role of temperature, other abiotic factors, and co-occurring species in shaping the fine-scale distribution of Pugnose Shiner within Thompson’s Bay in the upper St. Lawrence River, Ontario. Multivariate analyses and occupancy modelling were used to assess how environmental variables and species co-occurrence accounted for variation in the distribution of Pugnose Shiner. Environmental variables, including temperature, were significant predictors of community composition. Pugnose Shiner was associated with clear, cool sites far from the inner bay with high vegetation density, including Chara vulgaris. Pugnose Shiner was negatively associated with Brook Silverside (Labidesthes sicculus), Largemouth Bass (Micropterus salmoides), Pumpkinseed (Lepomis gibbosus), and juvenile Lepomis spp. This has important conservation implications because some native centrarchids are increasing in abundance with climate warming, which may threaten persistence of rare and imperiled cyprinids, including Pugnose Shiner. Although Pugnose Shiner showed little evidence of thermal compensation (SMR increased with acclimation temperature), it did show acclimation capacity in terms of thermal tolerance and increased gill size. Captive rearing may provide opportunities for thermal hardening in reintroduction efforts, and targeted hunting of expanding species may buffer Pugnose Shiner from the negative effects of elevated water temperature. Overall, my research highlights the importance of examining multiple traits at multiple scales of biological organization to quantify effects of elevated water temperature on freshwater fishes and guide conservation recommendations for imperiled species

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