Influence of introduced vs. native parasites on the body condition of migrant silver eels

Because parasitism is among the reasons invoked to explain the collapse of Anguilla anguilla, we evaluated the parasitic constraint on body condition (BC) of migrant silver eels as a proxy of fitness with inter-site comparisons. Metazoan parasites were studied in 149 silver eels from five sites (northern Europe). In total, 89% were infected by 13 species including Myxozoa, Monogenea, Cestoda, Nematoda, and Acanthocephala. Anguillicoloides crassus was most common (56%), then Acanthocephalus clavula (30%), and Pseudodactylogyrus sp. (17%). BC, calculated for 58 females, was negatively correlated by abundance of the introduced Pseudodactylogyrus sp. but not by other parasite taxa. Nevertheless, the introduced A. crassus was considered as a severe pathogen based on previous data, whereas the native A. clavula was supposed to have limited impact. Parasite component communities and BC were different between sites. Silver eels from Stockholm Archipelago (Sweden) were the least parasitized (40% vs. 90-95% for other sites) with no parasites on the gills. Burrishoole (Ireland) differed by the absence of A. crassus and high prevalence of A. clavula (84%) but without consequences on BC. Gudenaa (Denmark), Corrib (Ireland), and Frémur (France) were close due to high prevalence of A. crassus (89-93%). Gudenaa and Corrib were the most similar because Pseudodactylogyrus sp. was also highly prevalent (respectively 71% and 60%) whereas absent in Frémur. Our results suggest that the fitness loss induced by the introduced parasites could affect the spawning success of migrant silver eels from Gudenaa and Corrib, and to a lesser extent from Frémur, but probably not those from Stockholm Archipelago and Burrishoole.