Seasonal and inter-annual variations in the abundance and biomass of Neocalanus plumchrus in continental slope waters off Oregon.

Seasonal and inter-annual variability in the abundance and biomass of copepodid stages of the sub-arctic oceanic copepod, Neocalanus plumchrus, was studied during the January–May growth season, using an 11-yr time series of zooplankton samples collected over the upper 100 m of the water column. Abundance and biomass peaks occur in March/April. Abundance and biomass of N. plumchrus were significantly negatively correlated with sea surface temperature and significantly positively correlated with sea surface chlorophyll a, salinity, and density above the pycnocline. The seasonal integrated abundance and biomass of N. plumchrus declined during the warm years (2003–05), and increased during the cold years (2006–08). The date when 50% of the population had passed through stage C5 was significantly negatively correlated with temperature – earlier in warm years and later in cold years. In 3 yr (2003, 2007 and 2008), a second cohort appeared in mid-May, as indicated by the presence of stages C1 and C2 in the samples. Unusually high abundances of N. plumchrus in the spring of 2007 and 2008 were associated with cool ocean temperatures and an early spring transition in the NCC ecosystem, suggesting that the NCC ecosystem has returned to a cold phase. We discuss the merits of a hypothesis that the N. plumchrus population observed off Oregon is a local population as opposed to one that is expatriated from the Gulf of Alaska. [ABSTRACT FROM AUTHOR]