Understanding the Ins and Outs of American Eel Migration in the Bronx River, a Fragmented Tributary of the Hudson Estuary

Understanding the Ins and Outs of American Eel Migration in the Bronx River, a Fragmented Tributary of the Hudson Estuary nd View of the 182 Street dam on the Bronx River. Photo © WCS FINAL REPORT TO THE HUDSON RIVER FOUNDATION FEBRUARY 2018 Grant Award: $71,630 Grant Number: 001/15A Award Date: May 8, 2015 Grant Period: March 1, 2015 – December 31, 2017 Report Period: March 1, 2015 – December 31, 2017 For more information, please contact: Merry Camhi, PhD Catherine Grippo Director, New York Seascape Senior Development Officer, Foundation Relations Wildlife Conservation Society Wildlife Conservation Society New York Aquarium 2300 Southern Boulevard Surf Ave at West 8th St Bronx, NY 10460 Brooklyn, NY 11224 718.220.6828 631.581.9011 [email protected] [email protected] Executive Summary Over the past three years, thanks to the generous support of the Hudson River Foundation, the Wildlife Conservation Society (WCS), in partnership with CUNY Queens College, has conducted extensive monitoring of the American eel in the Bronx River, the only freshwater river in New York City. Our objectives were to measure the seasonal presence, abundance, densities, and size distribution of eels in various life stages across the nine dams that bisect this 23-mile river, and to determine the impact that these barriers pose to the upstream and in-stream movements of resident eels. Based on this study, we draw the following conclusions about American eels in the Bronx River, which are discussed below and will be expanded on for publication in professional journal articles: Prior to implementation of an eel pass in 2016, glass eels did not make it past the first dam, though some elvers did. Eels get larger with distance upstream, with the largest size increase between the second and third dams. Few eels great than 250 mm in total length are present past the second dam. Eel abundance stabilizes past the second dam. Biomass appears to be relatively consistent among sites; dams in the Bronx River appear partially limiting to upstream penetration. Eels appear to remain within their river segments and largely in place, indicating site specificity and small home ranges. It remains unclear whether upriver individuals are larger because of greater age correlated with the length (and likely corresponding time) of the journey or because of a relative lack of competition. Eel ladders on additional dams above the first dam would likely facilitate additional ingress. A limiting factor in the interpretation of our results is that it is not known how eel size and abundance vary in an undammed system. Progress towards Goals The target of this project is the American eel (Anguilla rostrata), a species that is considered depleted in U.S. waters (Atlantic States Marine Fisheries Commission, 2017) and is listed as globally Endangered under the IUCN Red List (Jacoby et al., 2017). It is a catadromous fish that is born in the ocean but spends most of its lifetime maturing in freshwater rivers and streams, including the Bronx River. Diadromous fishes, like eels, are fundamentally important to both marine and freshwater ecosystems: They serve as conveyors of energy and nutrients, as prey for many other species, and as indicators of ecosystem health. Increasing river connectivity, by removing barriers or providing passage to allow these fish to gain access to freshwater habitat needed for their growth and reproduction, is key to their restoration. The Bronx River is the only freshwater river in New York City. It stretches 23 miles from its source at the Kensico Dam in Westchester to its mouth at the eastern end of Long Island Sound. Dammed since the 1600s and suffering centuries of pollution and other assaults in one of the most urbanized places in the world, the Bronx River is slowing being restored through the hard work of many community partners. There are nine dams on the Bronx River, all four feet or more in height (Figure 1, Table 1). The largest (height of 14 feet), the 182nd Street dam, is the first dam and located three miles from the mouth of the river. The last dam, Haubold’s Dam, sits just above river mile 18. Despite these dams, American eels of 1 various life stages including glass/elver, yellow, and silver eels are found in the Bronx River. The lower dams on the river represent a formidable barrier to young eels, and all dams may play a role in restricting in-stream movements of yellow and silver eels (although the latter are able to pass downstream over the dams when they permanently exit the river on their return to the Sargasso Sea to spawn). To improve American eel