Dormancy in an eastern boundary current copepod

Dormancy and migration to deep water allow oceanic copepods to escape unfavorable surface conditions resulting in high mortality risk, low reproductive rates, or transport out of a favorable region. Dormancy was examined in Calanus pacificus, a dominant California Current copepod. Hypotheses about onset of dormancy and transport and retention of dormant C. pacificus fifth copepodid (CV) stages were examined in the region off southern California. Specific questions addressed include: (1) When does the abundance of dormant copepods increase and decrease in deep, open water off southern California? (2) When does onset of dormancy occur in surface water? (3) What environmental conditions are associated with onset of dormancy? (4) Do dormant copepods occupy the California Undercurrent? (5) Is dormant copepod abundance different in basins and deep, open water? Vertical distribution and abundance of dormant C. pacificus were described over eleven months at San Diego Trough (SDT) and at basin and open-water stations during three months. The abundance of dormant C. pacificus at SDT increased from June until October, and decreased from October until March. Variation in molting hormones was characterized through the CV molt cycle, and both molting-hormone level and jaw morphology were used as indicators of onset of dormancy in surface CVs. Although differences in mean molt-phase indices between the period of increasing deep CV abundance and other dates suggested onset of dormancy in surface water when deep CV abundance was increasing, variability in molt-phase indices among dates suggested a heterogeneous dormancy response in surface CVs. Dormant copepods were present at all stations examined between June and January. At the SDT, they occupied the California Undercurrent at the beginning and end of the dormant season and were deeper in the middle of the dormant period, indicating poleward transport for part of the year. The abundance of dormant CVs was not different in basin and open-water stations. This study indicates that the dormant phase of the C. pacificus population off southern California, both in basins and open water, contributes to seasonal population dynamics.