• Arctic charr;
  • niche segregation;
  • parasites;
  • secondary sex traits;
  • sexual selection;
  • speciation

The relationship between parasite infection and breeding ornamentation in an Arctic charr Salvelinus alpinus population was examined. Interrelationships between the intensities of parasites with different dietary transmission routes suggested that hosts were segregated by habitat and diet preferences. That is, intensities of parasites transmitted through intermediate hosts that share habitat were, in both sexes, positively interrelated. Negative interrelationships in the intensities of parasites transmitted through different hosts were found only in females. The intensity of breeding colouration was positively correlated with intensities of amphipod-transmitted parasites in both sexes. Intensities of parasites transmitted through copepods showed no relationship to morphological or ornamental characters. Seen in the context of morphology and ornaments of previously described sympatric populations of Arctic charr, these relationships suggested that individual variation in niche background may provide a source for the initial variation in morphology and secondary sexual traits leading to sympatric speciation through sexual selection.