Parasite-induced mortality in three host populations of the roach Rutilus rutilus (L.) by the tapeworm Ligula intestinalis (L.)

Patterns of parasite infestation by plerocercoid forms of the tapeworm Ligu la intestinalis (Cestoda, Pseudophyllidea) in the second intermediate host, the roach Rutilus rutilus, were analysed in three fish populations of Sout h-Western France during 1998. In all three studied sites, i.e. Pareloup Lak e, Lavernose-Lacasse and Muret gravel pits, roach were commonly parasitized with plerocercoids. A strong increase in prevalence, mean number of parasi tes and parasitic load was observed in the youngest hosts with a maximum pe ak reached in the medium-age (2+, 3+)roach classes. Such trends as a functi on of host fish age would indicate the occurrence of parasite-induced host mortality, with the most heavily infected fish specimens being removed from the three roach populations. The three roach populations infected by L. in testinalis exhibit different patterns of parasite-induced mortality which c ould correspond to different possible ecological and evolutionary responses associated with local variation in host-parasite interactions. The parasit e-induced host mortality observed, i.e. direct mortality caused by pathogen ic effects and /or enhanced fish predation by piscivorous birds to increase parasitic transmission is discussed within the context of the adaptative p arasite-induced manipulation hypothesis.