Observations of American Eels Using an Upland Passage Facility and Effects of Passage on the Population Structure ROBERT E. SCHMIDT* Bard College at Simon’s Rock, 84 Alford Road, Great Barrington, Massachusetts 01230, USA CATHERINE M. O’REILLY Biology Program, Bard College, Annandale, New York 12504, USA DANIEL MILLER New York State Department of Environmental Conservation, Hudson River National Estuarine Research Reserve, Norrie Point Environmental Center, Post Office Box 315, Staatsburg, New York 12580, USA Abstract.—Over two seasons, we observed passage of American eels Anguilla rostrata at an eel passage facility (eel ladder) placed adjacent to an upland mill dam on a small tributary to the Hudson River, New York. American eel use of the ladder was significantly correlated with freshets and moon phase, similar to other studies of eel migration. Most of the migrants were young (,20 cm total length). Several large American eels (.35 cm) exhibited local movements, including use of the ladder, as shown by recaptures of individuals marked with coded wire tags. Operation of the eel ladder greatly reduced the density of small American eels below the mill dam. Placing eel ladders on upland obstructions has the potential to enhance the productivity of silver (sexually mature) eels, an important consideration in the Hudson Valley, where hundreds of barriers have been constructed on upland tributaries. A decrease in landings of American eels Anguilla rostrata since the 1980s has raised concerns about the health of the population (Haro et al. 2000). The U.S. Fish and Wildlife Service recently considered listing the American eel as a threatened or endangered species but determined that listing was not warranted at this time. Several factors can affect the American eel population size, including changes in ocean conditions, overfishing, habitat degradation, and habitat loss. In many rivers, habitat loss has been caused by dams, which effectively restrict or eliminate access to critical upstream habitats where eels mature and grow. In addition, obstacles to migration may reduce the number of female American eels because the proportion of females increases in upstream areas (Oliveira and McCleave 2000). Small dams are common barriers to upstream habitat in the northeastern USA. The tributaries to the Hudson River have almost 800 recorded dams (and many more unrecorded; Swaney et al. 2006) that are spread throughout the watershed, mostly in upland locations rather than at the mouth of the tributaries (Schmidt and Cooper 1996). The annual run of age-0 eels (elvers) for the Saw Kill, a small Hudson River tributary, has been estimated to be as high as 10,000 (Yozzo et al. 2005). This leads to high eel densities (131–169 eels/100 m 2 ; Schmidt et al. 2006; Machut et al. 2007) at the mouth of the Saw Kill. Upstream of the first dam on Hudson River tributaries including the Saw Kill, eel densities are significantly lower (1.7–5.5 eels/100 m 2 ; Machut et al. 2007). These upstream densities are lower than those found in other northeastern rivers with eel passages (8–22 eels/100 m 2 ; Oliveira and McCleave 2000), suggesting that this upstream habitat might be able to support higher densities of American eels. Improving access to upland habitat for American eels can be done by removing (partially or completely) the barrier or by installing eel passage facilities (eel ladders). Removing or modifying dams is difficult and expensive and although ultimately this may be the best solution for American eels and other species, eel ladders are relatively cheap and can provide passage quickly. Most eel passage facilities have been situated on large dams, often in tidal water, where large numbers of immigrating elvers can be passed (Verdon and Desrochers 2003; Verdon et al. 2003). In this study, we placed a small eel ladder on a small upland dam in the Saw Kill to determine what effects the facility would have on local American eel populations and to associate American eel passage with environ- mental conditions. Methods Study site.—The Saw Kill is a small tributary, about 8 m wide at its mouth, to the fresh-tidal Hudson River estuary, which is located in northern Dutchess County, * Corresponding author: schmidt@simons-rock.edu Received March 6, 2008; accepted October 7, 2008 Published online May 14, 2009 715 North American Journal of Fisheries Management 29:715–720, 2009 Ó Copyright by the American Fisheries Society 2009 DOI: 10.1577/M08-050.1 [Management Brief]