Shallow-water, nearshore current dynamics in Algoa Bay, South Africa, with notes on the implications for larval fish dispersal

P Pattrick, NA Strydom & WS Goschen

Nearshore currents play a vital role in the transport of eggs and larval stages of fish. However, little is known about
their complexity and the implications for dispersal of fish larvae. The study describes the complexity of the shallow
nearshore environment in eastern Algoa Bay, on the south-east coast of South Africa, and its effect on larval fish
ecology. An ADCP was used to assess short-term spatial current variability across the nearshore (4–20 m depth) during
intensive one-day-transect profiling surveys. Data showing long-term temporal variability was collected half-hourly
from a bottom-moored (15–20 m depth) ADCP over the course of one year (May 2006–May 2007). The short-term
profiling revealed complexity in both current speed and direction with largely wind-driven flow patterns identified.
The long-term data showed that the currents were aligned approximately west/east with prevailing winds and local
shoreline. Modal speeds of 8 cm s–1 near the surface (4 m) and 6 cm s–1 near the bottom (14 m) of the water column are
much lower than the average swimming speeds of postflexion larvae known to occur in the area. Mean current speed
decreased with depth from ~30 cm s–1 (4 m) to ~10 cm s–1 (14 m), suggesting opportunities for depth refuge from current
displacement in older larvae. Potential nett displacements were greater during the spring and summer, coinciding with
peak fish breeding with passive eggs and early stage larvae travelling distances of up to 475 km near the surface of
the water column. These maximum dispersal distances recorded during the present study differ considerably by half
that from other calculations made for the area, highlighting a data error in a previous study. These predictions provide
preliminary indications of potential transport from a point source and do not factor in anomalous local oceanography,
benthic topographic impediments to transport or the complexities of larval fish behaviour.