Trophic flows in the southern Benguela during the 1980s and 1990s

Mass-balanced models of trophic flows in the southern Benguela ecosystem suggest a 10% increase in zooplankton biomass between the 1980s and the 1990s, in agreement with observed trends of increased zooplankton abundance off South Africa over the last few decades. Minimum hake biomass in balanced trophic models is substantially larger than survey and other model estimates, suggesting undersampling of hakes in surveys and underestimation of juvenile hake mortality. Model biomass and mean annual production of five important small pelagic fish groups were larger in the 1990s, and total catches were smaller than in the 1980s. Estimates of biomass per trophic level, transfer efficiencies, mixed trophic impacts and many other ecosystem attributes suggest that trophic functioning of the southern Benguela ecosystem was similar in the 1980s and 1990s. Because catches were lower and model zooplankton and small pelagic fish biomasses were larger in the 1990s, the ecosystem was less tightly constrained by predators (including fishers) and food availability than in the 1980s. Fishing took place at low trophic levels compared to other systems. Despite smaller total catches in the 1990s, fishing was ecologically more expensive (from higher trophic levels) during the 1990s than in the 1980s because snoek and hake catches were large. There was greater shared niche overlap of small pelagic fish predators in the 1990s than in the 1980s. Mean transfer efficiency was 12%. Transfer of biomass at trophic levels III-V appears to be more efficient in the southern Benguela than in other upwelling ecosystems. Primary production required to sustain catches in the southern Benguela ecosystem is 4% of total primary production, i.e. more similar to estimates for open ocean and coastal regions than for other upwelling or shelf systems averaging more than double this value. (C) 2002 Elsevier Science B.V. All rights reserved.