Abundance and Distribution of American Eel in a Heavily Dammed Urban River

Abstract

The American Eel Anguilla rostrata is a catadromous fish found in rivers along the East Coast of North America. American Eel populations have declined throughout their range for several reasons, including habitat fragmentation caused by damming, which hinders access to much of their historic lotic habitats. This study examined differences in abundance and size structure across river segments divided by nine dams in the Bronx River, New York, a heavily urbanized system. American Eels were sampled via multipass electrofishing at nine sites over the course of 3 years. American Eels 250 mm TL or larger were implanted with passive integrated transponder tags to estimate survival and movements. Estimates of abundance were calculated for electrofishing data using the Huggins closed‐capture model. Density was estimated based on area surveyed, abundance data, and estimated weights of American Eels. Eel mops were used to evaluate the presence and relative abundance of different life stages of American Eels (glass, elver, and small yellow phases). Our results showed decreased abundance in upstream areas, with a corresponding increase in average size. Density, by weight, was consistent among sites. Tagging data showed minimum apparent survival of up to 0.29 and only a single movement between dammed segments, demonstrating high site fidelity. Eel mop sampling revealed that although glass‐ and elver‐stage American Eels were common downstream, relative abundance decreased rapidly above the first dam. These results suggest that the Bronx River American Eel population is structured by physical barriers to passage, density‐dependent limitations, high stationariness, and high apparent survival.

Relating American Eel Abundance to Environmental DNA Concentration in the Bronx River

“…Electrofishing abundance estimates.-The seven lower sites (182nd Street to Strathmore) were surveyed asynchronously (Camhi et al 2021) by depletion electrofishing during the summer and autumn seasons from 2014 to 2016 using a backpack electrofisher (Halltech Aquatic Research, Guelph, Ontario). Numerical abundance of FIGURE 1.…”

“…In the Bronx River, New York, American Eels were the subject of a multiyear electrofishing survey using standardized depletion techniques (see the companion paper Camhi et al 2021, https://doi.org/10.1002/nafm.10620). The Bronx River is a highly urbanized 37-km-long river with nine dams that begins in Westchester County, New York, flowing into the East River tidal strait in New York City's Bronx County.…”

Thirteen novel ideas and underutilised resources to support progress towards a range‐wide American eel stock assessment

“…Evaluations of dam impacts on eels assume that stream densities that are lower above than below a dam are caused by passage impairment at the dam site (Camhi et al, 2021; Cooney & Kwak, 2013; Itakura et al, 2020; Machut et al, 2007). However, decreased upstream densities do not prove passage impairment because dams increase upstream habitat, which allows eels to spread over a larger area, decreasing their density.…”

Patterns in longitudinal distribution of American Eel (Anguilla rostrata) population characteristics in rivers of Puerto Rico

“…The Río Sabana has large natural barriers in downstream sections (Roghair et al, 2014). Natural and artificial barriers can decrease eel density in upstream environments (Goodwin & Angermeier, 2003; Machut et al, 2007) while dams can extirpate eel from suitable upstream habitats (Camhi et al, 2021; Cooney & Kwak, 2013).…”

“…Studies from temperate regions indicate trends in size, sex, and density distributions along estuaries, rivers, and latitudinal gradients in the American Eel population (Goodwin & Angermeier, 2003; Jessop, 2010). Most commonly, abundance and density decrease inland compared to environments closer to the ocean, especially in dammed rivers (Camhi et al, 2021; Goodwin & Angermeier, 2003; Smogor et al, 1995; Wiley et al, 2004). Low proportions of females occur in downstream and brackish water environments, which are characterized by high American Eel densities and overcrowding (Krueger & Oliveira, 1999) whereas, upstream freshwater environments harbor predominantly female eel, which are typically longer and heavier than males (Bonvechio et al, 2018; Goodwin & Angermeier, 2003; Hansen & Eversole, 1984; Helfman et al, 1984; Morrison & Secor, 2003).…”