The sardine run : investigating sardine and predator distribution in relation to environmental conditions using GIS and remotely sensed products.

Abstract

The sardine run is a spectacular but poorly understood natural phenomenon. This research aims to broaden scientific knowledge pertaining to sardine, Sardinops sagax, distribution, both in relation to their predators and environmental conditions. Sardine distribution was closely related to sea temperature. Sardines were sighted every year along the Lower Wild Coast, where continental shelf conditions were cooled by the Port Alfred upwelling cell. To the north of Mbashe River, shelf conditions were dominated by the warm Agulhas Current, and sardine distribution varied annually in close relation with sea temperature conditions. Along this coastline sardine abundance always peaked between Waterfall Bluff and Port St Johns with favourable conditions caused by the westward inflection of the coastline and the shelf bathymetry. Topographically-induced upwelling was concluded to be the cause of cooler sea temperatures and elevated chl a concentrations. Although chl a concentration appeared to be associated with east coast sardine distribution, the uncertainty with regards data accuracy hindered their usefulness as a predictor of suitable biological conditions for sardine. Sardine northward movement along the KZN coastline was impeded adjacent to the Durban Eddy, where they were forced shorewards by the warm conditions. This coincided with the peak in beach seine catches. The Cape Gannet, Morus capensis, was very closely associated with sardine along the entire east coast. Their abundance declined substantially adjacent to the Durban Eddy. Prevailing atmospheric conditions affected gannet behaviour: they travelled more frequently during strong alongshore winds and foraged more upon cessation of such winds. Gannets were closely associated with feeding dolphins at both coarse and fine scales, and responded to changes in dolphin behaviour. Common dolphin, Delphinus capensis, abundance and group size peaked between Waterfall Bluff and Port St Johns. Along this stretch of coastline they travelled more slowly, and in pods more perpendicular to the bathymetry of the region. Bottlenose dolphin, Tursiops aduncus, abundance increased during the sardine run with the influx of a migrant stock which reached the KZN Mid South Coast. Humpback Whale, Megaptera novaeangliae, and sardine distributions did not appear to be related.