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JPR Advance Access published online on January 28, 2008
Journal of Plankton Research, 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 Hsieh2

California Current Ecosystem LTER site, Scripps Institution of Oceanography, La Jolla, CA 92093-0218 U.S.A. Email: 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 7 CalCOFI cruises (3 spring, 4 late autumn-early winter). Copepodid stage durations were described as a continuous function of both food concentration and temperature from reanalysis of Vidal (1980). This function was combined with vertical profiles of temperature and chlorophyll 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 2-3 times higher than corresponding female mortality rates. There was no significant difference between seasons (spring vs. 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 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.

Key Words: Copepoda • population dynamics • mortality • predation • trade-offs

2 present address: Institute of Oceanography, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd. Taipei, 10617, Taiwan Email: chsieh@ntu.edu.tw

Communicating Editor: Dr Roger Harris


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