Does vertical migratory behaviour retain fish larvae onshore in upwelling ecosystems? A modelling study of anchovy in the southern Benguela.

A spatially explicit individual-based model (IBM) forced by 3D temperature and current fields simulated by a hydrodynamic model of the southern Benguela upwelling region was used to test two hypotheses concerning the role of diel vertical migration (DVM) by Cape anchovy Engraulis encrasicolus larvae and pre-recruits. These hypotheses were that: (1) DVM enhances alongshore transport of anchovy eggs and larvae from the spawning grounds to the nursery area while avoiding the lethal effect of low water temperatures in the upwelling system, and/or (2) DVM enhances the transport of larvae and pre-recruits from the offshore to the onshore domain of the nursery area, and then counteracts offshore advection by favouring retention. We tracked the trajectories of virtual particles in the model and calculated a pre-recruitment index as a proxy for transport success to the nursery area (onshore and offshore) and found that the index increased from 10% to 20% after the incorporation of larval vertical migration into the IBM, with virtual individuals held at depths of around 60 m showing maximal pre-recruitment index values. Hence, DVM does appear to enhance transport to the nursery area (offshore) for early and late larvae. Model outputs showed coarse-scale horizontal distribution patterns of larvae by age/size class that are similar to field observations for early, small larvae but not for large larvae and pre-recruits. Observations show that early/small larvae are located offshore whereas older/larger larvae and pre-recruits are found closer to the continental shelf and the inner nursery grounds. This disparity between model results and field observations does not support the hypothesis that DVM is one of the mechanisms involved in the onshore movement of early life-history stages, especially for large larvae.