Parasites transmitted by non-native species can have direct impacts on the resident species and threat biodiversity. In recent years, the piscivorous birds are expanding their distribution throughout Japan. In the northern Nagano Prefecture, which is the southern limit of the distribution of the endangered Pseudorasbora pumila, the colony of gray heron increased from one to 16 during the last four decades. Gray heron not only prey on P. pumila, but carry the yellow grub which is metacercariae of Clinostomus into the P. pumila habitat.

To evaluate the negative impacts of parasitic pollution on population growth of P. pumila, we investigated about relationships between infection rates, condition factor, steroid hormones, male traits, and gonad-somatic index using individuals from two P. pumila populations in the Nagano Prefecture.

As a result, 21.5 % and 97.4 % of fish collected from two ponds were infected, respectively. The maximum total weight of parasites　accounted for 5 % of body weight of the host. Sex difference in the infection rates was not observed, but smaller females received more impact from the parasites. The other indexes did not correlate with infection rates. The average number of parasites in a host decreased significantly from spring to autumn, suggesting that intensive infections could negatively affect the mortality rate of P. pumila.