Numerical simulations of tidally cued vertical migrations of flatfish larvae in the North Sea

A coupled EulerianeLagrangian Hybrid Ecological Model was applied to model the transport of flatfish larvae at different scenarios of tidally cued activity. Hydrodynamic data were supplied by a two-dimensional hydrodynamic model of the southern North Sea and were used as input in a Lagrangian particle model in which the flatfish larvae were represented by particles, which responded differently to tidal cuesdin this case, hydrostatic pressure. It was used to investigate the interference of vertical migration, in phase with the tidal rhythm, with the specific circulation pattern of the North Sea, by comparison with passive particle advection and diffusion. Instead of what was anticipated after tests in a simple one-dimensional tidal flow, the horizontal advection was not increased, which would allow a greater distance between spawning ground and nursery area. Rather, tidally cued vertical migration turns the direction of transport towards the nearest coast in the North Sea. This is caused by the phase difference between the onshore component of the water velocity and the along-shore component in this area. Tidally cued vertical migration would also be a very efficient mechanism to enter the Wadden Sea through a tidal inlet. The application of the present model looks promising but needs fine-tuning and calibration with real larval distributions and behaviour during the transport phase.