Caligus Clemensi prevalence and counts on Clupea pallasii from Port Angeles Harbor, WA and from a controlled laboratory experiment conducted at USGS Marrowstone Marine Field Station, WA

We characterized a natural sea louse epizootic of Caligus clemensi and the effects of parasitism on Pacific herring Clupea pallasii in Port Angeles Harbor, WA, USA. Infestation prevalence on newly metamorphosed age0 Pacific herring reached 100% prevalence by mid-August. At this time, the mean louse intensity was 4.6 lice and a positive correlation occurred between louse intensity and Pacific herring body length. The epizootic then waned, with infestation prevalence decreasing to less than 25% and the mean parasite intensity falling below 1 louse by October. In controlled experiments, louse-induced mortality was negligible in controlled experiments. While skin injuries were not detected, motile lice preferentially aggregated around head and anterior dorsal areas. However, louse tropism became evenly distributed over the body as the parasite intensity increased. These results indicate that C. clemensi epizootics reach high prevalence, but also fade from mid-summer to early fall. Due to the predominant presence of motile copepod stages, we suggest that the epizootic fades because lice complete their life cycle and dislodge from the host, however, multiple explanations for epidemic fading are discussed.