The importance of seasonal vertical migration in across shelf transport ofCalanus finmarchicus

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Abstract
The horizontal distribution and development of the copepod Calanus finmarchicus off the coast of northern Norway was simulated using a 3-dimensional hydrodynamic model. The transport of C. finmarchicus between the Norwegian Sea and the mid-Norwegian shelf was investigated in relation to ontogenetic vertical migration behaviour and the circulation pattern on the shelf. It was found that the offspring of the overwintering oceanic stock of Calanus is able to colonize the mid-Norwegian shelf by utilizing the cross-shelf currents. These currents emerge from topographic steering around bank areas. The spring bloom occurs much earlier on the shelf than in oceanic waters. Hence, the development of the new generation on the shelf commences prior to development in the oceanic waters. An important transport mechanism is the warm Norwegian Atlantic Current that flows north along the shelf break. Since the temperature decreases to the west of this current, there is a westward delay in the propagation of the recruiting generation. This is amplified by a greater mixed layer depth in the west which delays the spring bloom. Simulations specifying a deep vertical distribution of the overwintering stage (CV) led to an offshelf transport and a concentration of animals outside Lofoten (Lofoten basin), and it appears that most of the leftover from the production on the mid-Norwegian shelf is transported into the Lofoten basin. The fate of this stock will depend on the circulation pattern at the overwintering depth (between 500 and 1500 metres) which is poorly known.