A field experiment to assess passage of juvenile salmonids across beaver dams during low flow conditions in a tributary to the Klamath River, California, USA.

Across Eurasia and North America, beaver (Castor spp), their dams and their human-built analogues are becoming increasingly common restoration tools to facilitate recovery of streams and wetlands, providing a natural and cost-effective means of restoring dynamic fluvial ecosystems. Although the use of beaver ponds by numerous fish and wildlife species is well documented, debate continues as to the benefits of beaver dams, primarily because dams are perceived as barriers to the movement of fishes, particularly migratory species such as salmonids. In this study, through a series of field experiments, we tested the ability of juvenile salmonids to cross constructed beaver dams (aka beaver dam analogues). Two species, coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss), were tracked using passive integrated transponder tags (PIT tags) as they crossed constructed beaver dams. We found that when we tagged and moved late-summer parr from immediately upstream of the dams to immediately downstream of them, most of them were detected upstream within 36 hours of displacement. By the end of a 21-day field experiment, 91% of the displaced juvenile coho and 54% of the juvenile steelhead trout were detected on antennas upstream of the dams while <1% of the coho and 15% of the steelhead trout were detected on antennas in the release pool below the dams. A similar but shorter 4-day pilot experiment with only steelhead trout produced similar results. In contrast, in a non-displacement experiment, juveniles of both species that were captured, tagged and released in a pool 50 m below the dams showed little inclination to move upstream. Finally by measuring hydraulic conditions at the major flowpaths over and around the dams, we provide insight into conditions under which juvenile salmonids are able to cross these constructed beaver dams, which should help guide future restoration efforts.