Acute loss of European silver eel fitness by introduced (vs native) parasitic helminthes threatening spawning success

International audienceHelminth parasites were studied in 149 silver eels from five sites in northern Europe. In total, 88% were infected by 12 species including Monogena, Cestoda, Nematoda, and Acanthocephala. Anguillicoloides crassus was most common (56%), then Acanthocephalus clavula (30%) and Pseudodactylogyrus spp. (17%). The body condition (BC) was negatively influenced by the abundance of the introduced Pseudodactylogyrus spp. but not by the other parasite taxa. However, 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. Eels from Stockholm Archipelago (Sweden) were the less parasitized (30% vs ≥ 90% for other sites) with no parasites in gills. Burrishoole (Ireland) differed by absence of A. crassus and high prevalence of A. clavula (84%) but without consequences on BC. Gudenaa (Denmark), Corrib (Ireland) and Frmur (France) were close due to high prevalence of A. crassus (≥ 89%). Gudenaa and Corrib were the most similar because Pseudodactylogyrus spp. was also highly prevalent (≥ 60%) whereas absent in Frmur. Our results suggest that introduced parasites may threaten the spawning success of migrant silver eels from Gudenaa, Corrib and Frmur, but not of those from Stockholm Archipelago and Burrishoole