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

Abstract

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.

Thirteen novel ideas and underutilised resources to support progress towards a range‐wide American eel stock assessment

“…eDNA concentrations explained 57% of the variation in estimated aquatic species abundance in 25 studies conducted in natural waters (Yates et al, 2019). For anguillid eels, studies have reported low (32%, Japanese eel, Itakura et al, 2019), variable (43%–95%, European eel, Weldon et al, 2020) and high (99%, American eel, Chin et al, 2021) explained variation of abundance estimates. Models that treat eDNA release as an allometric function of body size increase explained variation (Yates, Glaser, et al, 2021).…”

“…qPCR detects eDNA of targeted species with greater sensitivity and also measures its concentration (Kasai et al, 2021). Measured eDNA concentrations can be used to predict organism abundance by calculating their relation to populations estimated by independent methods (Chin et al, 2021). The inability of eDNA concentrations to fully explain the variance of independent population estimates is attributed to factors, other than organism abundance, that affect eDNA concentrations (notably variable rates of eDNA release, degradation and dispersal) (Caza-Allard et al, 2022).…”

Optimised protocol for the extraction of fish DNA from freshwater sediments

“…As well as spatial heterogeneity, fish biomass can be low in the environment and because eDNA concentrations are often positively related to fish biomass or abundance, eDNA concentration also may be relatively low (Chin et al, 2021;Itakura et al, 2019Itakura et al, , 2020Klymus et al, 2015;Thomson-Laing et al, 2021;Weldon et al, 2020). In this study, irrespective of the extraction method used, the concentration of fish sedDNA was low for all sediment types and fish species analysed.…”

“…Heterogeneity in fish DNA is not only a feature of sedDNA, but also of water eDNA, highlighting a challenge of using eDNA approaches to detect fish (Baldigo et al, 2017; Buxton et al, 2018; Eichmiller et al, 2014; Shaw et al, 2016; Turner et al, 2015). As well as spatial heterogeneity, fish biomass can be low in the environment and because eDNA concentrations are often positively related to fish biomass or abundance, eDNA concentration also may be relatively low (Chin et al, 2021; Itakura et al, 2019, 2020; Klymus et al, 2015; Thomson‐Laing et al, 2021; Weldon et al, 2020). In this study, irrespective of the extraction method used, the concentration of fish sedDNA was low for all sediment types and fish species analysed.…”

Development and validation of two environmental DNA assays for American Eel (Anguilla rostrata)

“…Currently, point or transect‐based electrofishing strategies (Reid, 2011) are used by a variety of American Eel monitoring programs (e.g., Laffaille et al, 2005; Newhard et al, 2021); however, American Eels have been shown to be highly susceptible to injury from these methods (Reynolds & Holliman, 2004). Recently Cairns et al (2022) highlighted the importance of eDNA for American Eel monitoring and Chin et al (2021) showed that American Eel eDNA concentration can be used to predict abundance; however, there is still no validated eDNA assay for this organism. Therefore, the goal of our study was to develop and validate two eDNA assays for the American Eel that could serve as a noninvasive method to aid in conservation efforts of the species (e.g., determination of fish passage and reintroduction success).…”