Population biology of euphausiids off northern California and effects of short time-scale wind events on Euphausia pacifica

Variability in upwelling conditions has been shown to change the physical and biological characteristics of the water over the California shelf, including the population biology of the dominant euphausiid Euphausia pacifica. However, on a short time-scale (ca. weekly), far less is known, especially for larger planktonic animals like euphausiids. We examined E. pacifica abundance, size structure, oocyte composition, and euphausiid egg abundance in an upwelling region off northern California in June 2000, May and June 2001 and January 2002. Euphausiid egg abundance increased on a short time-scale during and after periods of relaxation and larval E. pacifica abundance decreased significantly during upwelling in June 2000 and January 2002, presumably due to interaction with offshore surface currents. Diel vertical migration out of the advective surface layer during the day by adults and juveniles may enhance shelf retention of euphausiids during upwelling events. Seasonally, a reduction in reproductive effort of E. pacifica (no. of oocytes female(-1)) was observed during January 2002. Interannually, E. pacifica abundance was significantly higher during June 2000, a year of less intense upwelling, than in May and June 2001, a year of more intense upwelling. Thus, upwelling-favorable winds following periods of relaxation can lead to rapid increases in euphausiid egg abundance, but larval abundance, and perhaps juvenile and adult abundance as well, are likely to be negatively impacted locally by strong upwelling off Northern California.