Keeping it R(eel): A Survey of the American Eel in NYC Rivers

Discussion

Results

Research Questions & HypothesesQuestion: How does distance from the mouths of the Bronx and Hutchinson rivers affect eel abundance?Hypothesis: There will be a decreased abundance of eels at sites further upstream from the mouths of the rivers.Question: How do dams affect abundance and size of eels in the Bronx River and the Hutchinson River?Hypothesis: Eels will be more abundant and smaller before the dams and less abundant and larger above the dams.Question: How does macroinvertebrate species richness affect eel abundance in the Bronx and Hutchinson Rivers?Hypothesis: Sites with greater macroinvertebrate species richness will have higher abundance and size of eels.Question: How does water quality affect eel abundance in the Bronx and Hutchinson Rivers?Hypothesis: At sites where water quality is lower as indicated by lower levels of dissolved oxygen, then there will be a decrease in eel size and abundance.

References Acknowledgements

The American Eel (Anguilla rostrata) is the only catadromous fish in North America. Born in the Sargasso Sea in the Atlantic Ocean, young eels then swim up rivers, and upon reaching maturity, adult eels return to the Atlantic where they spawn and die1. The American Eel functions as both a predator and prey to other organisms living in North American riparian ecosystems; specifically, the American Eel preys on smaller animals such as macroinvertebrates and small fish, while also serving as a key prey species to larger aquatic organisms2. These key roles that the American Eel plays in the regulation of both oceanic and riparian ecosystems make it an important species to study. However, due to its unique migration patterns and variation in habitat, it has been difficult for researchers to study the American Eel, resulting in a lack of information about its life cycle and the factors affecting its recently declining populations3.

Introduction

Methods

Figure 3 shows a negative linear dependence between distance from the mouth of the river to abundance of eels.

Figure 8 demonstrates an average increase in the size of eels as dissolved oxygen level increases.

Figure 4 demonstrates a slight positive linear relationship between distance from the mouth of the river and eel size.

Figure 5 demonstrates the mean number of eels found above and below the first dam of the river. Eels tend to be more abundant below the first dam than above it.

Figure 6 shows the mean size of eels found below and above the first dam. There was a statistically significant increase in mean eel size at sites above the first dam compared to sites below the first dam.

Effect of distance to mouth of river on size of eels- Our data demonstrated a slight relationship between quantity and size of eels and distance from the river’s mouth; we found larger numbers of eels at sites closer to the mouth, compared to sites further upstream. Furthermore, we found a decreased mean eel size further upstream, demonstrating a positive relationship between eels size and distance upstream. Both aspects of our data support our original hypothesis.Effects of Dams on Eel Abundance and Size- Our data revealed a strong linear dependence between eel abundance and size below and above dams. Our E. 182nd street and Hutchinson River sites where we found eel abundance to be highest are located below dams. Our Twin Dams site however, is above the first dam in the Bronx River, and demonstrated a notably smaller number of eels. Eels at this location were also shown to be statistically significantly larger than the eels found at our two sites below dams. These findings supports our hypothesis, as well as other research claiming that eels are less abundant and generally larger above dams than below them, suggesting that human-made habitat modifications such as dams are hindering the catadromous migration patterns of eel populations moving upstream5.Macroinvertebrate species richness and eel abundance- Analysis of our data revealed no significant relationship between macroinvertebrate species richness and eel size or abundance. While this data did not support our hypothesis, there was a significant increase in macroinvertebrate species richness, as well as pollution-intolerant macroinvertebrate species at the Hutchinson River site compared to the Bronx River, which may be of interest in further research on water quality in urban rivers. Water Quality and Eels- While our data did not support our hypothesis regarding eel abundance and dissolved oxygen, our results do demonstrate a relationship between eel size and dissolved oxygen. We found a trend of increased mean eel size at sites with higher levels of dissolved oxygen, attributed to the fact that eels depend on dissolved oxygen for their survival, both in their own respiration and that of the macroinvertebrates and small fish that they prey on6. However, chemical contamination from anthropogenic sources can lead to decreased dissolved oxygen levels and water quality that is unsuitable for aquatic life, including eels7.

Conclusions

Figure 7- A 6 cm eel at the Hutchinson River

from humans, especially in highly urbanized areas such as the Bronx. We hope that our research, alongside further work studying the American eel, can spread awareness about the American Eel and the ways that humans are impacting it. In light of our findings, we suggest that further steps be taken to preserve and improve the quality of water in Northeastern rivers such as the Bronx and Hutchinson, particularly in urban areas, in order to protect the already threatened and highly sensitive American Eel. Furthermore, we encourage both concerned civilians and government officials to consider possible impacts on the American Eel as well as other migratory fish species when making decisions regarding physical modifications to riparian ecosystems.

