Prevalence and abundance of Anguillicola crassus in the European eel (Anguilla anguilla) at a thermal discharge site on the Swedish coast

The accidental introduction and establishment of a population of the sanuivorous swimbladder nematode, Anguillicola crassus , in eels in an area of the Baltic Sea off the coast of Sweden that is affected by thermal water discharge was studied bi-annually over a 5-year period. In 1987, none of 387 eels examined was found to be affected. The first case of infection was recorded in 1988 and within 3 years the prevalence of infection had increased to about 60 %. By 1991, this was the most heavily infested area in Swedish waters. Whereas for yellow eels in the surrounding archipelago records have been kept only sporadicall since the infestation became established, in 1989–91 it was observed that both the prevalence and the abundance of infection were significantly higher in spring than during the second half of the year. When the frequency distributions of parasites for this period 1989–91 are compared, it is evident that this seasonality is connected with an autumn increase in the proportion of eels free from infestation, rather than a reduced proportion of those heavily infected. Furthermore, no seasonality was noted in the proportion of developmental stages of the parasite. Consequently, the observed seasonality is suggested mainly to be an effect of immigrating heathy eels.