Guide to Waterfall Fish Barrier Assessment
Jason Blackburn, Brad Hurkett, and Logan Redman
Invasive species are a significant threat to native trout populations, and potentially the greatest threat to Westslope cutthroat trout (WSCT) in Alberta, through hybridization and competition. To effectively safeguard against extirpation, it is essential that existing pure populations remain protected from invasive species, and new pure populations are established. Several sub-populations of native trout remain genetically pure because of waterfall barriers that impede upstream migration of invasive fish. Similarly, habitats above barriers represent opportunities to expand the ranges of native trout and their total habitat areas through introduction/re-introduction of pure stocks. To date there is no single assessment method to identify and rank barriers in the context of invasion risk. Our objective was to develop a standard method for assessing natural fish passage barriers. After evaluating approximately 100 known barrier locations and approximately 200 barrier features, we developed a waterfall barrier assessment methodology to identify, measure, classify, and rank a complex range of natural waterfall barriers to upstream fish invasion. We developed methods to quantify and characterize four primary variables that affect upstream invasion potential at waterfall barriers: leaping obstacles, stream velocity, swimming depth, and turbulence. We generated feasible swimming and leaping performance charts to determine theoretical barrier passage by fish size category, and a barrier scoring framework that ranks passage difficulty. Updates to this manual are planned as opportunities for improving the assessment methodology are identified.