© 2005 International Council for the Exploration of the Sea
Variability in retention of Calanus finmarchicus in the Nordic Seas
a University of Bergen, Department of Biology PO Box 7800, N-5020 Bergen, Norway
b Institute of Marine Research Box 1870 Nordnes, N-5817 Bergen, Norway
*Correspondence to T. Torgersen: tel: +47 55584439; fax: +47 55584450. e-mail: thomas.torgersen{at}bio.uib.no.
Using a regional ocean circulation model and particle tracking, we have studied the probability of the copepod Calanus finmarchicus being retained within the Nordic Seas' population as a function of its initial location, its vertical migration pattern, and the interannual variability in physical forcing. Defining a retention index in terms of the number of particles remaining within the Nordic Seas divided by the initial number of particles released, we found that spatial location had the greatest effect on the retention index during the study period, 1988–1991. Variability as a result of differences in physical forcing among years and among different seasonal vertical migration patterns had smaller but similar effects. The seasonal vertical migration behaviours with the highest advective loss rates and the most sensitive to interannual physical forcing were those that ascended early and descended late from a shallow summer depth. Average retention within the Nordic Seas was 0.40 after one year in simulations with diffusion and advection, and 0.42 in simulations with advection only. The average retention at the end of the four-year sequence was 0.10 and 0.12 with and without diffusion, respectively. Particles located in the western areas of the Nordic Seas had the highest retention, while those along the Norwegian coast showed little or no retention after four years. Initial location has a larger influence on retention than interannual variability in advective fields. C. finmarchicus offspring tend to reside in areas different from their parents, with different probabilities of retention. This spatial variability in retention rate is also experienced as inter-generational variability by members of the population. Model results suggest that almost all of the C. finmarchicus that are advected into the Barents Sea originate from off the Norwegian coast. Thus, predicting C. finmarchicus inflow into the Barents Sea requires knowledge of their abundance on the Norwegian Shelf.
Keywords: Barents Sea, Calanus finmarchicus, models, Norwegian Sea, ocean circulation, particle tracking, retention
Received 2 July 2004; accepted 30 May 2005.
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