Maximum swimming speed of European smelt Osmerus eperlanus and European eelAngudla 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.

ABSTRACT. From October 1987 to September 1988, l778 eels were sampled at monthly intervals in the Elbe estuary, Germany, and examined for parasites in the swimbladder and muscle. Juveniles and adults of the nematode Anguillicola crassus were the only parasites found in the swimbladder and L-111 stages of the nematode Pseudoterranova decjpiens were the only parasites found in the muscle. Averaged over all samples, 57.7 % of the eel were infected with A. crassus and 3.7 % with P. decipiens. Prevalence decreased for A. crassus with increasing fish length, but increased for P. decipiens. No clear seasonal fluctuations in parasite frequency were detected. Infection with A. crassus could not be related to any change in condition factor or liversomatic index. Some P. decipiens from the muscle of smelt after experimental oral transfer settled in the body cavity and muscle of eel. The majority of these nematodes, however, penetrated through the stomach wall, muscle and skin and left the eel.