Colonisation of freshwater habitats by the European eel Anguilla anguilla

Abstract

SUMMARY 1. The spatial distribution of European eels in 18 U.K. rivers was related to distance from tidal limit using a negative exponential model. This function accounted for between 19 and 90% of the variation in eel density where quantitative data was available. For semiquantitative data the negative exponential function was a significant predictor of eel densities in only six out of 10 cases, although all rivers showed a consistent decline in abundance with distance upstream from the tidal limit. 2. The spatial distribution of different age groups of European eel in River Severn showed an initial rapid dispersion into freshwater followed by a much slower dispersion rate. Movement of the population upstream by a wave‐form migration process does not occur in this system. Instead colonisation of freshwaters can be seen as a two‐phase dispersion. Phase‐1 is a rapid dispersion upstream driven by density at the point source. Phase‐2 commences once the eels become yellow eels and is equivalent to random diffusion of particles. 3. These processes have important implications for the penetration of freshwaters with reduced numbers of eel larvae arriving on the coast of Europe and North America. Eel abundance will decrease more in freshwaters in an upstream direction whilst it may remain stable or decrease to a lesser extent in estuaries. They are also able to explain the demography of eels migrating upstream over weirs and the observations of varying sex ratios within catchments. We conclude that a dispersion model dependent on age, temperature, difficulty of migration, habitat quality and density of eels should be an important part of freshwater eel management.

Assessing the freshwater distribution of yellow eel

“…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.…”

“…For instance, the size structure observed could be biased if all the different habitats are not investigated , or if the gear used is not the same (Baisez, 2001). Despite some exceptions (Lamson et al, 2006), large eels generally exhibit little migration behavior and tend to become sedentary ("home-range dwellers"), though some remain "nomadic" (Baisez, 2001;Ibbotson et al, 2002;Feunteun et al, 2003;Laffaille et al, 2005b). In UK rivers, Ibbotson et al (2002) showed that density-dependent mechanisms mainly apply to eels aged 0-4 years, whereas larger ones disperse randomly.…”

“…Despite some exceptions (Lamson et al, 2006), large eels generally exhibit little migration behavior and tend to become sedentary ("home-range dwellers"), though some remain "nomadic" (Baisez, 2001;Ibbotson et al, 2002;Feunteun et al, 2003;Laffaille et al, 2005b). In UK rivers, Ibbotson et al (2002) showed that density-dependent mechanisms mainly apply to eels aged 0-4 years, whereas larger ones disperse randomly. The environmental and intrinsic factors involved in migration behavior change when individuals grow up, and it is these pressures that determine the distribution of different size classes along the course of freshwater rivers .…”

“…Moriarty, 1986;Ibbotson et al, 2002;Feunteun et al, 2003;Rigaud et al, 2008). In other words, the more the downstream areas are already occupied, the more eels are forced to go upstream to settle in suitable unoccupied places.…”

Corner and sloped culvert baffles improve the upstream passage of adult European eels (Anguilla anguilla)

“…Barriers to the upstream migration of juvenile eels (elvers) (Moriarty and Dekker, 1997;Feunteun, 2002), and dispersal of resident adults (yellow eels) (Ibbotson et al, 2002;Feunteun et al, 2003), have likely contributed to this decline. In 2007, the European Union adopted council regulation number 1100/2007 for establishing recovery measures for the European eel.…”