Multiscale investigations into the thermal habitat use and conservation of Arctic grayling (Thymallus arcticus) in the Parsnip River watershed, Canada

,The drivers of the abundance and distribution of riverine species are often multiscale; they depend on both the local heterogeneity of habitats within the river and the context of the larger landscapes through which they flow. Methodological constraints can limit the ability of river ecologists to conduct studies over large scales, often producing results over fine scales that must then be extrapolated over unsampled units. Multiscale sampling approaches have been increasing in riverine studies, but the applications of their findings to conservation management – which operates over its own scales – are not always clear. In this dissertation, I investigate the use of physical and thermal habitats by Arctic grayling (Thymallus arcticus) at the reach, river, and watershed scales. Through experiments and observational studies, I investigated how this species used habitats in a degraded watershed. As a component of my research, I developed new statistical models that were parameterized with data from acoustic telemetry and applied new approaches using drone technology. Results from those models will allow fisheries ecologists to better understand the distribution of fish in river networks. I found strong associations between the distribution of Arctic grayling and pool habitats at the reach and river scales, and nonlinear relationships with temperature at the reach (11.0–16.0 °C) and watershed scales (11.1–17.1 °C). Unexpectedly, I found that the combination of upstream distance and pool habitats were a stronger predictor of Arctic grayling abundance than temperature at the river scale. This was explained by the fact that upstream distance accounted in part for the effects of temperature but also likely explained variation in other important predictors of Arctic grayling abundance (e.g. forage density, site fidelity, and territoriality). I quantified a thermal preference range (10.1–13.0 °C) for Arctic grayling in the laboratory and related this metric to the in-situ thermal habitat use of tagged but free-ranging individuals to determine the effectiveness of behavioural thermoregulation as a strategy for maintaining body temperature. I found that Arctic grayling used behavioural thermoregulation to effectively maintain their body temperatures during their summer feeding window, that heat transfer in Arctic grayling was slightly more efficient when cooling than warming, and that heat transfer was more rapid in smaller fish. I found novel evidence that Arctic grayling may be single-direction thermoregulators that will invest energy into cooling but not into warming. Contrary to my expectations, I found that the effectiveness of behavioural thermoregulation was more strongly related to thermal heterogeneity in time (along diel and seasonal axes) than in space. My findings related to the thermal ecology of Arctic grayling emphasized the continued conservation of coldwater-producing habitats (i.e. headwaters). I also found through my reach and river-scale studies that additional conservation of highly structured instream habitats that support thermal refugia and feeding opportunities across local scales would support better outcomes for the conservation of this population. I identified potential future risks, namely territoriality and dominance hierarchies that may keep Arctic grayling from leaving unfavorable thermal habitats during heatwaves.

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