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.

为了研究co足类Penellidae的pe足类Pennella sp。的寄生虫对太平洋秋刀鱼Cololabis saira死亡率的影响，我们研究了以下指标的季节性变化：患病率（％；受感染鱼的数量×100） /受检鱼的数量），平均强度（受感染Pennella sp。的总数/受感染鱼的数量），相对强度（受感染Pennella的总数）sp./没有 的鱼的数量），以及按年龄划分的一条鱼上的最大寄生虫数量。2016-2019年5月至12月，共从北太平洋中部和西部收集了32,376个太平洋秋刀鱼，并进行了分析。在5月或6月，所有指数均在1岁鱼中最高，然后急剧下降。我们还记录了904个1岁鱼类的宿主体内遗留的寄生虫碎片（头胸部）的数量，但仅发现了6个头胸部。由于寄生虫死亡后不太可能容易出现头胸部胸腔积液，因此我们的研究结果表明，流行率和强度的降低不仅是由寄生虫死亡引起的，而且是宿主死亡的结果。我们需要定量评估Pennella sp。的影响。秋刀鱼资源暴发。