Sea lamprey control agents from USFWS and DFO have spent the past several years identifying, inventorying, and ground truthing a list of barriers important to the Sea Lamprey Control Program (SLCP; hereafter referred to as lowermost barriers). A lowermost barrier is defined as the first manmade structure a migrating adult sea lamprey encounters in a tributary. The structure prevents upstream movement and, in most cases, protects upstream habitats from sea lamprey infestation. These barriers limit the distribution of sea lamprey larvae throughout the Great Lakes, enabling a much more focused, effective lampricide application program. The term ‘lowermost barrier’ can also describe an important piece of infrastructure that enables the SLCP to effectively conduct its work such as hosting a barrier integrated trap or serving as a platform from which lampricides are applied.

Over 10,000 structures are known to exist on Great Lakes tributaries, but sea lamprey control agents consider fewer than 500 lowermost barriers as important to maintaining present levels of sea lamprey control. The list is expected to evolve as it is shared with a wider audience and as barriers are built, modified, or removed. The GLFC has agreed to host a living list of these structures for reference at any time (www.glfc.org/pubs/slcp/lowermost-barriers.xlsx). Updates will be made on a regular basis to ensure completeness and each revision will be date stamped.

This list contains only man-made structures - waterfalls were excluded. We acknowledge that some lowermost barriers are built upon natural falls that would likely block sea lamprey migrations but chose to include those structures since we cannot confirm this. Not all lowermost barriers are located on the mainstream of a tributary. Some systems have multiple lowermost barriers if the primary structure does not block lampreys 100% of the time or if lowermost barriers are located on secondary tributaries.

This list also contains a few additional system descriptions to help describe the impact of each lowermost barrier. Length of stream blocked (km) are provided for the entire upstream watershed and to the assumed next upstream barrier. All barriers included in the National Inventory of Dams or the Ontario Dam Inventory were considered candidate second order barriers. We acknowledge that these inventories are not comprehensive and are working toward validating and updating next upstream barriers. We estimated upstream lengths using the National Hydrography Plus High-Resolution Dataset (NHDPlus HR) and the stream tracing process continues to be refined. Finally, we have included estimates to treat any newly opened stream segments with lampricides using a model developed by sea lamprey control agents. This model incorporates length of stream, watershed area, and stream alkalinity to estimate staffing and lampricide costs (in USD). This simple, deterministic bioeconomic assessment is meant to provide a starting point for future discussions regarding investments in selective fish passage and barrier removal or construction negotiations. Assessments incorporating increased ecological and economic realism and stochastic model elements would be valuable future work.