Spatial and seasonal variations of methylmercury in European glass eels (Anguilla anguilla) in the Adour estuary (France) and relation to their migratory behaviour.

Overall recruitment of European glass eels (Anguilla anguilla) has decreased significantly since the early 1980s. Due to their long life cycle, benthic/demersal habits and high lipid content, eels might accumulate high concentrations of contaminants, but data concerning glass eels are still scarce. This study provides original data on methylmercury (MeHg) concentrations in glass eels at spatial (marine and estuarine), annual and seasonal scales. The relationship between MeHg concentrations in glass eels and their propensity to migrate up estuaries was also investigated. MeHg data were individually related to the eels' energetic condition which was estimated by dry weight. Glass eel migratory behaviour was investigated in an experimental flume and related to the MeHg concentration and dry weight at the individual scale. Marine and estuarine glass eels were caught from 2004 to 2011. There was a strong inverse correlation between MeHg concentrations and dry weight. MeHg concentrations increased in marine and estuarine glass eels from 2004 to 2009 and from 2004 to 2010, respectively, and then, both groups decreased in 2011. On a seasonal time scale, MeHg concentrations were higher at the end of the fishing season (April). MeHg bioaccumulation is likely to result from different sources, but the lack of significant differences between marine and estuarine glass eels suggests that direct contamination during estuarine migration is low. Other sources such as maternal transfer or oceanic contamination are discussed.