Anguillicoloides crassus infection of European eel, Anguilla anguilla (L.), in inland waters of Estonia: history of introduction, prevalence and intensity

Abstract

Summary Eel fishery in Estonian inland waters depends entirely on the stocking of glass eels or pre-grown (farmed) eels. Via importation of live eels of 20–30 cm length the non-indigenous swimbladder nematode Anguillicoloides crassus was probably introduced via Germany into Lake Vortsjarv in 1988, and has since spread to many inland waters of Estonia. In 1992, the parasite was found in eel caught from Lake Vortsjarv. Between 1992 and 2002 and additionally in 2008, we examined in total 870 eels from Lake Vortsjarv (270 km2) and in 2008, 63 eels from three small lakes for adult A. crassus. The aim of the study was to obtain information on the variation of A. crassus infection in eels in Estonian lakes, to determine the temporal dynamics of prevalence and intensity of infection, and to establish a relationship between the length of host and intensity of infection in the eels in Lake Vortsjarv. There appeared to be a pronounced variation in prevalences of infected eels (from 3.7% to 100%) between the four investigated lakes. However, in Lake Vortsjarv, the prevalence of adult A. crassus infection remained stable (mean about 65%) for many years. The average number of nematode per infected eel (mean intensity) ranged from 12.6 ± 2.5 in 1993 to 4.0 ± 0.6 in 1999 in Lake Vortsjarv, while it was significantly higher (P < 0.0001) in the period 1992–1998 compared to 1999–2002 and 2008. The mean number of parasites per swimbladder was not related to eel length and no statistical difference was found in the condition factor of infected and non-infected eels. Although under normal environmental conditions A. crassus has not caused serious disease problems to eels in the study area, high intensity of parasite infection may contribute to eel kills due to oxygen deficiency in winter under the ice in Lake Vortsjarv.