The impact of Diel Vertical Migration on larval dispersion and recruitment of Cape anchovy and sardines

Abstract

Understanding how Diel Vertical Migration (DVM) shapes the dispersal pathways and retention of larval anchovies ( Engraulis encrasicolus ) and sardines ( Sardinops sagax ) is vital for managing these species in South Africa, because vertical movements determine whether larvae remain in productive coastal nursery areas or are exported offshore. We used a 3 km fine-grid Coastal and Regional Ocean Community (CROCO) model to simulate the transport of larvae in the upper 25 m of the water column across the East Coast, Agulhas Bank, and Southern Benguela between 2010 and 2014, under DVM and non-DVM scenarios. Results indicate that the surface layer (0–5 m) is highly unstable, with larvae frequently being pushed offshore by fast-moving currents and wind. In the Natal Bight, surface larvae were advected by the Agulhas Current, whereas increasing depth to 25 m improved retention. On the Agulhas Bank and West Coast, offshore loss was similarly reduced at deeper fixed strata. However, the DVM scenario consistently provided the highest and most stable particle densities across all years. By migrating into relatively slower-moving subsurface waters (10–25 m), larvae utilised the stratified velocity field to decouple their movement from unpredictable surface anomalies. DVM acts as a critical biological mechanism, allowing larvae to find retentive strata and maintain a high-density connection between the South and West coasts. This behaviour minimised export beyond the 200 m shelf break, ensuring larvae reached productive nursery grounds even during years of high environmental variability. Ultimately, DVM stabilises recruitment and supports the population resilience of these ecologically important fish stocks against shifting oceanographic conditions. • High wind stress and velocity in the 0–5 m layer frequently export larvae offshore. • Increased depth to 25 m buffers against the Agulhas Current, enhancing larval retention. • Diel Vertical Migration (DVM) consistently yields the highest particle densities by utilising stratified velocity fields. • Subsurface migration (10–25 m) allows larvae to decouple from unpredictable surface anomalies. • DVM ensures connectivity to nursery grounds, stabilising population resilience against environmental variability.