Modelling regional and local-scale larval seeding strategies for abalone (H. midae) ranching in South Africa

Understanding connectivity between abalone populations and residence times for pelagic larvae within suitable settlement habitats is critical for sustainable management of wild abalone stocks and ranching schemes. In the Eastern Cape region of South Africa, poaching has severely depleted wild abalone Haliotis midae populations. Rebuilding stock through larval seeding is being considered as a potentially viable method, though is hampered by a limited understanding of abalone habitat connectivity through larval dispersal, both at a regional and local scale. To address this shortcoming, we apply a multi-scale approach to investigate potential larval dispersal along the coastline of Eastern Cape, South Africa, examining both intra- and inter-site connectivity, with the primary focus at the local nearshore scale (< 40 m water depth) where surf zone processes have a strong influence. Delft3D is applied at a regional scale (400-km domain) and XBeach is used at a local scale (5-km domain) to model a known abalone habitat at Cape Recife at high-resolution (5-m cell size). Models were validated using field observations. Complex topo-bathymetry data for the local-scale model domain around Cape Recife were obtained using a state-of-the-art low-cost multi-method surveying approach. At a regional scale, sites with abalone habitat have the potential to be connected across distances of 50-100 km. However, a large sandy embayment (Algoa Bay; 100-km arc length) is likely to present an obstacle to larval transport in all but the most extreme and sustained wind and wave forcing conditions. The central Cape Recife abalone ranching area has the potential to act either as a self-seeding or meta-population connected site, depending on forcing conditions. At the local scale around Cape Recife, a zone of shallow gullies, under lower-energy waves, was determined to be the optimal release point to maximise residency times within the preferred rocky, shallow subtidal habitat of H. midae. Competent larvae released in this zone are predicted to have an order of magnitude greater residency than randomly seeded larvae. This study provides site-specific connectivity and residence time information for the Eastern Cape abalone fishery, but more generally it provides a novel multi-scale method to aid in planning and managing of abalone larval seeding for ranching and stock enhancement programs that could be applied in other locations.

Automated summary

Understanding connectivity and residence times for abalone larvae within settlement habitats is crucial for managing wild abalone stocks. Research in the Eastern Cape region of South Africa highlights the importance of shallow gully zones near Cape Recife as optimal release points for maximizing larval residency times.