Modelling drift of pelagic offspring: the importance of egg surveys in providing a realistic model initialization

Having valid information about the location and dynamics of biological processes is important for coastal management. In this context modelling, the pelagic drift of early life stages has been shown to be an important tool for understanding the spatial scale of population dynamics in marine systems. Often simulated particles are released in hypothetical quantities at assumed spawning grounds with no or few field data to guide the model parametrization. In this study, we combine high-resolution field data and state-of-the-art oceanographic modelling and use a probabilistic approach to construct kernel density distributions of the dispersal of pelagic fish eggs. Specifically, the potential drift of pelagic offspring of cod (Gadus morhua) was investigated in a large, open fjord system in northern Norway by combining field observations of newly spawned cod eggs with simulations of particle movement using a semi-Lagrangian trajectory model with inputs from high-resolution hydrodynamic simulations. The larger part of the distribution of eggs after drift was located in the fjord, suggesting fjord scale recruitment dynamics. Finally, we also examined the drift of eggs released in a uniform distribution and found that lack of egg survey data gave an unrealistically large spread of egg particles within this fjord system