Raby, Graham

Lake Huron's food web has experienced drastic changes in response to multiple stressors including declines in offshore productivity, decreased trophic transfer efficiency and a transformation of the benthic food web. However, how these changes have affected the diets and isotopic niches of predatory fish is largely unexplored. My diet study analysed stomach contents from five predatory fish species (lake trout, lake whitefish, chinook salmon, rainbow trout, and walleye) from the Ontario waters of Lake Huron. My isotopic study focused on lake trout and lake whitefish, based on community concerns that recovering lake trout are competing with or consuming lake whitefish. By contrast, I found that lake whitefish were a minimal component of lake trout diets, and the diet and isotopic overlap between these two species was low overall, but varied regionally. Both the dietary and isotope analyses reflect the high regional diversity of energy sources used by predatory fish.

Author Keywords: Food web, Isotopic Niche, Lake Huron, Predator-prey, Regime shift, Regional Energy Sources

The Great Lakes represent nearly a fifth of the global freshwater surface supply, and support culturally, economically, and ecologically important fish species such as Walleye (Sander vitreus). With water temperatures projected to rise by several degrees in the coming century, understanding the energetic impacts on fishes is crucial for effective habitat and fisheries management. I measured the swimming performance, accelerometer-based activity, and metabolism of walleye in relation to body size, sex, and water temperature to assess potential responses to climate variability. Both acceleration and swim speed predicted metabolic rate with reasonable accuracy. Temperature had a positive effect on oxygen consumption whereas body mass had a negative effect. Critical swimming speed increased with temperature, and tailbeat frequency had positive relationships with swim speed and acceleration. My laboratory-based calibrations should enable remote field monitoring of energy use of walleye (via accelerometers) and help conserve an important species through bioenergetic modelling.

Author Keywords: accelerometry, bioenergetics, climate change, metabolism, respirometry, walleye

I tracked summer movements and activity of snowshoe hares (Lepus americanus) in southwestern Yukon, Canada, during their population cycle (2015–2022) to assess the primary drivers of movement. Hares exhibited variable movement and activity during the cycle, exhibiting increased home range size and higher daily displacement during low population densities. Males exhibited more dramatic increases in their home ranges (>3-fold), and had greater and more variable movement rates and time spent travelling than females. The ratio of predators to hares was highest at low hare densities when hares were moving most, and seasonal activity of hares seemed unrelated to that of predators. Differences between sexes imply that reproductive activities likely were the main driver of hare movement variation rather than food availability. These findings reinforce that, even in highly variable environments, potential rewards associated with successful mate search and reproduction may outweigh the risks associated with reproductive behaviour.

Author Keywords: behaviour, boreal forest, GPS telemetry, home range, movement ecology, predation risk

In situ chlorophyll sensors are beneficial for monitoring of long-term impacts of algal blooms and accessing water quality issues in bodies of water. However, more research is needed to validate their efficacy and understand how environmental conditions can influence sensor measurements. I assessed the performance of an in situ chlorophyll sensor under controlled environmental conditions and used the same sensor to collect vertical phytoplankton patterns in south-central Ontario boreal lakes. The performance of the sensor was assessed by examining the precision of chlorophyll measurements and determining the suitable timing length that would produce precise results. In general, the sensor was relatively insensitive to conditions under lower algal concentrations and the decent of the sensor should be slowed for vertical lake profiling in lakes with higher algal biomass. Most variation resulted from the movement of particle bound algal cells. We described chlorophyll profile characteristics including surface chlorophyll levels and chlorophyll peak depth and width and investigated the relationships of these features with environmental controls. The lakes showed a typical chlorophyll profile of low phytoplankton biomass lakes. Our results showed that dissolved organic carbon was a strong predictor of epilimnetic biomass while light attenuation and dissolved organic carbon were both strong predictors of peak depth. Light attenuation and surface area were small but significant predictors of peak width. We acknowledged that any uncertainties in sensor chlorophyll readings were not an issue in our lakes due to the overall low chlorophyll biomass.

Author Keywords: chlorophyll, chlorophyll fluorescence, in situ profiling, lakes, phytoplankton biomass, water quality