Modeling krill aggregations in the central-northern California Current

In the California Current ecosystem, krill availability is a well-known influence on the demography of commercially and ecologically valuable fish, seabirds, and marine mammals. Modeling factors that enhance or inhibit krill aggregations, or 'hotspots', will benefit management of marine predators of conservation concern and contribute to ecosystem approaches to fisheries. Here, we link an oceanographic model (ROMS) and an individual-based model (IBM) parameterized for the krill species Euphausia pacifica to test the hypothesis that occurrences of krill hotspots are disassociated from centers of upwelling along the central-northern California coast due to strong advective currents that transport zooplankton away from the productive continental shelf environment. We compare the distribution of modeled to observed hotspots derived from hydroacoustic surveys from 2000 to 2008. Both acoustic data and modeled hotspots show the greater Gulf of the Farallones and Monterey Canyon as areas of persistent krill hotspots. In this large retention zone, we found no clear relationships between krill hotspots and proxies of upwelling. In contrast, modeled hotspots were associated with reduced upwelling (warmer sea surface temperature [SST] and lower alongshore currents) to the north of Pt. Reyes, and with enhanced upwelling (cooler SST and greater alongshore currents) south of Pt. Sur. Our model highlights the role spatial variability of physical forcing plays in determining the likelihood of krill hotspots forming in particular regions. Notably, our model reproduced the spatial organization of krill hotspots using only simple oceanographic forcing mechanisms and diurnal vertical migration behavior.