Infection by the parasitic copepod Pennella sp. induces mortality in the Pacific saury Cololabis saira

Abstract

To investigate the influence of parasitism of the copepod Pennella sp., which belongs to the family Pennellidae, on mortality in the Pacific saury Cololabis saira, we examined the seasonal changes in the following indices: prevalence (%; no. of infected fish × 100/no. of examined fish), mean intensity (total no. of infected Pennella sp./no. of infected fish), relative intensity (total no. of infected Pennella sp./no. of examined fish), and the maximum number of the parasites on a fish by age. A total of 32,376 Pacific sauries were collected from the central and western North Pacific, from May to December in 2016–2019, and analyzed. All indices were the highest in age-1 fish in May or June, and then decreased drastically. We also recorded the number of parasite debris (cephalothorax) left behind in the host bodies of 904 age-1 fish, but only six cephalothoraxes were found. Since it is unlikely that the cephalothoraxes could easily fall out after parasite death, our results indicated that decreases in prevalence and intensity were not caused simply by parasite death, but also as a result of host mortality. We need to quantitatively evaluate the impact of the Pennella sp. outbreak on resources of Pacific saury.