Variation between European eel Anguilla anguilla (L.) stocks in five marshes of the Thames Estuary (United Kingdom)

Abstract

The European eel (Anguilla anguilla, L.) was historically widely distributed throughout the United Kingdom, in coastal waters, lakes, rivers and wetlands. Recruitment has declined in recent decades and the species is now listed as 'Critically Endangered' on the International Union for Conservation of Nature and Natural Resources (IUCN) Red List. Management of suitable wetland habitats may contribute to species recovery; however, little is known about the stocks in these areas. In this study, yellow (adult stage [ 300 mm) eels were sampled in ditches in five marshes bordering the Thames Estuary in England, UK. Ecological variables, including ditch characteristics, invertebrate abundance and water quality parameters were measured. Habitat features were also observed and recorded, including access, land use and water management regimes. Eels were found in all marshes, but at varying catch-per-uniteffort (CPUE). There were no significant correlations between CPUE and the ecological variables, except ditch width. However, a significant difference in CPUE was found between two of the marshes, which may be explained by variations in local habitat management. Mean lengths showed a high proportion of females and mean body condition of four of the marshes was also found to be greater than in three rivers in the same region. These findings suggest that the marshes are potentially favourable eel habitats and that factors influencing habitat quality, such as land use and water management, may affect eel abundance, production of females and body condition. Effective management of such wetlands may therefore contribute to the conservation of European eel. Keywords North Kent marshes Á Thames RBD Á CPUE Á Yellow eel Á Ecological variables While the continental life stage of European eel (Anguilla anguilla, L.) has attracted substantial research interest over the last century, the majority of published studies have focused on eels in freshwater ecosystems, such as rivers and lakes (Aprahamian and Walker 2009; ICES 2009; Jacoby et al. 2015). However, it is thought that coastal wetlands, including estuaries, lagoons and marshes may provide K. Steele (&) Royal Veterinary College,

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

“…In agreement with the findings of Steele et al (2018) from the Thames estuary, the coastal paludal network of ditches and pools appears to be an important habitat for eels in the Glaven. This was indicated by the highest fyke net catches, suggesting that densities, particularly of yellow eels, may be higher than those estimated from the limited PAS surveys.…”

“…However, paludal habitat was highly variable, ranging from small channels with abundant macrophytes in clear waters to larger structureless channels with turbid waters. It is of note that Steele et al (2018) were unable to link eel abundance with habitat characteristics, other than a lack of channel management (i.e. dredging and weed-cutting) being beneficial.…”

Important questions to progress science and sustainable management of anguillid eels

“…Such habitat flexibility facilitates their broad intra‐ and interspecific distributions (Helfman et al, 1987); however, they also demonstrate specific microhabitat preferences and requirements. Factors driving habitat selection include water depth, temperature, salinity, substrate, water velocity, oxygen concentration, vegetation cover and biotic features such as prey availability, threat of predation and the presence of other eels (Greene et al, 2009; Jellyman & Arai, 2016; Laffaille et al, 2003, 2004; Steele et al, 2018). Knowledge of tropical anguillid habitat use is more limited than for temperate eels (Arai & Abdul Kadir, 2017).…”