Documenting historical anchorworm parasitism of introduced warmwater fishes in the Willamette River Basin, Oregon

Anchorworms (Lernaea spp.) are freshwater parasitic copepods that use a wide range of hosts. Yet little is known about their prevalence, distribution, and which species are their primary fish hosts in the state of Oregon. Institutional fish collections serve as banks which allow investigators to look at historical fish specimens and ascertain their health status at the time of their collection. We examined 1,039 specimens collected between 1941 and 2016 from the Oregon State Ichthyology Collection to detect the presence of anchorworms on non-native warmwater fishes from the Willamette River Basin, Oregon. Adult female anchorworms were found on 11 of the 17 fish species that we examined. The most infected species included common carp (Cyprinus carpio), bluegill (Lepomis macrochirus), and smallmouth bass (Micropterus dolomieu). We suggest these introduced warmwater fishes can act not only as hosts, but also as potential reservoirs for these under-studied parasites posing a potential risk for Endangered Species Act (ESA)-listed native fishes. Our findings reveal unique insights that will serve as a baseline to detect future changes in parasite loads in the Willamette River Basin.