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

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.