Tidal effects on transport and dispersion of particles, cod eggs and larvae in the Lofoten and Vester ålen region, Norway

Eli Børve ^1,2* Pål Erik Isachsen ^1,3 Ole Anders Nøst ⁴ Peygham Ghaffari ⁵ Stig Falk-Petersen ²

• ¹ University of Oslo, Oslo, Oslo, Norway
• ² Akvaplan niva AS, Tromsø, Troms, Norway
• ³ Norwegian Meteorological Institute, Oslo, Norway
• ⁴ Akvaplan niva AS, Trondheim, Norway
• ⁵ Akvaplan niva AS, Oslo, Norway

The study investigates the net impact of nonlinear tidal dynamics on transport and dispersion of particles, resembling cod eggs and larvae, out of a large embayment called Vestfjorden in Northern Norway. Vestfjorden is a major spawning area for Northeast Arctic cod. The large embayment is partially sheltered from the continental slope and open ocean by the Lofoten-Vester ålen archipelago. The dynamical ocean processes that control transport paths and transport times of cod eggs and larvae out of the embayment are of major importance, not only for the fish stock, but in general the marine ecosystem in the region. The coastal ocean circulation in and around Vestfjorden is simulated with a variable mesh model, both with and without tides. The two different flow fields are used to advect passive particles seeded near Henningsvaerstraumen, a key spawning location within the embayment. A comparison of the two transport calculations reveals that nonlinear tidal dynamics clearly impact the particle drift in the region. When including tides in the model simulation, transport through the various straits that cut through the Lofoten-Vester ålen archipelago becomes more important, causing the total particle drift out of Vestfjorden to increase by about 10%. One strait in particular, Moskstraumen, contributes with a significant transport (∼ 30%) in the model simulation with tides included, but other straits contribute as well. For comparison, in the simulation where tides are excluded, almost 90% of the particles are transported out of the embayment around the southern tip of the archipelago and only 6% through Moskstraumen. A key implication of the tidal transport through the straits is that the journey for a large fraction of the cod eggs/larvae from Vestfjorden to the shelf is considerably shortened.