page_title: North American beaver
section_title: Effects on trout and salmon
hierarchical_section_title: North American beaver / Ecology / Effects on trout and salmon
caption_attribution_description: English: Sockeye salmon jumping over beaver dam Lake Aleknagik, AK by Kristina Ramstad 1997.
context_page_description: The North American beaver is one of two extant beaver species. It is native to North America and introduced in South America and Europe. In the United States and Canada, the species is often referred to simply as "beaver", though this causes some confusion because another distantly related rodent, Aplodontia rufa, is often called the "mountain beaver". Other vernacular names, including American beaver and Canadian beaver, distinguish this species from the other extant beaver species, Castor fiber, which is native to Eurasia. The North American beaver is an official animal symbol of Canada and is the official state mammal of Oregon and New York.
context_section_description: Beaver ponds have been shown to have a beneficial effect on trout and salmon populations. Many authors believe that the decline of salmonid fishes is related to the decline in beaver populations. Research in the Stillaguamish River basin in Washington found that extensive loss of beaver ponds resulted in an 89% reduction in coho salmon (Oncorhynchus kisutch) smolt summer production and an almost equally detrimental 86% reduction in critical winter habitat carrying capacity. This study also found that beaver ponds increased smolt salmon production 80 times more than the placement of large woody debris. Swales and Leving had previously shown on the Coldwater River in British Columbia that off-channel beaver ponds were preferentially populated by coho salmon over other salmonids and provided overwintering protection, protection from high summer snowmelt flows and summer coho rearing habitat. Beaver-impounded tidal pools on the Pacific Northwest's Elwha River delta support three times as many juvenile Chinook salmon (Oncorhynchus tshawytscha) as pools without beaver.
  The presence of beaver dams has also been shown to increase either the number of fish, their size, or both, in a study of brook trout (Salvelinus fontinalis), rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) in Sagehen Creek, which flows into the Little Truckee River at an altitude of 5,800 feet (1,800 m) in the northern Sierra Nevada. These findings are consistent with a study of small streams in Sweden, that found that brown trout were larger in beaver ponds compared with those in riffle sections, and that beaver ponds provide habitat for larger trout in small streams during periods of drought. Similarly, brook trout, coho salmon, and sockeye salmon (Oncorhynchus nerka) were significantly larger in beaver ponds than those in unimpounded stream sections in Colorado and Alaska. In a recent study on a headwater Appalachian stream, brook trout were also larger in beaver ponds.
  Contrary to popular myth, most beaver dams do not pose barriers to trout and salmon migration, although they may be restricted seasonally during periods of low stream flows. In a meta-review of studies claiming that beaver dams act as fish passage barriers, Kemp et al. found that 78% of these claims were not supported by any data. In a 2013 study of radiotelemetry-tagged Bonneville cutthroat trout (Oncorhynchus clarki utah) and brook trout (Salvelinus fontinalis) in Utah, both of these fish species crossed beaver dams in both directions, including dams up to 2 metres (6.6 ft) high. Rainbow, brown, and brook trout have been shown to cross as many as 14 consecutive beaver dams. Both adults and juveniles of coho salmon, steelhead trout, sea run cutthroat (Oncorhyncus clarki clarki), Dolly Varden trout (Salvelinus malma malma), and sockeye salmon are able to cross beaver dams. In southeast Alaska, coho jumped dams as high as two meters, were found above all beaver dams and had their highest densities in streams with beaver. In Oregon coastal streams, beaver dams are ephemeral and almost all wash out in high winter flows only to be rebuilt every summer. Migration of adult Atlantic salmon (Salmo salar) may be limited by beaver dams, but the presence of juveniles upstream from the dams suggests that the dams are penetrated by parr. Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows. Two-year-old Atlantic salmon parr in beaver ponds in eastern Canada showed faster summer growth in length and mass and were in better condition than parr upstream or downstream from the pond.
  The importance of winter habitat to salmonids afforded by beaver ponds may be especially important in streams without deep pools or where ice cover makes contact with the bottom of shallow streams. Enos Mills wrote in 1913, "One dry winter the stream ... ran low and froze to the bottom, and the only trout in it that survived were those in the deep holes of beaver ponds." Cutthroat trout and bull trout were noted to overwinter in Montana beaver ponds
caption: Salmon (Oncorhynchus nerka) jumping beaver dam
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