The spatial distribution of euphausiid aggregations in the Northern California Current during August 2000

The location and size of euphausiid aggregations (patches) were identified using acoustic volume backscattering strength at 38 and 120 kHz and vertically stratified MOCNESS sampling during a US GLOBEC cruise off southern Oregon and northern California in August 2000. Euphausiid patches were clustered inshore of Heceta Bank (44.0°N) and off Cape Blanco (42.8°N), but were not common elsewhere. The distribution of euphausiid patches throughout the study area was correlated with that of near-surface chlorophyll. We suggest that Heceta Bank and Cape Blanco were zones of plankton retention and concentration along the shelf and slope, and also that large offshore meanders in the equatorward upwelling jet carried upwelled water, chlorophyll, and euphausiids offshore. In particular, the complex interaction of mesoscale physical features, shoaling bottom topography, and diel vertical migration created large patches of euphausiids. These euphausiid patches, probably persisting on timescales of days to weeks, could function as relatively large and persistent resources for euphausiid predators.