Low potential for restraint of anadromous salmonid reproduction by beaver Castor fiber in the Numedalslågen River catchment, Norway

Abstract

After a long absence, beaver Castor fiber are rapidly returning to Europe. Their dam-building and tree-felling behaviour may have consequences for salmon Salmo salar and sea trout Salmo trutta management. In 2003 we investigated the parallel use of stream sections by beaver, sea trout and salmon and determined the potential hindrance that beaver dam-building presented for reproducing salmon and sea trout along 65 km of the Numedalslågen River and tributaries, a major Norwegian catchment. We also surveyed landowner attitude to having beaver on salmon and sea trout streams. Most salmon spawned in the river and most sea trout in 51 tributaries. Nine of these tributaries also hosted spawning salmon. 15 (29%) of the 51 tributaries with spawning sea trout and six (67%) of the nine with spawning salmon had intermittently been occupied by beaver. Though beaver preferred to colonize the same sections of stream used for spawning, only 15% of the stream length navigable by salmon and sea trout on the 51 tributaries had actually been used by beaver, and only three colonies were occupied autumn 2003 (1 colony/25.0 km). Five dams were functioning during autumn 2003 on the 51 tributaries (1 dam/14.3 km). These potentially hindered sea trout and salmon from reaching 18% and 3%, respectively of their potential spawning habitat, though all dams were low ( 0.5 m). Though the autumn density of occupied beaver colonies along the river (1 colony/2.5 km) was 10.0 times the density on the 51 tributaries, no dams were built on the river. Thus most salmon reproduction in the catchment was unhindered by beaver. Nine of 14 landowners were unequivocally positive about having beaver together with salmon and sea trout. We conclude that the presence of beaver on similar catchments will likely have only an insignificant negative impact on the reproduction of sea trout and salmon.

Qualitative and quantitative effects of reintroduced beavers on stream fish

“…In the North‐western United States, beaver dams represent temporary structures often washed out during the same freshets used by Pacific salmon species to reach their spawning grounds (Taylor 1999). In Norway, it is suggested that any impacts of beaver dams on the upstream and downstream movements of Atlantic salmon and sea trout would be negligible because of their low density, small size and short life‐span (Parker and Ronning 2007).…”

“…In Norway, Atlantic salmon are thought to primarily spawn in the main Numedalslågen river, recolonized by beaver in 1957, while sea trout tend to utilize tributary streams (Parker and Ronning 2007). As a result, the impact of beaver activity on access to spawning grounds of the two salmonid species may differ substantially.…”

“…This can prove detrimental to fish, such as trout (Salyer 1935; Knudsen 1962; Guignion 2009), when temperatures rise towards the upper limit of thermal tolerance for the species (Swanston 1991). Unlike other impacts, which may be localized, the reduction in shade because of tree felling can occur over considerable stream length (Parker and Ronning 2007). Problems associated with high stream temperatures are most acute during summer months (NRC 1995; Guignion 2009) and can affect fish downstream of dams in addition to those frequenting beaver impoundments (Shetter and Whalls 1955).…”

“…1998; Scruton et al. 1998; Mitchell and Cunjak 2007; Parker and Ronning 2007; Guignion 2009; Taylor et al. 2010), although little research quantifying the magnitude of this impact was found (see Tables 3 and 4).…”

Beavers in Russian forest-steppe — characteristics of ponds and their impact on fishes and amphibians

“…But just few countries had studies about that topic, e.g. Canada, USA, Germany, Lithuania (Naiman et al, 1988;Nummi, 1989;Balciauskas et al, 2001;Halley & Lamberg, 2001;Dalbeck et al, 2007;Parker & Rønning, 2007;Dalbeck & Weinberg, 2009;Hill & Duval, 2009). The majority of research about beaver's influence concerns North American populations of Castor canadensis (Baker & Hill, 2003;Gallant et al, 2004;Westbrook et al, 2006;Hood & Bayley, 2008;Karraker & Gibbs, 2009;Hood & Larson, 2014;Anderson et al, 2015).…”