Why does Calanus sinicus prosper in the shelf ecosystem of the Northwest Pacific Ocean?

Abstract

Across the continental shelf of the eastern Inland Sea of Japan and the adjacent Pacific Ocean, the Calanus sinicus population is centred in the shelf waters and declines inshore and offshore. The reasons why this species prospers in the shelf ecosystem are discussed in terms of its biological attributes and pattern of water circulation. Offshore in deep water, the surface temperature near the Kuroshio Current is lethally or sublethally high for C. sinicus, and the food supply in the form of phytoplankton is poor. Inshore in shallow water, C. sinicus is replaced by small species such as Paracalanus sp., Acartia omorii and Oithona davisae, although its reproductive potential, represented by fecundity and development time, is comparable to or even higher than theirs. Shallow water may enhance the rate of mortality of C. sinicus, because its eggs tend to descend through the water column and die quickly when they come into contact with bottom muds, and also because its older stages, particularly CVs and adults, cannot perform a full-scale diel vertical migration to avoid visual predators. In contrast, shelf waters may provide a suitable habitat for C. sinicus because temperature, phytoplankton food supply, and depth are ideal. © 2000 International Council for the Exploration of the Sea.