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

Abstract

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 fish movement, 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 dam analogues. We found that when we tagged and moved these fishes from immediately upstream of the dams to immediately downstream of them, most 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. In contrast, during the final week of the 21-day experiment, just 1 of 158 coho salmon and 6 of 40 (15%) of the steelhead trout were still 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. Further, by measuring hydraulic conditions at the major flowpaths over and around the dams, we provide insight into low-flow conditions under which juvenile salmonids are able to cross these constructed beaver dams, and that multiple types of flowpaths may be beneficial towards assisting fish movement past instream restoration structures. Finally, we compared estimates of the number of juvenile salmonids using the pond habitat upstream of the dam relative to the number that the dam may have prevented from moving upstream. Upstream of the dams we found an abundance of juvenile salmonids and a several orders of magnitude difference in favor of the number of juveniles using the pond habitat upstream of the dam. In sum, our study suggests beaver dams, BDAs, and other channel spanning habitat features should be preserved and restored rather than removed as perceived obstructions to fish passage.