A model‐based meso‐zooplankton production index and its relation to the ocean survival of juvenile coho (Oncorhynchus kisutch)

The ocean survival of coho salmon (Oncorhynchus kisutch) off the Pacific Northwest coast has been related to oceanographic conditions regulating lower trophic level production during their first year at sea. Coastal upwelling is recognized as the primary driver of seasonal plankton production but as a single index upwelling intensity has been an inconsistent predictor of coho salmon survival. Our goal was to develop a model of upwelling‐driven meso‐zooplankton production for the Oregon shelf ecosystem that was more immediately linked to the feeding conditions experienced by juvenile salmon than a purely physical index. The model consisted of a medium‐complexity plankton model linked to a simple one‐dimensional, cross‐shelf upwelling model. The plankton model described the dynamics of nitrate, ammonium, small and large phytoplankton, meso‐zooplankton (copepods), and detritus. The model was run from 1996 to 2007 and evaluated on an interannual scale against time‐series observations of copepod biomass. The model’s ability to capture observed interannual variability improved substantially when the copepod community size distribution was taken into account each season. The meso‐zooplankton production index was significantly correlated with the ocean survival of hatchery coho salmon from the Oregon production area, although the coastal upwelling index that drove the model was not itself correlated with survival. Meso‐zooplankton production within the summer quarter (July–September) was more strongly correlated with coho survival than was meso‐zooplankton production in the spring quarter (April–June).