Long-term changes in the population size and geographical distribution of Pennella sp. (Copepoda) on the saury, Cololabis saira, in the western North Pacific Ocean and adjacent seas*

Abstract

Changes in the population size and geographical distribution of the mesoparasitic copepod Pennella sp. (Siphonostomatoida: Pennellidae) on the saury, Cololabis saira, were studied for a period of six years (1981–86) in the western North Pacific Ocean and adjacent seas. The parasite was first recorded in the western North Pacific in 1981. Its population size increased rapidly in 1982–83 and declined slightly in 1984. During these years, infected fish were widely distributed in the western and central North Pacific and also found in the Sea of Okhotsk and the Sea of Japan. However, the population declined dramatically in 1985 and its distributional range was reduced. The parasite disappeared in 1986. The observed frequency distributions of parasites on the host population were the Poisson in 1981 and 1985, but those during 1982–1984 were over-dispersed and fitted the negative binomial.