M. J. Armstrong , P. Chapman , S. F. J. Dudley , I. Hampton & P. E. Malan (1991) Occurrence and population structure of pilchard Sardinops ocellatus, round herring Etrumeus whiteheadi and anchovy Engraulis capensis off the east coast of southern Africa. South African Journal of Marine Science. Volume 11, 1991 – Issue 1
The distribution, abundance and population structure of pilchard, round herring and anchovy off the east coast of southern Africa were investigated during hydroacoustic and midwater trawl surveys in August 1986, June 1987 and June 1990. The 1986 survey revealed the distribution of the three species to be almost continuous between Cape Town and East London, with about 13 per cent of the pilchard, 21 per cent of the round herring and 4 per cent of the anchovy biomass east of Cape Town occurring on the East Coast. Abundance and distribution of pelagic fish off the East Coast was similar in 1986 and 1987. All three species spawn off the East Coast. Few temperate pelagic fish were detected off the Natal coast during either the 1986 or 1987 surveys, but during June 1990, pelagic fish targets, thought to have been mainly temperate clupeoids, were encountered over most of the shelf from East London to some 70 nautical miles north-east of Port St Johns, and along the shelf-edge in the vicinity of Port Shepstone. During all three surveys, pelagic fish in the vicinity of Port St Johns had descended into the cool water underlying the Agulhas Current. It is proposed that the annual “sardine run” off Natal is a phenomenon arising from an expansion of pilchard habitat during the cooler conditions that prevail during winter. The expansion brings the fish east into Transkei waters at roughly the same time each year. Phenomena such as current reversals, upwelling of cool water onto the narrow shelf or even pursuit by predators then results in “leakage” of schools along the shelf between Port St Johns and Durban.