The passability of barriers was informed by the Scottish Obstacles to Fish Migration data set (see: https://www.sepa.org.uk/environment/environmental-data/). The dataset contains information on the location of barriers on the river network, whether they are natural or manmade and whether they are impassable or passable under certain conditions. All analyses were performed on a topologically corrected version of the Centre for Ecology and Hydrology (CEH) digital river network (hereafter DRN). Atlantic salmon fry densities were predicted for each river segment using landscape covariates (upstream catchment area (UCA), river distance to sea, and altitude) and the national juvenile salmon density model for Scotland developed by Malcolm et al. (2019). Given the focus on a diadromous species, the Dendritic Connectivity Index (DCI) was used to assess the impacts of barriers on longitudinal connectivity (Cote et al. 2009). Where values returned are between 0 – 100%. A value of 100% would be in a river network with no barriers, where all potential habitat (i.e., rivers below natural impassable barriers) is accessible from the outflow. The increase in connectivity associated with barrier removal was recorded as delta DCIScot. This indicates the percentage increase in connectivity at a national level and thus provides a basis for ranking and prioritising the removal or easing of barriers to migration based on environmental gain at both local and national scales. Delta DCIScot assumes all downstream IMBs are also removed. Cumulative gain is calculated by summing the Delta DCIScot of the barrier of interest with the deltaDCIScot of downstream IMBs.