21. Response of plankton and fish production dynamics to sardine abundance regime shifts in the Oyashio Current region

First order estimates of annual production, seasonal biomass patterns, and production to biomass ratios were determined for major components of the pelagic food-web observed during the high sardine abundance regime of the 1980s in the Oyashio current region off Hokkaido using a one-dimensional ecosystem model. The model consists of diatoms, copepods, euphausiids, walleye pollock, and Pacific sardine, and it was forced using time dependent functions of sea surface temperature, mixed layer depth, mixing rate, surface light, and sardine migration. The model estimates diatom production during the 1980s to be about 2293 t km super(-2) y super(-1), zooplankton production between 119 to 271 t km super(-2) y super(-1), sardines 23 t km super(-2) y super(-1), and pollock production 7 t km super(-2) y super(-1). Sensitivity analyses indicated that euphausiid and pollock production were most sensitive to changes in zooplankton feeding parameters. Diatom and sardine production were most sensitive to changes in physical forcing parameters, while copepod production was sensitive to both zooplankton feeding parameters and physical forcing parameters. To address the relative importance of sardine biomass on whole system production dynamics we conducted a model experiment whereby sardine biomass was set to zero. Euphausiid production responded drastically to a reduction in sardine biomass. In addition, pollock production was enhanced with the removal of sardines. These results imply that the presence of high biomass of sardines may act as competitors to other planktivores and to euphausiids for limited diatom and copepod production in feeding grounds.