First Lagrangian ROMS–IBM simulations indicate large losses of chokka squid Loligo reynaudii paralarvae from South Africa's Agulhas Bank

Abstract

Present knowledge of ocean currents based on in situ observation and models suggests that passive biological material such as eggs and larvae can be advected offshore away from the Agulhas Bank, South Africa, and hence removed from the ecosystem on which their survival and recruitment depends. Such losses have been cited as the root cause of the sudden drop in annual squid catches experienced in 2001. In this study, a Lagrangian IBM (individual-based model) coupled to a ROMS (regional ocean model system) model was used to investigate this hypothesis. Three simulations were performed for 12 model months using neutrally buoyant particles released from the seabed (120 m) every second day on the mid-shelf of the eastern, central and western regions of the Agulhas Bank. Boundary effects and resolution precluded the release of virtual particles on the inshore spawning grounds. Particles were given lifespans of 40 days. Results demonstrated large particle losses from the eastern Agulhas Bank (76%) and the western Agulhas Bank (64%). In contrast, few particles were lost from the central Agulhas Bank (2%), making this, in terms of the model, the most suitable place on the Agulhas Bank for spawning. Visualisation of the ROMS outputs revealed that leakage on the eastern Agulhas Bank was caused by a cyclonic eddy resident in the Agulhas Bight. Similarly, leakage from the western Agulhas Bank was caused by deep-water cyclonic eddies in the adjacent Atlantic Ocean.