In this study from Fjellfrøsvatn, an oligotrophic lake in northern Norway, the parasite communities in two sympatric Arctic charr populations were compared. The dwarf morph, which inhabits the profundal zone, exhibited the lowest parasite diversity, seven species, and 72% of these charr harboured only one or two parasite species. In contrast, 10 parasite species were encountered in the larger normal charr, and between 5 and 8 species were present in 73% of these fish, which also utilised a broader food and habitat niche. Proteocephalus sp. was by far the most abundant species in the dwarf charr, probably because this morph fed intensively upon the benthic copepod Acanthocyclops gigas. On the other hand, parasites that are transmitted with littoral benthic prey (i.e., Phyllodistomum umblae, Cyathocephalus truncatus, Cystidicola farionis, and Crepidostomum spp.) were almost absent in the dwarf charr, though they were common in the normal morph. Also, Diphyllobothrium spp. were more prevalent in the normal charr, and this was attributed to their feeding upon limnetic copepods in the pelagic zone. The only recorded parasite with a direct life cycle, the copepod Salmincola edwardsii, had relatively similar abundances in the two morphs. The considerable differences in parasite community structure and abundance between the two charr populations were closely related to differences in the width and composition of the habitat and food niches between the morphs.
