Recently Poulin (1991), using published data on Canadian freshwater fishes (60 species, three families), concluded that there is no difference in parasite species numbers between solitary and social species. Nor could he associate parasite diversity to host size, age or range of distribution. Analysing Poulin's data with somewhat differing methods I reached different conclusions. Recognising that reported parasite species number per host species is a function of research effort (also noted by Poulin), I use residuals of the regression model as a measure of standardized parasite species number. First, it turned out that solitary and social salmonids differ in parasite faunal richness. That is, solitary species harbour a smaller diversity than species that school outside the breeding season. Second, host distribution range clearly correlates with the richness of total parasite fauna of the species in the three families. This observation was recently confirmed for North American freshwater fish species by Chandler and Cabana (1991). Third, in Percidae host size and age are also correlated with parasite species diversity.
Keywords: Evolutionary conclusions; Freshwater fishes; Parasitism; Research effort; Sociality.