Towards a more complete understanding of the life cycle of brown shrimp (Crangon crangon): modelling passive larvae and juvenile transport in combination with physically forced vertical juvenile migration

In this study we developed and utilized a complex model approach to investigate the impact of stage-specific transport processes on the development and spatial distribution of brown shrimp (Crangon crangon) post-larvae and juveniles in the German Bight. First, we focused on drift processes during the pelagic larval stage by coupling an individual-based model for egg and larval development 'off-line' to a 3D hydrodynamic model utilizing the Lagrangian method. Secondly, we investigated tidal-induced transport processes after juvenile settlement. To determine the tidal cycle, the model coupling was accomplished 'on-line' by resolving the individual-based model and hydrodynamic model with the same time step. The vertical migration of juveniles, a prerequisite for the selective tidal stream transport (STST), was modelled as a sub-grid scale physical process (balance of forces: gravitation, buoyancy, Stoke's friction and dynamic uplift) and considered complex particle dynamics. We applied the model to test temperature and salinity cues as possible tidal indicators utilized by juvenile brown shrimp. Our results indicated that transport processes could significantly change the timing and spatial distribution of post-larval abundance. We also showed that the small-scale hydrodynamic forcing acting on the bodies of juvenile brown shrimps was sufficient to account for the vertical migration required to use STST. For both investigated tidal cues STST performing juvenile brown shrimp were transported on-shore. A faster and more continuous STST was calculated for the salinity cue, resulting in larger abundances of brown shrimp in estuarine areas.