Project Spotlights

American Eel Within the Delaware River

The project described in this article was funded by the National Park Service through the Chesapeake Watershed CESU. 

By: Sarah Stalcup, Drexel University

Map of the Delaware River Basin
Map of the Delaware River Basin modified from US Geological Survey 1:100,000-scale digital data. Source: USGS

In an effort to better understand the age, health, and distribution of juvenile American eel populations in the Delaware River, scientists at the Academy of Natural Science of Drexel University (ANSDU) have begun a multi-year research project to inform future management of the species in the upper portion of the Delaware River.


Eel sampling took place at four sites in the Delaware River during 2018. There were two upper sites near Buckingham, PA., and Callicoon, NY, and two lower sites near Westcolang and Lackawaxen, Pa., for the purpose of measuring and comparing upstream and downstream health. Since beginning the 2019 field season, the ANSDU team has also sampled several other sites including those at Hankins, NY;  Lordville, NY; and Damascus, PA.


The first stage of sampling took place in the fall of 2018 by electrofishing, which temporarily stuns the fish and allows them to be collected and measured. Electrofishing was conducted both with boats and shoreline backpacks.

The team set out to collect and measure every eel they encountered in the field. At the Callicoon, Westcolang, and Lackawaxen stations, most eels collected were euthanized, individually tagged, preserved on dry ice, and returned to the Academy for further research. Collection at the Buckingham station yielded 71 eels in less than an hour – a high density. Therefore, the scientists released 21 eels back into the stream while maintaining 50 for further analysis.

Field sampling resulted in 118 eels that were returned to the Academy to be analyzed for age, swim bladder condition, stomach contents and sex. Once thawed in the lab, they were measured for total length, weight, and sexed.

Backpack electrofishing in Barryville, NY
Backpack electrofishing on Delaware River in Barryville, NY, in 2019.

To determine age, the otoliths were removed from each animal. Otoliths are small calcium carbonate structures in the inner ears of vertebrates that help the animal hear, sense gravity and movement, and can be used similarly to tree rings to determine the age of some vertebrates like eels. Otoliths were sent to John Casselman of Queens College for analysis, which began in summer 2019.

Swim bladders are internal gas-filled organs found in many fish species that help the animal control its buoyancy, allowing it to maintain its depth in the water without expending energy to swim. A parasite, Anguillicola crassus, can damage eel swim bladders, forcing them to use extra energy to stay at their desired depth, and can even force them to float upside down. In eels, damage to the swimbladder by infection can include thickened walls, decreased volume, and shrinkage. To assess the impact of the parasite on the health of the eel swimbladders, Academy scientists first counted, recorded and preserved any parasites found. Next, they calculated the ratio of swimbladder length to total eel length for each specimen. Finally, they used the “swimbladder degenerative index,” which scores the opacity, thickness and pigmentation of the bladder and gives an overall assessment of the impact of A. crassus on the swimbladder. The swim bladders assessed from this first stage of sampling revealed that the collected animals varied in parasite presence and swim bladder damage.

Eel being measured and PIT tagged.
Eel being measured and PIT tagged.

Next Steps

The second year of field sampling began in August 2019. As in the first year of sampling, during week one, Academy scientists will capture another 100 eels for laboratory analysis. Unlike the first sampling season, selected eels will be marked with an internal wire tag and released. These fish will be anesthetized with a new drug, authorized by the U.S. Fish and Wildlife Service to minimize the stress placed on the eels when handled for measurement. As this is a new drug, the Academy will document the time it takes the eels to become sedated and the time it takes them to recover from sedation. These data will be used to help support and provide information for the new drug’s application. Sampling small blocked areas near the shoreline will be done as another potential approach for estimating trends in eel collection.

In the second week of sampling, sites where marked eels were released will be resampled to document the number of recaptured individuals and estimate the population of American eel at those sites. Additional eels will be marked and released, and a third sampling trip will be done to look for marked eels. Flora and fauna, such as aquatic vegetation, algae, and macroinvertebrates, will also be collected throughout the study for stable isotope analysis to better understand the food web and the American eel’s place in it.