Habitat preferences of different European eel size classes in a reclaimed marsh: A contribution to species and ecosystem conservation

Abstract

Freshwater reclaimed marshes along the European Atlantic coast are highly suitable for European eels (Anguilla anguilla). However, European eel stocks have declined, and the coastal marshes have been subjected to major disturbances. The objective of our study was to analyze the processes governing patterns of European eel microhabitat distribution of four eel size classes (from Ͻ160 mm to Ͼ360 mm) in a reclaimed marsh (France). Analyses were conducted using artificial neural network (ANN) techniques and ecological profiles. Our ANN results showed that eel densities were significantly related to three major influencing variables: the width of ditch section, the silt depth, and the density of emergent plants. Such ecological profiles were significantly different between small (Ͻ240 mm) and large eels (Ͼ360 mm): small eels were more widespread than large eels. Large eels were absent or at low densities in shallow ditches with a high aquatic plant cover obstructing the water column and a large quantity of silt. These characteristics seem to define the ditches not directly connected with the main river where dredging operations were rare. Management of regular dredging operations in the channels by maintaining a mosaic of permanent aquatic habitats and avoiding the heavy silt loads in most ditches should be promoted. This dredging operation was probably one of the most promising ways for restoring inland eel stocks.

Assessing the freshwater distribution of yellow eel

“…However, today it is declining sharply throughout its distribution area (Moriarty and Dekker, 1997;Dekker, 2003b). Although we do not understand the exact causes of this decline, it is speculated that the main factors involved are probably changes in oceanographic conditions that are assumed to be linked to climate changes (Desaunay and Guérault, 1997;Knights, 2003;Bonhommeau et al, 2008), habitat changes and the obstruction of migration by dams and other physical obstacles (Feunteun, 2002;Laffaille et al, 2004Laffaille et al, , 2007, and the impact of parasites and chemicals (Robinet and Feunteun, 2002;Kirk, 2003). In this context, Russel and Potter (2003) suggested that the precautionary approach is the most appropriate way to consider European eel stock management.…”

“…Yellow eel stock assessments and surveys are common, but they have been based on different sampling methods (Naismith and Knights, 1990;Knights et al, 1996;Jellyman and Graynoth, 2005;Laffaille et al, 2005a). They provide evaluations in terms of the relative or absolute abundance, spatial distribution and size-or age-class distribution patterns (LobonCervia et al, 1995;Smogor et al, 1995;Ibbotson et al, 2002;Laffaille et al, 2004;Laffaille et al, 2005c;. However, they often only report on a small geographical area.…”

“…More precisely, several studies suggest that around the 300-mm size several changes occur (Michel and Oberdorff, 1995;Baisez, 2001;Feunteun et al, 2003;Laffaille et al, 2003Laffaille et al, , 2004. Firstly, elvers and small yellow eels are mainly invertivorous and live in shallow water, whereas larger eels are found in deeper water, where they feed on other fish including small eels.…”

“…These patterns result mainly from a combination of different parameters: freshwater recruitment (above the tidal limit; see Laffaille et al, 2007), the stock already present (i.e. catchment occupancy according to a patchy fluid mosaic as proposed by Feunteun et al, 2003), habitat accessibility and habitat suitability (Laffaille et al, 2004). Generally speaking, monitoring small eel distribution patterns should provide information about the state of continental stocks.…”

Un Outil D’aide Á La Gestion De L’anguille : Le Tableau De Bord Anguillle Du Bassin Loire

“…Sur le plan ichtyologique, l'anguille européenne (Anguilla anguilla L.) est l'une des espèces occupant la plus grande diversité d'habitats au sein du territoire français (LAFFAILLE et al, 2003b ;2004a). Elle est en effet susceptible de coloniser tous les types d'habitats accessibles depuis la mer.…”