Communicating changes in state of the southern Benguela ecosystem using trophic, model-derived indicators

Abstract

An updated trophic model of the southern Benguela ecosystem in the period 2004−2008 was constructed, complementing trophic models for earlier periods. The model repre- sents the trophic structure of the system after a southward shift of major resources. There was an increase in biomass of small pelagic fish and cephalopods between the 1980s and 2004-2008 peri- ods, accompanied by declines in several higher trophic level groups. A 3 step process was fol- lowed: (1) a series of snapshots of the ecosystem was used to explore changes in the food web structure over time, (2) trophic indicators were extracted from these models to detect changes, and (3) model-derived ecological indicators which were deemed most meaningful for management within the ecosystem were selected for use in decision trees, providing a logical framework in which to access synthesised information on trends in ecosystem status as a result of fishing. Three decision trees were developed which examined the ecosystem at the community level (pelagic- caught fish and demersal-caught fish community decision trees) and at the system level (ecosys- tem decision tree). The decision trees classified one period of the southern Benguela ecosystem as 'Deteriorating' (the period after the onset of industrial fishing: 1960s vs. 1900s), and 3 periods as 'Not improving' (1980s vs. 1960s, 1990s vs. 1980s, and 2004−2008 vs. 1980s). The current manage- ment strategy has ensured that the offshore southern Benguela ecosystem has not deteriorated further since the 1960s in terms of ecosystem functioning and at the scale of the fish community. Although the decision trees are conservative with regard to trends, the logic they employ is sound and robust, providing fisheries managers with a coherent framework to access synthesised infor- mation and the reasoning behind conclusions reached.