Changes in the trophic structure of the northern Benguela before and after the onset of industrial fishing

Abstract

Exploitation of marine resources has been occurring in the northern Benguela ecosystem for centuries. Understanding the cumulative long-term effects of this exploitation is important toward effective management of the modern system. Retrospective mass-balanced models of the ecosystem have been constructed, using Ecopath with Ecosim, for each of the aboriginal (1600), pre-industrial (1900), industrial (1967) and post-industrial (1990s) eras of exploitation in order to ascertain the nature of changes that may have occurred in ecosystem structure and functioning. Biomass of most exploited groups, specifically sardine, hake and seabirds, declined considerably over time. The dominant small pelagic fish, characteristic of upwelling systems, were replaced by a wider range of species, and biomass of gelatinous zooplankton appears to have increased dramatically in recent decades. Catches declined, and mean trophic level of the catch increased from 3.18 in 1967 to 3.68 in 1990, as did the weighted trophic level of the community (excluding plankton), after the collapse of small pelagic stocks in the 1970s. Environmental anomalies experienced in the 1990s had a greater influence over already depleted stocks than previous events, both directly and indirectly affecting a number of stocks negatively. This compounded any effects of fishing and prevented mitigation of declining stocks by management measures implemented after Namibian independence in 1990. Changes in ecosystem structure prior to the 1990s, as a result of heavy fishing, may have altered the trophic control mechanism operating in the system, allowing environmental effects to exert a greater influence. Although fishing certainly influenced ecosystem structure and functioning in the northern Benguela, environmental events have had a considerable, and possibly even greater, impact.

Keywords: Ecopath; ecosystem model; fishing impacts; foodweb; northern Benguela; trophic flows

African Journal of Marine Science 2008, 30(2): 383–403