Global freshwater systems are severely threatened by pollution, climate change, invasive species, and habitat fragmentation. Artificial barriers such as dams and weirs are longstanding features of global landscapes, with several anthropogenic uses including flood mitigation and recreation as well as cultural value. Many publications have highlighted river fragmentation as one of the most prominent threats to native species.

Thus, conservation groups and policies are focused on removing barriers that are no longer necessary to societal function to aid movement and migration of native species. Despite the large amount of support for holistic remediation of rivers to their natural state, debates have arisen around flood risks and safety, sediment load and pollution, and invasive species spread. Paradoxically, the removal of barriers is largely assumed to address the issues of native species migrations and movement, while simultaneously, removing the first line of physical defense against aquatic invasive species. In rivers where invasive species have spread post-removal of barriers, new barriers (e.g., chemical, electrical, physical) and costly management strategies have been reinstated or started, creating a ‘wicked problem’ for conservation.

For the most cost-effective and time-efficient plans, a trade-off between native species movement in freshwater systems and the spread of non-native species must be incorporated into barrier removal decisions and strategies.

Throughout literature and policy, there is a bias towards the negative outlook of instream barriers. However, there remains little to no quantitative evidence to support or deny the expansion of native or invasive species after the removal of barriers. This PhD is aimed at developing national and international levels of understanding the effects of instream barriers and their removals on aquatic species, with specific interest on the widely overlooked invasive populations. Each chapter will involve novel and innovative methods to answer cutting-edge questions and will involve a holistic approach by assessing species from individual to community levels through the collection and analysis of large data resources coupled with field surveys and experiments.