Spatial differences in mortality of Calanus pacificus within the California Current System

A vertical life table method was used to estimate patterns of mortality of late developmental stages of Calanus pacificus in the southern sector of the California Current System from seven California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruises (three spring, four late autumn-early winter). Copepodid stage durations were described as a continuous function of both food concentration and temperature from reanalysis. This function was combined with vertical profiles of temperature and Chl a to estimate stage durations in the field and solve for instantaneous mortality rates of two stage pairs: C5/adult females and C5/adult males. Male mortality rates averaged two to three times higher than the corresponding female mortality rates. There was no significant difference between seasons (spring versus winter) in either male or female mortality rates. There was a consistent spatial pattern of higher mortality rates of both males and females in the inshore region relative to the offshore region. This inshore increase in mortality rates is the opposite of that expected from the effects of food limitation, but is correlated with an index of abundance of zooplanktivorous fishes obtained from the CalCOFI ichthyoplankton series (a fishery-independent data source). Elevated predation risk may mitigate the inferred benefits to zooplankton of increased food supply.