Hydrodynamic backtracking of fish larvae by individual-based modelling

We discuss methodological and implementation issues of spatial, temporal and combined spatio-temporal backtracking and illustrate larval backtracking for North Sea lesser sandeel Ammodytes marinus larvae, using a combined hydrodynamical and individual-based model. It was found that dispersal effects are important for larval backtracking predictions. Our results show large differences in average transport distance, as well as in shape and extent of predicted hatch areas, when backtracking advected larval cohorts in different regions of the North Sea, thus emphasizing the importance of using realistic, spatially and temporally resolved diffusivity fields in simulations of larval transport. In all cases, biologically likely hatching areas have been predicted. We discuss issues of methodological consistency and present a new scheme for including life-history stochasticity effects on growth in backtracking in a consistent way, as well as procedures for assessing the effects of larval mortality. Finally, fundamental limitations of larval backtracking are clarified, most importantly the time horizon and spatial resolution limit for backward prediction.