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JPR Advance Access originally published online on January 28, 2008
Journal of Plankton Research 2008 30(4):359-366; doi:10.1093/plankt/fbm110
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© The Author 2008. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

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

Mark D. Ohman* and Chih-Hao Hsieh{dagger}

California Current Ecosystem Lter Site, Scripps Institution of Oceanography, La Jolla, CA 92093-0218, USA

* CORRESPONDING AUTHOR: mohman{at}ucsd.edu

Received on November 15, 2007; revised on December 28, 2007; accepted on December 29, 2007


  Abstract

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.

{dagger} Present Address: Institute of Oceanography, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10 617, Taiwan

Corresponding editor: Roger Harris


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