The influence of food competition and host specificity on the transmissionof Triaenophorus crassus (Cestoda) and Cystidicola farionis (Nematoda) to Coregonus lavaretus and Coregonus albula (Pisces : Coregonidae) in Finland

As a superior competitor for planktonic food, vendace (Coregonus albula), w hen abundant, is expected to displace whitefish (Coregonus lavaretus) from feeding on plankton and to force it to rely more on benthic food. The predi cted result would be a reduced abundance of the Copepod-transmitted cestode Triaenophorus crassus in whitefish, but an increase in the abundance of th e nematode Cystidicola farionis transmitted via benthic amphipods. We studi ed the occurrence of both parasites in whitefish during 1991-1996 in three interconnected areas at Lake Saimaa, Finland, where the densities of the ve ndace stocks varied due to natural fluctuation in year-class strengths. In accordance with our hypothesis, some indication of the effect of the densit y of the vendace population on abundance of C. farionis infection in whitef ish was found, but not in the case of T. crassus. Only 0.2% of vendace were infected with T. crassus, while up to 100% of the whitefish in the yearly samples harboured the parasite. In further experiments we clarified which c opepod species in Lake Saimaa act as first intermediate hosts of T. cl cras sus, and verified from stomach samples the exposure of both whitefish and v endace to those species. Experimental infections indicated that the infecti vity of T. crassus is lower for vendace than for whitefish. We suggest that the reason why vendace stock density does not affect T. crassus infection in whitefish is that T, crassus is transmitted in littoral areas during a s hort period in spring. At that time of the year copepods are abundant and a vailable to both whitefish and vendace, but since the parasite is less infe ctive to vendace, they do not become infected. At other times of the year, dense vendace stocks may force whitefish to shift to benthic food, which in cludes amphipods transmitting C. farionis. During vendace stock decline, wh itefish may, however, continue to feed on plankton and avoid exposure to C. farionis. (C) 1999 Australian Society for Parasitology Inc. Published by E lsevier Science Ltd. All rights reserved.