© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
A comparative study of Calanus finmarchicus mortality patterns at five localities in the North Atlantic
a Integrative Oceanography Division, Scripps Institution of Oceanography, University of California 9500 Gilman Drive, La Jolla, CA 92093-0218, USA
b The University Courses on Svalbard NO-9171 Longyearbyen, Norway
c Graduate School of Oceanography, University of Rhode Island Narragansett, RI 02882, USA
d Ocean Process Analysis Laboratory, University of New Hampshire Durham, NH 03824, USA
e Alfred-Wegener-Institute for Polar and Marine Research D-27568 Bremerhaven, Germany
*Correspondence to M. D. Ohman. e-mail: mohman{at}ucsd.edu.
We compare the patterns of stage-specific mortality of Calanus finmarchicus at five localities across the North Atlantic Ocean during the spring–summer period of active population growth: Georges Bank, a continental shelf locality in the NW Atlantic, based on 30 broadscale survey cruises in the US GLOBEC program; the northern North Sea, studied during the historic FLEX program with sampling four times daily for 73 days; Ocean Station M in the central Norwegian Sea, based on an 80-day daily time-series; and Lurefjorden (sampled weekly in late winter–early summer) and Sørfjorden (sampled monthly), two fjords in southwestern Norway characterized by markedly different guilds of predators. The mortality estimation methods included Wood's Population Surface Method, the Vertical Life Table (VLT) method, and a modified VLT, according to the study site and copepod recruitment schedules. Contrary to assumptions implicit in many simulation models and indirect methods for estimating zooplankton mortality, both rates and stage-specific patterns of mortality of C. finmarchicus vary appreciably across the North Atlantic. Characteristics of local environments, including the predator field in particular, appear to strongly influence mortality schedules in different regions. In at least two sites (Georges Bank and Ocean Station M), mortality rates of early stages of C. finmarchicus are density-dependent. We attribute this density-dependent mortality to egg cannibalism, which introduces non-linear population responses to changing environmental conditions. Region-specific biological interactions can substantially modify the effects of physical climate variability and render simple linear relationships between climate and zooplankton abundance unlikely.
Keywords: population dynamics, mortality, density-dependence, copepod, Calanus finmarchicus
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