Dam Removal Increases American Eel Abundance in Distant Headwater Streams

Abstract

American eel Anguilla rostrata abundances have undergone significant declines over the last 50 years, and migration barriers have been recognized as a contributing cause. We evaluated eel abundances in headwater streams of Shenandoah National Park, Virginia, to compare sites before and after the removal of a large downstream dam in 2004 (Embrey Dam, Rappahannock River). Eel abundances in headwater streams increased significantly after the removal of Embrey Dam. Observed eel abundances after dam removal exceeded predictions derived from autoregressive models parameterized with data prior to dam removal. Mann–Kendall analyses also revealed consistent increases in eel abundances from 2004 to 2010 but inconsistent temporal trends before dam removal. Increasing eel numbers could not be attributed to changes in local physical habitat (i.e., mean stream depth or substrate size) or regional population dynamics (i.e., abundances in Maryland streams or Virginia estuaries). Dam removal was associated with decreasing minimum eel lengths in headwater streams, suggesting that the dam previously impeded migration of many small‐bodied individuals (<300 mm TL). We hypothesize that restoring connectivity to headwater streams could increase eel population growth rates by increasing female eel numbers and fecundity. This study demonstrated that dams may influence eel abundances in headwater streams up to 150 river kilometers distant, and that dam removal may provide benefits for eel management and conservation at the landscape scale.

Synergistic patterns of threat and the challenges facing global anguillid eel conservation

“…Another major threat which was thought to affect 85% of anguillid species was Natural system modifications, a broad descriptor that encompasses the abstraction of water and the installation of dams and other water management systems. Despite the ability of eels to climb moist surfaces, large dams can restrict juvenile upstream movements, impacting localised population densities (Hitt et al, 2012) and thus sex ratio and reproductive output from specific localities (McCleave, 2001). In addition, they can also prevent adult escapement of silver eels to the ocean and cause direct mortality of silver eels passing through hydropower turbines as they emigrate to their spawning grounds (ICES 2002;Jansen et al, 2007 andMacGregor et al, 2009).…”

Landscape context and the biophysical response of rivers to dam removal in the United States

“…In addition, most dam removal studies are short in duration and focus on a single response metric [2]. Despite these limitations, some patterns have emerged from the literature: there may be a lag between geomorphic and ecological responses [17, but see 18]; aquatic species typical of flowing rivers (lotic habitats) tend to replace stillwater (lentic) communities in the reservoir after dam removal [15]; and upstream fish migration that was formerly impeded by the dam may occur swiftly after dam removal in some cases [19–22]. …”

Comparative riverscape genetics reveals reservoirs of genetic diversity for conservation and restoration of Great Plains fishes

“…This set of observations justifies a conservation focus on reconnection of fragment reaches regardless of their position in the riverine network to restore both community and genetic diversity (Perkin et al 2014a). Natural colonization may occur rapidly once habitat connectivity and quality is restored (Catalono et al 2007; Hitt et al 2012; Archdeacon et al 2012), but if not, it is appropriate for managers to translocate fishes to speed local recovery. As an example, reconnection of Platte basin fragments would increase the length of available habitat and may increase the distribution of pelagic spawning species.…”