Catchment‐scale distribution, abundance, habitat use, and movements of European eel (Anguilla anguilla L.) in a small UK river: Implications for conservation management

Abstract

Effective conservation management of the Critically Endangered European eel (Anguilla anguilla) is hindered by incomplete understanding of distribution, abundance, and habitat requirements at the catchment scale. All habitats available to eels within a small, highly regulated river catchment, representative of many used across the species' range, were sampled using several methods (including point abundance sample electric fishing and fyke nets) and supplemented by individual telemetry to investigate movements. Eels were found throughout the catchment (59% of n = 131 sites) from the coastal marshes to the headwaters, although the probability of presence declined with distance from the estuary. The lack of a clear relationship with perceived barriers may illustrate a mismatch with the reality experienced by eels, as telemetry identified connectivity across obstacles between paludal habitat and estuary, and detected escapement of mature silver eels from both lotic and lacustrine habitat. Different size/age classes occurred in different parts of the catchment, partly linked to different habitat associations, with coastal paludal habitat supporting >50% of the catchment population and especially smaller (possibly male dominated) yellow eels. Recently recruited elvers were most abundant in the lower reaches of lotic habitat. The largest (probably female) eels were concentrated in lacustrine sites, especially at the ‘end‐of‐the‐line’ in the headwaters. Experiences here suggest that conservation management for eels in small catchments is best focused on: (i) improving connectivity and assisting migration of elvers across ‘problem’ barriers that cannot be removed or modified, (ii) river restoration and rewilding, especially measures that increase instream woody material to provide refuge habitat and, (iii) enhancement or creation (where necessary) of suitable lacustrine habitat to benefit large females in particular. Such action across numerous small river catchments may ultimately help support the recovery of eel stocks.

Ontogenetic habitat shift of Anguilla marmorata and A. japonica in the rivers of southern Japan: Implications for habitat use evaluation and conservation

“…However, these gradual shifts have not been sufficiently considered when evaluating their habitat use. Many previous studies have examined differences in habitat use only between two or more size classes (Itakura & Wakiya, 2020; Matsushige, Yasutake & Mochioka, 2020) or between two or more life stages (Degerman et al, 2019; Kume et al, 2020; Shuai et al, 2021b; Harwood et al, 2022). These classifications make it difficult to capture gradual changes in habitat use and may lead to inaccurate evaluation of habitat use.…”

“…However, many studies of anguillid eels have only evaluated the habitat use within only two or more separate size classes or life stages. For example, many studies have classified individuals into ‘small’ and ‘large’ classes according to body size (Itakura & Wakiya, 2020; Matsushige, Yasutake & Mochioka, 2020) or into elver and yellow eel stages only (Degerman et al, 2019; Kume et al, 2019; Harwood et al, 2022). These classifications could make it difficult to capture gradual changes in habitat use with growth or even lead to the misevaluation of habitat use if the classifications are not appropriate.…”

The drivers of anguillid eel movement in lentic water bodies: a systematic map

“…Although rarely used across our 36 studies, sex was found to be significant in 100% (n = 4) of cases. Its low inclusion rate is probably due to difficulties in sexing eels quickly in situ, as well as sexual dimorphism which can lead to females being preferentially selected for tagging studies (Béguer-Pon et al 2018a, b) and female biased preferences for lentic, lacustrine habitats (Oliveira et al 2001;Hagihara et al 2018;Harwood et al 2022). Where investigated, monthly and seasonal variance are seen between the timing of movement behaviours between male and female eels (Todd 1981;Stuart et al 2019;Lagarde et al 2021).…”