Characterizing the structure, dynamics, and ecology of predator populations is essential for understanding their role in ecosystems and for predicting their potential impact on prey. Killer whales (Orcinus orca) are a genetically and ecologically diverse marine predator with ecosystem-level influence. Globally, they occupy a broad ecological niche, but are often locally specialized and categorized as ecotypes or morphotypes. In the eastern Canadian Arctic, their seasonal presence is apparently expanding and is predicted to negatively impact Arctic-endemic prey, but certain aspects of their demographics have yet to be determined. Further, it is unknown whether genetically distinct populations demonstrate differences in ecology. In this thesis, I use physical and molecular markers to address these questions, investigating the demographics and relative ecology of killer whales in the eastern Canadian Arctic and Northwest Atlantic. First, I used photographic and genetic-identification to assess the abundance and population trend. Mark recapture analysis of photo-identification data estimated an abundance of 217 individuals and a growing population, but genetic-identification data was insufficient to produce reliable estimates. Epigenetic aging suggested a population age structure skewed to juveniles, while the proportion of calves and young juveniles based on group composition in photographs was comparable to other stable or growing populations. Second, I used whole genome sequences and compound-specific stable isotope analysis of amino acids to evaluate genetic and ecological differentiation between killer whale populations in the Northwest Atlantic. Analysis of genetic population structure confirmed two previously identified, genetically-isolated populations in Eastern Canada and Greenland. d13C and d15N of amino acids, used to infer relative distribution and diet, respectively, revealed moderate differences in source carbon values and a considerable difference in trophic level between the two populations. Taken together the stable isotope data show that the two genetic populations differ in ecology, but further research is needed before classification as ecotypes or morphotypes. This work contributes to our knowledge of killer whale demographics and ecology in the eastern Canadian Arctic, with implications for the management and conservation of this population as they expand in the Arctic.