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Aquatic Life

The Bronx River and surrounding areas support a variety of wildlife species. The presence or absence of a particular species depends on its ability to tolerate the altered conditions of an urban river system. The composition of wildlife communities within streams (i.e. fish and benthic macroinvertebrates) is sensitive to changes in hydrology, fluvial geomorphology, water quality, and in-stream and overhanging vegetation. Wildlife in neighboring forests, on the other hand, is sensitive to changes in the stream communities, as well as changes to their forest habitat and landscape conditions. Information about wildlife along the Bronx River is uneven, with some groups such as benthic invertebrates fairly well studied and others such as amphibians, reptiles and mammals less studied. The following sections summarize our understanding of various groups.

Benthic Macroinvertebrates

Benthic macroinvertebrates include insects, worms, bivalves and crustaceans that live on and in the bottom (benthic) substrate of a water body. They play important and diverse roles in the aquatic food web, breaking down leaf litter, grazing on periphyton, a matirx of algae and heterotrophic microbes attached to submerged substrata in almost all aquatic ecosystems, removing organic matter from the water column and in turn serving as prey for many fish species. Assessments of the benthic community are commonly used to evaluate stream health (Barbour et al., 1999). Benthic macroinvertebrates re useful indicators of habitat and water quality because they are affected by multiple and synergistic effects of different pollutants, are sensitive to both chemical and physical impacts on their habitat, and are less mobile than fish, making it difficult for them to avoid stresses (NYS DEC, 2002).

Piss clam

The benthic species composition and abundance also provides an indication of water quality, because certain species are sensitive to pollutants, while others thrive under altered conditions. The aquatic invertebrate population of the Bronx River has been studied by NYS DEC in both Westchester County and northern Bronx County and NRG has studied benthic invertebrates in the Bronx County portion of the Bronx River. Both NYS DEC and NRG analyzed their data on benthic macro-invertebrates to determine the values of four different indices of water quality at different sites. According to NYS DEC criteria, the biotic index values calculated for the Bronx River classify the river as moderately impacted, which is defined as “altered to a large degree from the pristine state” (Bode et. al., 1999 and 2003). Annual sampling between 2002 and 2004 by NRG at eight sites suggests that the trend is more complicated within the Bronx County portion of the river. Among the eight sites sampled the lowest water quality was observed within the Botanical Garden (at approximately river mile 4.1), a finding which we are currently further investigating. Conversely the best water quality was observed within the Bronx River Forest (river mile 5). There have been no significant changes in the water quality indices as determined by the benthic invertebrate community, since monitoring began. Benthic invertebrate monitoring has also been conducted as part of the pilot oyster reef project.


Despite the construction of many dams, which have artificially fragmented and disconnected fish populations since the 1600s, and the history of polluting industries lining the river’s banks, the Bronx River fish community is surprisingly intact—although species diversity appears to be slightly lower today than in the past. More than 30 species of fish have been collected in recent surveys, including several pollution-sensitive species (Schmidt, 1984; Larson, 2004). The current fish assemblage in the Bronx River, while representative of an urban system, is quite diverse. The fish assemblage found in the estuarine section, for example, is similar to that found at the less degraded Pelham Bay Park. Although many of the expected species are present, their abundance is low. The predominant species are tolerant of disturbed and polluted environments, but variability in species composition and abundance from year to year indicates system instability. For example, conditions in 2003, including high flows and dredging in the East River, resulted in extremely poor fish sampling results in the estuary, a stark change from previous years..

Anadromous fish reintroduction

In the Spring of 2003, NRG and Lehman College were began work on a WCS/NOAA Regional Partnership Grant to study the feasibility of restoring anadromous fish to the Bronx River. The study verified that dams on the river have effectively cut off all anadromous fish migration since the 1600’s. Today, the presence of species, such as striped bass and river herring at the mouth of the river suggest suitable estuarine conditions for anadromous fish. Water quality data upstream also indicates that water temperature and suspended sediment concentrations are within the appropriate range. Suitable spawning and rearing habitat is available for blueback herring (Alsoa aestivalis) and alewife (Alosa pseudoharengus) upstream, although habitat may be restricted under extreme low flows, or by sedimentation, particularly in impoundments.

Confident that alewife would be able to survive in the Bronx River, NRG, WCS, and the Connecticut Department of Environmental Conservation began depositing alewife to spawn in the Bronx Zoo impoundment in 2006. If these fish spawn successfully, the surviving young will swim out to sea after a few months, and return in three to five years. In the meantime, project partners are studying possible designs for fish passage that would allow returning fish to reach their spawning ground while satisfying aesthetic and structural requirements. To date, no alewife mortality has been observed, and but juvenile alewife feeding behavior has been witnessed, suggested spawning was successful. Fish passageways are currently being designed for the dams on the river.

Amphibians, Reptiles and Mammals

Though there is very little quantitative data on the abundance and distribution of amphibians, reptiles and mammals in the Bronx River watershed, there are strong indications that many of these animals do live in and around the river. For example, muskrat populate the river in sufficient numbers that sightings are frequent. During the Bronx River BioBlitz in 2005, snapping turtles of varying sizes were found in the impoundment above the 182nd St. Dam and students from a local middle school discovered a red-back salamander during a field trip to the river. Since these organisms play an important role in natural ecosystems, the Bronx River system could be better assessed if more data on Bronx River communities existed.


The plankton communities of the Bronx River have yet to be intensively studied. Plankton play an important role in nutrient cycling and energy transfer in aquatic environments. Lehman College’s LaMER is monitoring Bronx River plankton as part of its ongoing research efforts to characterize the fauna of the Bronx River. Additional collection and analysis of plankton samples are necessary before conclusions about health of the Bronx River’s plankton communities can be drawn.

Aquatic Life Monitoring

The following table provides links to other restoration or monitoring projects focusing on aquatic life in the Bronx River.

Section Subject Title Source Year
All Fish Distribution and Number of Fish Sampled on the Bronx River Lehman College/LaMer 2001- 2003
All Benthic Inverts Urban Riparian Wetland Monitoring - Benthic Invertebrates NRG 2002-2004, 2007
Estuary Shellfish Fish and Shellfish Habitat Creation and Seeding Project - Pilot Oyster Reef NRG 2006-Present
Estuary Fish Recreationally Important Species Lehman College/LaMer 2001- 2003
Estuary Plankton Bronx River Plankton Sampling Tows Lehman College/LaMer 2003

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