PubMed1999
As a superior competitor for planktonic food, vendace (Coregonus albula), when abundant, is expected to displace whitefish (Coregonus lavaretus) from feeding on plankton and to force it to rely more on benthic food. The predicted result would be a reduced abundance of the copepod-transmitted cestode Triaenophorus crassus in whitefish, but an increase in the abundance of the nematode Cystidicola farionis transmitted via benthic amphipods. We studied the occurrence of both parasites in whitefish during 1991-1996 in three interconnected areas at Lake Saimaa, Finland, where the densities of the vendace stocks varied due to natural fluctuation in year-class strengths. In accordance with our hypothesis, some indication of the effect of the density of the vendace population on abundance of C. farionis infection in whitefish 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 copepod species in Lake Saimaa act as first intermediate hosts of T. crassus, and verified from stomach samples the exposure of both whitefish and vendace to those species. Experimental infections indicated that the infectivity 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 short period in spring. At that time of the year copepods are abundant and available to both whitefish and vendace, but since the parasite is less infective 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 includes amphipods transmitting C. farionis. During vendace stock decline, whitefish may, however, continue to feed on plankton and avoid exposure to C. farionis.