access to its developmental habitat in the Bronx River requires, in part, a better understanding of the presence and timing of baby eels (glass and elvers) entering the river and the abundance, size distribution, and movements of juvenile and adult eels in various river segments, as well as the impact that dams and other barriers have on eel movements and productivity. These questions were the focus of this project, begun in 2014 and generously funded by the Hudson River Foundation from 2015 to 2017. Table 1. Dams on the Bronx River including their distance from mouth of the river and their height. Dam Dam River Dam Name Height # Mile (ft.) 1 182nd Street Dam 3 14 2 Twin Dams 3.9 10 3 Stone Mill Dam 4.5 7 4 Bronxville Dam 11 5 5 Old Stone Mill Dam 11.8 4 6 Hodgman Dam 12 4 7 Strathmore Dam 14.6 4 8 Popham Road Dam 15.8 10 9 Haubold’s Dam 18.4 6 Figure 1. Map of nine dams along the Bronx River. Throughout the project we used two distinct survey techniques to assess the American eel population and impacts of dams on their movements and distribution in the Bronx River: Eel mops: Also known as “medusas,” hand-constructed eel mops (Figure 2) were used to assess the timing and abundance of young eels (glass and elvers) entering the rivers (Silberschneider et al., 2001; Sullivan et al., 2009), and the potential impact that the dams posed to their upstream passage. Eel mops provide shelter to young eels as they seek day-time cover. A set of three eel 2 mops was placed near the base of the first six dams in 2015 and 2016, and only below the first two dams (182nd Street and Twin dams) in 2017. Mops were checked twice weekly from April to August each season. Figure 2. New eel mop as it was being deployed (right) and Jake Labelle collecting young eels from an eel mop in the Bronx River (left). Photos © Merry Camhi/WCS Electrofishing tag-and-recapture surveys: These labor-intensive surveys, requiring teams of 6- 10 people, were conducted using a Halltech backpack shocker and a two- or three-pass depletion approach (Carle and Strub, 1978) at each site (Figure 3). All captured eels were measured and recorded, and those greater than 250 mm were implanted with a Passive Integrated Transponder (PIT) tag, which enabled us to identify individual eels and subsequent recaptures. All eels were released at the site of their capture. The benefit of PIT tags is that they do not require a battery and so can be read throughout the eel’s lifetime. This will enable us to return to the river periodically in subsequent years to reassess the size, distribution, and survivorship of previously tagged animals. We conducted 66 electrofishing surveys between 2014 and 2017 (Table 2). We initially conducted electrofishing surveys below all dams, but when no eels were caught at the eighth and ninth dams, future surveys were constrained to the first seven. Figure 3. Electrofishing survey in the Bronx River (left; © Jake LaBelle/WCS) and measuring and checking eel for PIT tag (right; Julie Larsen Maher © WCS). 3 Table 2. Summary of eel mop and electrofishing sampling by year. Measure 2014 2015 2016 2017 Total Eel mops deployed 9 18 18 6 42 Eel mops sites 3 6 6 2 n/a* Eels captured in mops n/a 515 121 351 987 Electrofishing surveys 6 26 19 15 66 Eels captured during electrofishing 180 379 255 219 1033 Eels tagged (> 250 mm) and released 31 152 46 93 322 Eel size range (mm) (electrofishing) n/a 112-897 102-790 58-756 58-877 *Eel mop sites are not cumulative since mops were placed in a similar location each year. The following sections, organized by our original objectives, provide a brief explanation of our findings to date. These results will be expanded on for inclusion in a manuscript for journal publication. Objective 1. Assess extent of upstream penetration and the permeability of barriers to instream movements of all eel life stages We used eel mop arrays along the lower stretches of river to assess the ability of glass eels and elvers entering the Bronx River to pass various dams. Electrofishing and tagging of yellow and silver eels (i.e., eels longer than 250 mm) were used to assess the distribution of eels of various sizes across river segments, as well as the movement of recaptured (tagged) eels across dam barriers. Our research had two main conclusions regarding the permeability of barriers to upriver movements of American eels in the Bronx River: (1) Some eels do make it past the first six dams, but (2) there is a pronounced falloff in eel numbers from dam to dam, particularly after the first two dams (Figure 4). The first dam at 182nd Street is three miles from the mouth of the Bronx River with no barriers to impede passage along the way.

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