Concave relationship between growth of masu salmon <i>Oncorhynchus masou</i> and abundance of the parasite <i>Salmonema</i> cf. <i>ephemeridarum</i> (Nematoda, Cystidicolidae)

Under the common definition, parasites generally exploit and have negative effects on hosts. Intestinal parasites, including species of nematodes, are believed to be less harmful, but the impact on the host generally depends on the degree of parasitic infection. We explored the relationship between the number of Salmonema cf. ephemeridarum (Linstow, 1872), a nematode parasite, infecting masu salmon Oncorhynchus masou (Brevoort, 1856) and growth of the salmon in an outdoor enclosure system . We found a concave relationship between the growth of masu salmon and the number of S. cf. ephemeridarum. This suggests that the negative effect of this parasite on host growth becomes apparent only when the parasite abundance exceeds a certain value. Although the mechanism behind this phenomenon remains unclear, this study demonstrated an uncommon relationship between host growth and parasite abundance. Our results imply that the effect of parasitism on host growth can reverse depending on parasite number. This concave function model can be a foundation for further comprehensive analysis.