The Influence of Seasonal and Decadal Trends in Coastal Ocean Processes on the Population Biology of the Krill Species Euphausia pacifica: Results of a Coupled Ecosystem and Individual Based Modeling Study

Krill of the California Current play a crucial role in the transfer of primary production up to many commercially important higher trophic levels. Understanding the short time scale (weeks to seasonal) and long time scale (decadal) variability in abundance, condition, and spatial patterns that results from changes in ocean conditions is critical if we hope to manage the fishery of any higher trophic levels from more than a single species approach. I have coupled a suite of models in an attempt to understand the impacts of changing ocean conditions on this important prey item. The coastal ocean was simulated with a commonly used oceanographic model (Regional Ocean Modeling System) coupled with an ecosystem model (Nutrient, Phytoplankton, Zooplankton, Detritus). The coastal ocean was simulated from Newport, OR to Point Conception, CA over an 18-year period (1991–2008). These model results were used to force a 3-dimensional individual based model (IBM) that was parameterized to represent the krill speciesEuphausia pacifica.Biological processes of the IBM (growth, life-stage progression, mortality, reproduction, vertical migration) were compared to laboratory data and field data under varying food and temperature conditions to understand how well the model can reproduced known biological rates and processes. The model performs well at simulating growth, life stage progression, and reproduction, as these are the areas from which there is an abundance of data from which to parameterize the model. Results from simulations of larval and adult populations indicated the greatest amount growth in both larval and adult populations was over the six- month period from April through October. Mortality was greatest for larvae during the winter (when food resources are typically lowest), but mortality was greatest for adults during summer due to offshore transport of individuals to regions of warmer surface waters and reduced food concentration. Condition of individuals and mortality of individuals correlated positively with the more productive phase of the Pacific Decadal Oscillation and the North Pacific Decadal Oscillation, providing evidence the impact of ocean basin scale atmospheric conditions on krill. The impacts of atmospheric forcing onE. pacificaare important factors that control the distribution, abundance and productivity of this important prey item for many commercially important fisheries of the West Coast of the United States