Benguela (Southern) - Ecology

Changes in the marine environment have been shown to impact a range of species in the Benguela simultaneously, with different lag periods being noted. For example, the warm event that occurred during the mid-1980s has been linked to changes in resources such as anchovy, sardine, hake, rock lobster and chokka squid. During the 1980s, anchovy was the dominant small pelagic fish in the southern Benguela sub-system. By the 1990s, anchovy biomass had declined and stocks of sardine, redeye, horse mackerel and both species of hake had increased in size. Sardine dominated the South African catch from 1950 to 1965, and anchovy from then until the mid 1990s. Sardine again began to recover in the late 1990s, and in the early 2000s, both anchovy and sardine were relatively abundant, although this“high pelagic fish biomass” situation has since been proved unsustainable, with stocks of both anchovy and sardine declining again in recent years. . It has been hypothesized that in the Southern Benguela, the water column stabilizes during warm periods as upwelling events are weak and less frequent, phytoplankton is dominated by small cells, leading to a zooplankton community dominated by small copepods that favour filter-feeding sardine. In cool periods when upwelling is stronger, phytoplankton biomass is high and dominated by large chain-forming diatoms, and zooplankton are larger and ideal for the biting feeding behaviour of anchovy. A single or a series of good year-classes may result from an episodic, favourable environmental event, and may be adequate to shift the stock into a high abundance/productivity regime. Off South Africa, two favourable episodic environmental events, resulting from wind-induced upwelling in the summer of 1999-2000, have been proposed as the reason for the sudden upsurge in abundance of small pelagic fish in the southern Benguela in the early 2000s. Indeed, trophic modelling simulations of the southern Benguela suggest that off South Africa, environmental factors have been more important drivers of ecosystem dynamics than fishing over the latter part of the 20th century. In particular, model simulation studies suggest that environmental forcing changing the relative availability of mesozooplankton to anchovy and sardine may be a key factor in driving changes in the abundance of small pelagic fish in the southern Benguela. Studies are underway to explore in greater detail the possible mechanisms of these ecosystem changes, initiated or mediated through physical forcing.