Our research suggests that due to its complex life cycle, the American Eel is sensitive to the impacts of physical and chemical changes to rivers caused by humans. As explained above and supported by other studies, the American Eel tends to be more abundant further downstream in rivers, likely a result of the impediment of their migration patterns by obstacles such as dams. Additionally, as our research showed eels to be sensitive to water quality factors such as dissolved oxygen, the American Eel may also be affected by pollution

Figure 9- An 18.5cm eel at the Bronx River

Eels- We used traps made from unravelled rope to mimic the aquatic plants that act as a habitat for eels. Three traps were stationed at each of our three sites (Twin Dams, E. 182nd street, and the lower Hutchinson River), and checked twice a week. To check a trap, we placed it in a bucket of water and strained the contents through mesh cloth. Any eels found were measured by length in centimeters, then released.

Some studies have pointed toward a multitude of negative impacts on American Eel populations caused by humans, such as pollution, commercial fishing, oceanic changes and habitat modification4. In this study, we sought to analyze the abundance and size of eels in relation to factors such as water quality, macroinvertebrate species richness, pollution, and dams. Through this research, we aimed to identify the ideal habitat for the American Eel and the factors threatening it.

Figure 2: an eel trap6

Water quality- Once a week we used a YSI 2030 PRO Dissolved Oxygen/Conductivity Meter to take measurements of Salinity, Conductivity, Temperature, and Dissolved Oxygen at each site.Trash- Once a week we conducted a trash survey along the river bank of each site. We used a random number generator to determine a distance downstream from our furthest trap each week. We then conducted a line transect survey for 5 meters, counting all visible trash. This was done on both banks of the river and the total number of pieces of litter was recorded. Macroinvertebrates- Bycatch: Once a week at each site, after straining the contents of each eel trap and removing all eels, we removed the remaining contents and examined the macroinvertebrate presence. Every species present in the sample was identified and the total number of different species at each site was added up to obtain a measure of species richness.Net Sample: Once a week we collected a net sample of macroinvertebrates. One team member held a net in the river facing the current while another picked up and scrubbed submerged rocks until all debris fell off, floating into the net. This was done for exactly one minute, and the resulting sample was examined on a tray. Every species present in the sample was identified and the total number of different species at each site was added up to obtain a measure of species richness.

Figure 1- Migration patterns of the American eel5

We would like to thank The National Science Foundation for funding our research, and the Bronx Zoo, the Wildlife Conservation Society and Fordham University for allowing us to use their spaces. We would also like to thank Jason Aloisio, Karen Tingley, Su-Jen Roberts, Dr. J.A. Clark, Dr. J. Lewis, Dr. J. Munshi-South, and Erich Eberhard for their continuous support and guidance throughout our research. Finally, we want to thank Jake LaBelle and the WCS Seascape Program for advising us and allowing us to use their site and traps on the Hutchinson River.

Emma-June Orth, Crystal Cabreja, Kendyll Ferguson, Maya Scott, Julianna Anglada, Joseph Svoboda, Conor Gilligan

1. DeMarte, R., Waldman, J., Queens, N., & Bednarski, M. S. (2014) The Effects of Dams on Densities and Sizes of American Eels in the Bronx River. Final Reports of the Tibor T. Polgar Fellowship Program, 2014. 1-22.2. Denoncourt, C. E., & Stauffer Jr, J. R. (1993). Feeding selectivity of the American eel Anguilla rostrata (LeSueur) in the upper Delaware River. American Midland Naturalist, 301-308.3. Johnson, J. H. & Nack, C. C. (2013) Habitat Usage of the American Eel (Anguilla rostrata) in a tributary of the Hudson River, New York. Journal of Applied Ichthyology, 29, 1073-1079.4. Castonguay, M., Hodson, P. V., Couillard, C. M., Eckersley, M. J., Dutil, J. D., & Verreault, G. (1994). Why is recruitment of the American eel, Anguilla rostrata, declining in the St. Lawrence River and Gulf?. Canadian journal of fisheries and aquatic sciences, 51(2), 479-488.5. The Nature Conservancy (2018) The Migration of the American Eel. Retrieved from https://www.nature.org/ourinitiatives/regions/northamerica/unitedstates/indiana/journeywithnature/incredible-journey-american-eel.xml6. St. Martin, A. J. (2011), Photo of Eel Mop, Hudson River Sloop Clearwater Inc. Retreived from https://www.clearwater.org/educator-blog/the-mystic-whaler-and-the-eels/ 7.DeMarte, et. al.8. Serafy, J. E., & Harrell, R. M. (1993). Behavioural response of fishes to increasing pH and dissolved oxygen: field and laboratory observations. Freshwater biology, 30(1), 53-61.9. Byer, J. D., Lebeuf, M., Trottier, S., Raach, M., Alaee, M., Brown, R. S., ... & Hodson, P. V. (2015). Trends of persistent organic pollutants in American eel (Anguilla rostrata) from eastern Lake Ontario, Canada, and their potential effects on recruitment. Science of the Total Environment, 529, 231-242.