Nurdle drifters as indicators of egg and early-stage larval fish dispersion and transport on the south-east coast of South Africa

There is considerable uncertainty about fish population connectivity and the trajectory for larval transport following adult spawning migrations from temperate to subtropical oceanic waters off the south and east coast of South Africa. In particular, the role of the Agulhas Current and local wind drift has not been fully elucidated as drivers for connecting coastal and offshore waters, and the results of a previous study that modelled the movements of plastic nurdles following a spill in Durban Harbour are used to infer dispersal and transport. Planktonic fish eggs and early stage larvae are passive and behave in a similar way to nurdles, and the rapid movement of the nurdles provided a unique opportunity to track coastal dispersal trajectories and provide insights into population connectivity in some migratory fish species. Under the variable wind conditions typical along the coast, the nurdles tended to remain localised but when winds blew consistently for a few days in one direction, the nurdles drifted from localised surrogate spawning areas and moved via Agulhas Current connectivity and onshore wind assistance to distal coastal locations hundreds of kilometres away. The rapid and extensive dispersal of nurdles served as a valuable surrogate for understanding dispersal dynamics of planktonic organisms in the surface layer of the ocean which has important implications for genetic dispersal in fishes.