The selection of spawning location of sardine (Sardinops sagax) in the northern Benguela after changes in stock structure and environmental conditions

Most reports on the distribution of spawning areas of sardine (Sardinops sagax) in the northern Benguela originate from the 1970s and 1980s. The northern Benguela system was in a high upwelling regime during those decades. Since the early 1990s upwelling favourable winds have decreased and a trend of increasing sea surface temperature (SST) has been observed. Changes in the structure of sardine stock in the northern Benguela have been observed and it has been suggested that a reduced biomass and changes in stock structure has led to decreased spawning in the favourable southern locations, thus preventing a recovery of the sardine stock. The present paper on the contrary shows that there has been a shift in spawning location from the less favourable northern areas in the early 1980s to spawning areas further south in the 2000s. Thus, the failure of the northern Benguela sardine stock to recover since its collapse in the late 1960s cannot be explained by spawning in less favourable areas. The shift in preferred spawning location to more southern areas since the 1980s was to be expected with a general warming of the northern Benguela system. Alternative explanations for the failure of the sardine stock to recover such as a reduction in average length as well as length at 50% maturity, leading to a reduction in reproductive output, increased predation pressure, and increased low oxygen waters are proposed.

The northern Benguela upwelling system is one of four major eastern boundary upwelling systems in the world. Its dynamics have been extensively reviewed by several authors (Nelson and Hutchings, 1983; Shannon, 1985; Lutjeharms and Valentine, 1987; Fennel, 1999). The system is characterized by strong seasonal signals with higher water temperatures during austral summer and autumn (December to March) and lower temperatures during winter and spring (August to November) (Bartholomae and van der Plas, 2007), when upwelling favourable southwesterly winds are strongest.

Published

Environment

Northern Benguela

Sardine

Spawning location

Stock structure