The origin and movements of juvenile anchovy in the Orange River region as deduced from acoustic surveys

Abstract

The results of 22 qualitative acoustic surveys of the region between the Olifants River and Lüderitz conducted between 1978 and 1985 are presented. The surveys have revealed that pelagic fish shoals, believed to be of anchovy, occurred around the Orange River throughout the year, and that there was a consistent gap in their distribution between Chamais Bay and Lüderitz. The findings of the present investigation support the concept of the Lüderitz upwelling cell as a barrier separating the Namibian and Cape anchovy stocks. It is postulated that the upwelling and offshore flow of cold water prevent southward penetration of Namibian recruits and in most years cause a portion of the north-drifting larvae from the Cape stock to be advected out of the system. In extreme years, such as 1986/87, warm-water intrusions inhibit this loss and provide an unusually large supplement to recruitment off Namibia. Samples of post-larval anchovy taken north of the Orange River in April 1983 showed a trend of increasing fish length with decreasing distance from the coast, consistent with an advection of these fish by the onshore flow of water, which is known to occur in this region. These fish presumably migrate or are advected southwards in coastal waters to join anchovy which have been retained farther south. The Orange River cone is seen as an oceanographically quiescent region in which young fish accumulate prior to their southward migration to the spawning grounds of the Cape south coast.