Parasitism of a native H awaiian stream fish by an introduced nematode increases with declining precipitation across a natural rainfall gradient

Climate change can promote disease emergence if shifting conditions favour infection of native fauna by introduced parasites and pathogens. In H awai'i, climate warming is predicted to reduce net precipitation and surface flow in streams, which in turn could increase parasitism of native stream fishes by non‐native parasites. In this study, we utilised a natural precipitation gradient across the H amakua coast on the island of H awai'i to assess the relationship between precipitation and infection of A waous stamineus , a native amphidromous goby, by the introduced nematode C amallanus cotti . We found that the abundance, intensity and prevalence of C . cotti in A . stamineus increased with declining rainfall. Our results also show that parasitism tracks precipitation patterns across the archipelago and that parasitism increases even with moderate decreases in rainfall. As the Hamakua coast precipitation gradient represents a proxy for predicted climate‐driven reductions in precipitation, these findings suggest that infection of native Hawaiian fishes by introduced parasites will increase if climate conditions change as expected. Our findings also suggest that parasitism may be exacerbated by other factors that reduce surface flow, including water extraction for agricultural and urban uses. If so, then adaptive management of minimum flow standards in Hawai'i and elsewhere could improve the well‐being of at‐risk native fishes by alleviating parasitism under current and future climate conditions.