Biology of the introduced copepod Pseudodiaptomus inopinus in a northeast Pacific estuary

Abstract

Compared to other regions, estuaries and coastal bays along the west coast of North America have experienced the largest number of invasions by nonindigenous planktonic copepods. Eight species of copepods from Asia, including 2 species of the genus Pseudodiaptomus, have been reported in coastal bays of northern California, and a third species of Asian Pseudodiaptomus (P. inopinus) has become established in the Columbia River estuary and many smaller estuaries in the northeast Pacific Ocean. It can dominate the plankton in fresh and oligohaline tidal waters of estuaries that are utilized as rearing grounds for a variety of larger invertebrates and fishes. In July 1998 we initiated a 16 mo study of P. inopinus in the Chehalis River estuary, Washington State, USA, to document its biology, ecological relationships with other holoplankton, and importance as prey for fish and invertebrate planktivores. In 1998 P. inopinus reached peak densities in the late summer/early autumn period of low river flow but a similar peak was not seen in the same period in 1999, when densities of the copepod were significantly lower. These interannual density differences did not appear to be caused by between-year differences in predation or river flooding, but could have resulted from cooler temperatures and higher river flows that occurred in the Chehalis River in 1999. Other abundant planktonic copepods were separated from P. inopinus either temporally (Eurytemora affinis) or spatially (Acartia spp., Eurytemora americana) within the estuary, over a relatively short segment of the estuary spanning salinities of 0 to 10 psu. This separation may imply that P. inopinus experienced little competition when it was introduced; alternatively, it may have altered the distribution of other copepods in the estuary. P. inopinus was rare in the diets of estuarine fishes, but comprised an important and sometimes dominant prey for mysid shrimp Neomysis mercedis and juvenile caridean shrimp Crangon franciscorum. In turn, N. mercedis was an important prey item for estuarine fishes, and thus the main impact of P. inopinus on the estuarine food web was via this pathway. © Inter-Research 2007.