J.A. Runge, School of Marine Sciences, University of Maine and Gulf of Maine Research Institute
A. Leising, NOAA, Southwest Fisheries Science Center
C. Johnson, University of British Columbia
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We seek to develop for inclusion in population dynamics modeling a mechanistic understanding of dormancy in the life cycle of Calanus finmarchicus. Our approach is to compile Calanus life cycle and environmental data sets across regions in the NW Atlantic, look for common patterns and cues and develop quantitative hypotheses using individual-based life-cycle models to explain the observed patterns. We compiled, with the assistance of colleagues (E. Durbin, R. Jones, S. Plourde, E. Head, P. Pepin), life history data sets, including ambient water temperature, food availability (measured as mean chlorophyll a concentration in the surface water column), photoperiod and life stage abundance cycles, from six areas spanning 9 degrees of latitude in the region. We use as proxies for dormancy entry and exit the proportion of stage CV (year day when the CV proportion is at one half of its climatological maximum proportion) and the proportion of adult females (yearday when adult females reach 10% of the population), res pectively. Our first results show that no single observed environmental cue explains the range of dormancy patterns; rather, dormancy entry and emergence occur over a broad range of times, both among years and regions. We propose a "Lipid-Accumulation Window (LAW)" hypothesis, in which dormancy entry and exit involves interaction of multiple environmental factors with the copepod's physiological responses, including lipid accumulation and development rates. We postulate that an individual enters dormancy when it has accumulated = 30% of its dry weight as lipid by the end of stage CIV; if this condition is not met, it continues molting to adult. The lipid accumulation rate is dependent on ambient temperature and food availability. There is, therefore, a seasonal window in time of conditions that will allow dormancy. Upon entry, the length of the dormant period is a function of both ambient temperature during dormancy and the lipid level. Predation, especially at the early life stages, as well as advection, can significantly alter the demographic pattern of dormant individuals. We are presently testing this hypothesis using an individual based Calanus life cycle model against the observed data.The model shows encouraging replication of the observed life cycles in the Anticosti Gyre and Rimouski data sets. We are presently testing the model against the other Canadian AZMP data sets and expect to refine a LAW hypothesis for general application across the region. A similar approach is being carried out to examine dormancy of Pacific species of the genus, as part of the Northeast Pacific GLOBEC synthesis phase.