The infection of the European eel with the parasitic nematode Anguillicoloides crassus in inland and coastal waters of northern Germany

Recruitment of the European eel is in decline since three decades. So far, the reasons of the decline have not been fully understood. Beside other factors, infection with the exotic swimbladder parasite Anguillicoloides crassus has been discussed as a threat to the species. In the present study monitoring results for A. crassus in eels from North German waters are presented. Between 1996 and 2011, the swimbladders of 17,219 eels from eight freshwater and coastal water areas were analyzed. Prevalence, abundance of parasites, infection intensity and severity of the damage to the swimbladder were recorded by visual inspection. In the freshwaters the prevalence was in the range of 65–83 %, whereas significantly lower values were found in the brackish waters. The differences were less clear for infection intensity but significantly lower values were found in the outermost location in the Baltic Sea. Mean damage to the swimbladders was highest in eels from the Rivers Weser and Elbe and lowest in the Baltic coastal waters. Prevalence and damage degree were stable in all waters except for two rivers, where a decreasing trend in infection intensity was found.