Modelling effects of fishing in the Southern Benguela ecosystem

A mass-balanced model of the Southern Benguela upwelling ecosystem was constructed using Ecopath. The effects of altered fishing on three abundant small pelagic fish, and on hake, are explored using Ecosim for three scenarios of top-down and bottom-up control: (1) bottom-up control of zooplanktivorous fish by zooplankton dampens effects of altered fishing; (2) wasp–waist control (top-down control of zooplankton by their predators and bottom-up control of predators by small pelagic fish) causes vigorous effects to propagate through the ecosystem; and (3) effects of mixed control (neither top-down 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 predicting ecosystem effects of fishing is understanding the way in which components of the ecosystem interact.