Changes in development timing and cohort width of Neocalanus plumchrus degree ' flemingeri copepods in the eastern North Pacific

Neocalanus plumchrus/flemingeri copepods make up a large proportion of spring mesozooplankton biomass and are a valuable nutritional source for many higher trophic levels. Copepodites through to sub-adult stage are present in surface waters for a relatively short period of time each spring, and the date of maximum biomass has been calculated as the date when 50% of the population were at the sub-adult, CV stage. This index allows quite a precise date to be calculated from relatively infrequent sampling and interannual comparisons between 1957 and 2004 have demonstrated that the timing of peak abundance is significantly advanced in warmer years. However, recent data from the Continuous Plankton Recorder survey, which samples the surface NE Pacific more frequently during spring, has found that maximum numbers of CV copepodites occur after the 50% point is reached so that maximum biomass occurs some weeks later than predicted by this index (although comparisons between years show that the magnitude of the timing shift is similar). Comparisons with depth-stratified profiles from the BIONESS show that this is not just due to single-depth near-surface sampling by the CPR. We speculate on the cause of this change which could be related to the width of the cohort (which appears to now be narrower, at least in warm years) or the length of time that the CV stage needs to spend in the surface accumulating lipid before beginning diapause. A narrower cohort has implications for predators who will have less time to take advantage of this food source.