Accumulation of plerocercoids of Triaenophorus crassus in the second intermediate host Coregonus lavaretus and their effect on growth of the host

Patterns of accumulation of Triaenophorus crassus in its second intermediat e host whitefish Coregonus lavaretus s.l. were studied between 1991 and 199 6 from two host populations in two separate areas of Lake Saimaa, Finland. Whitefish were infected commonly with several T. crassus plerocercoids and the parasites were aggregated into the oldest hosts. In one host population the annual parasite accumulation was 0.9 parasites in all host age groups between 3 and 8 years. In the other host population the annual accumulation was 1.6 parasites in 3-5-year-old fish, but increased up to 3 to 4 parasit es per year in fish over 5 years old. The increase did not coincide with th e period of maturation or any increase in whitefish growth, both of which c ould alter the food intake of the fish. The sharp increase in the annual ac cumulation suggests a threshold intensity above which the probability of ac quiring further parasites increases. In spite of a heavy aggregation of par asites there was no evidence of parasite-induced host mortality. The annual increase in mean abundance was not correlated with the mean annual weight increase in 2-4-year-old fish within cohorts. However, evidence of a negati ve effect of parasites on whitefish growth was revealed by back-calculation of lengths of uninfected and infected whitefish and correlations between l ength or weight of fish and intensity of infection with fish age. Both anal yses suggested that larger young fish harboured more parasites than the sma ller ones while in older fish the reverse was true, a pattern that has not been shown earlier for parasitized fish. (C) 1999 The Fisheries Society of the British Isles.