Assessing the relative influences of abiotic and biotic factors on American eel <i>Anguilla rostrata</i> distribution using hydrologic, physical habitat, and functional trait data

The American Eel Anguilla rostrata has experienced a marked population decline due to a multitude of threats, including pollution, overfishing, and damming. Government agencies responsible for American Eel management have identified gaps in survey coverage and understanding of habitat needs as critical obstacles to recovery. However, efforts to expand American Eel monitoring are hindered by this species’ resistance to capture, the wide variety of habitats in which they occur, and their extensive natural range, which would be cost prohibitive to survey using established techniques. Surveys based on analysis of environmental DNA (eDNA) may offer an affordable and efficient means of closing these gaps. While eDNA methods are increasingly used as a rapid, minimally invasive means of detecting species presence, attempts to use them to produce quantitative estimates of abundance have yielded mixed results. In order to evaluate the viability of eDNA as a quantitative American Eel monitoring tool, we compared the concentration of American Eel DNA in water samples to estimates of numerical and biomass density measured by electrofishing in the Bronx River, New York. Our results revealed a strong, positive association between DNA concentration and numerical density but no relationship with biomass density. Mass‐specific eDNA production rates were found to decline with mean American Eel weight. While further study will be required to determine whether the observed relationship between DNA concentration and numerical abundance is consistent across sites and whether it can be generalized to other systems, our findings suggest that eDNA will likely be a feasible and cost‐effective option for surveying American Eel abundance throughout their range.

Section: Discussionmentioning

confidence: 99%

Relating American Eel Abundance to Environmental DNA Concentration in the Bronx River

Chin

Waldman

Bednarski

et al. 2021

“…There is evidence that by acting as barriers to migration, dams slow the colonization of upstream reaches by European Eels (Lasne and Laffaille 2008). Habitat fragmentation by dams also has the strongest influences on American Eel distribution (Woods and McGarvey 2018). Dams may have important impacts on the characteristics of eels at a local scale, particularly because they restrain the distribution of small colonizing eels that are sensitive to density‐dependent mechanisms (Feunteun 2002).…”

Section: Discussionmentioning

confidence: 99%

Spatial Distribution Patterns of Japanese Eels Anguilla japonica in a Large Subtropical River (Pearl River), China

Shuai

Liu

et al. 2020

The Japanese Eel Anguilla japonica is an important migratory fish with great commercial and conservational significance in Asia. The Pearl River in southern China provides important natural growing and rearing grounds for glass eel recruits, but there is very limited information on their population characteristics. We studied the resource status and habitat preferences of Japanese Eels in the Pearl River from 2015 to 2018. In total, 181 Japanese Eels (89 males and 92 females) were collected during the study period. In terms of frequency of occurrence, Japanese Eels contributed less than 1% of the total number of collected fish specimens. Most of the eels were between 255 and 600 mm TL. Redundancy analysis indicated that eels were mainly distributed in the downstream portion of the river and preferred more complex habitats characterized by larger river fractals, higher coefficients of fluvial facies, and greater river widths. Adults preferred to inhabit areas with larger fractals and river widths, whereas juveniles inhabited areas with higher coefficients of fluvial facies. Young‐of‐the‐year eels preferred habitats near the estuary and with high water salinity. Dams have blocked the eels’ migration routes and affected their spatial distribution, in turn leading to a reduction in the population. This study was part of a long‐term investigation of wild Japanese Eels in the Pearl River, and the results showed that the Japanese Eel resource in southern China is extremely scarce and its conservation is greatly needed.

“…Because American Eels typically aggregate below dams (Machut et al. 2007; Woods and McGarvey 2018), sampling sites were chosen as near to the base of each dam as was safely possible. For electrofishing, 30‐m‐long sections of the river were blocked off using two 15.2‐ × 1.2‐m 3/8‐in delta knotless seine nets (Memphis Net and Twine, Memphis, Tennessee).…”

Section: Methodsmentioning

confidence: 99%

“…Also, existing dams have been major contributors to lowered abundances by causing habitat fragmentation, restricted upstream and downstream movements, and, in the case of hydrodams, downstream mortality from turbine blades (Carr and Whoriskey 2008; Eyler et al. 2016; Woods and McGarvey 2018). American Eels are now hindered from access to more than 84% of their historic inland waters along the U.S. Atlantic coast (Haro et al.…”

mentioning

confidence: 99%

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

Camhi

Bednarski

LaBelle

et al. 2021

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.