Modelling effects of fishing in the Southern Benguela ecosystem

A mass-balanced model of the Southern Benguela upwelling ecosystem was cons tructed using Ecopath. The effects of altered fishing on three abundant sma ll pelagic fish, and on hake, are explored using Ecosim for three scenarios of top-down and bottom-up control: (1) bottom-up control of zooplanktivoro us fish by zooplankton dampens effects of altered fishing; (2) wasp-waist c ontrol (top-down control of zooplankton by their predators and bottom-up co ntrol of predators by small pelagic fish) causes vigorous effects to propag ate through the ecosystem; and (3) effects of mixed control (neither top-do wn nor bottom-up control) are intermediate between the former two scenarios . Heavier fishing may be sustainable under one scenario of control, whereas fisheries may crash if another type of control is assumed. The key to pred icting ecosystem effects of fishing is understanding the way in which compo nents of the ecosystem interact. (C) 2000 International Council for the Exp loration of the Sea.