Beaver Impact
The decline in anadromous salmonid species over the last two to three centuries is correlated with the decline in the North American beaver and European beaver, although some fish and game departments continue to advocate removal of beaver dams as potential barriers to spawning runs. Migration of adult Atlantic salmon may be limited by beaver dams during periods of low stream flows, but the presence of juvenile Salmo salar upstream from the dams suggests the dams are penetrated by parr. Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows. A 2003 study of Atlantic salmon and sea trout (Salmo trutta morpha trutta) spawning in the Numedalslågen River and 51 of its tributaries in southeastern Norway were unhindered by beaver. In a restored, third-order stream in northern Nova Scotia, beaver dams generally posed no barrier to Atlantic salmon migration except in the smallest upstream reaches in years of low flow where pools were not deep enough to enable the fish to leap the dam or without a column of water over-topping the dam for the fish to swim up. The importance of winter habitat to salmonids afforded by beaver ponds may be especially important (and underappreciated) in streams of northerly latitudes without deep pools where ice cover makes contact with the bottom of shallow streams. In addition, the up to eight-year-long residence time of juveniles in fresh water may make beaver-created permanent summer pools a crucial success factor for Atlantic salmon populations. In fact, 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.
Read more about this topic: Atlantic Salmon
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