Do the Fish Return? A Qualitative Assessment of Anadromous Pacific Salmonids' Upstream Movement After Dam Removal

Basic assumptions about the ecological effects of dam removal remain untested hypotheses. In this work, I move to address this by assessing the effectiveness of dam removal as a tool to restore anadromous Pacific salmonids' (Oncorhynchus spp.) access to upstream habitat. I compiled all available post-removal data from all cases (N = 40) where anadromous salmonid restoration was a project goal in California, Oregon, and Washington from 1999 to 2011. In cases without empirical data, I contacted biologists and river managers and collected anecdotal information. I analyzed full barriers (N = 16), where upstream passage was not possible for fish, separately from partial barriers (N = 27), where fish were able to pass upstream of the dam. In three cases, dams were passable to some species but not others. In cases of full barrier removal, formerly excluded anadromous salmonids were observed to have recolonized upstream reaches in 9 of 16 cases. In cases of partial barrier removal, results were uneven; they are discussed on a case-by-case basis. There were no useful data on 14 of 27 partial barrier removals, and there was only anecdotal evidence of success in 9 of 27 cases. Dam removal remains a promising restoration action for anadromous Oncorhynchus species, but monitoring, especially in cases of partial barrier removal, has largely been inadequate to determine whether these fish expand into newly accessible habitat after dam removal.