Abstract-An optical plankton counter/CTD package was used with zooplankton net samples to map the distribution of fifth copepodid (C5) CuZanus paci$cus in the Santa Barbara Basin region during two autumn cruises. Diapausing C5 C. puci$cus were aggregated in a layer just above the basin’s oxygen-deficient bottom waters and below its sill depth. The maximal concentration measured was 6,900 ind. m- ’ from a net sample spanning a depth range double the thickness of the C5 layer. Although the C5 concentration varied, the layer was found at all stations of sufficient bottom depth within the basin. During November 1994, C5 C. pucijcus accounted for 95-97 % of all zooplankton caught in net samples from the layer. Relatively low concentrations of deep-dwelling C5 C. pacificus were observed at nearby stations outside the basin. We hypothesize that C5 C. paci$cus descend into the Santa Barbara Basin at diapause, are trapped, and accumulate in a region of relatively low predator abundance. The resultant aggregation is estimated to contain a significant fraction of the regional C. pucijcus population and thus assumes an important role in its dynamics. Cdanus paciJcus, a marine planktonic copepod, is one of the dominant zooplankton species in the waters off southern California (Fleminger 1964; Star and Mullin 1981). In this region, C
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