Infection by endoparasites reduces maximum swimming speed of European smelt Osmerus eperlanus and European eel Anguilla Anguilla

Maximum swimming speed of European smelt Osmerus eperlanus and European eel Angudla anguilla from the Elbe estuary was investigated in a circular experimental tank in relation to infect~on with 3 parasite species: the muscle parasites Pseudoterranova decipiens (Nematoda) and Pleistophora ladogensis (Microspora), both in smelt, and Anguillicola crassus, a parasite of the swim bladder in eel, were considered.All 3 parasites reduced the swimming performance of their hosts significantly.Smelt were seriously affected by double infection with P. decipiens and P. ladogensis.The reduction in maximum swimming speed of heavily infected fish was 32.2 % in smelt and 18.6 % in eel compared with uninfected fish.The reduced swimming ability of parasitized fish is assumed to make them more susceptible to predators and to entrainment by the cooling-water intakes of power plants than are uninfected fish, and it may reduce their ability to migrate.Especially in eel, which must migrate extensively to spawn, the poss~ble effect of paras~tization on population level has to be considered.