Trophic models of the Benguela upwelling system : towards an ecosystem approach to fisheries management

Periods of anchovy and sardine abundance alternate in many upwelling areas. In the southern Benguela ecosystem, anchovy were dominant in the 1980s whereas sardine abundance increased in the 1990s. Mass-balanced models were constructed of the trophic flows in the southern Benguela ecosystem for 1980-1989 and 1990-1997 and in the northern Benguela ecosystem for the 1980s, based on the best available data and estimates. Comparing biomass per trophic level, transfer efficiencies, mixed trophic impacts and several other ecosystem-level attributes suggests that trophic functioning of the southern Benguela ecosystem was similar in the 1980s and 1990s. However, because stocks and catches of small pelagic fish and model zooplankton biomass were larger in the 1990s, the ecosystem was less tightly constrained by predators and food availability than in the 1980s. An extension ofOdum's development theory also suggests that the ecosystem was more mature in the 1990s. Using the trophic models, indices quantifying interactions between species and trophic groups were developed to provide useful measures for the comparison of marine ecosystem structure and function. A new trophic similarity index for anchovy and sardine explains the strong similarities in trophic functioning of the southern Benguela ecosystem in the 1980s and 1990s. Modelling experiments using the dynamic ecosystem simulation tool Ecosim, suggest that observed changes in pelagic fish catches between the 1980s and 1990s are unlikely to have played a large role in driving changes in abundance of anchovy and sardine in the southern Benguela ecosystem. It is hypothesized that changed environmental conditions may have influenced the size composition of planktonic food of anchovy and sardine, thus changing their relative abundance, with some signs of a "regime shift' from anchovy to sardine dominance. Steady-state models show that there are structural and functional differences between the northern and southern Benguela sub-systems. Dynamic simulations of altered fishing strategies show that in the south, heavy fishing on model hake and small pelagic fish has larger negative effects on horse mackerel and large pelagic fish respectively than in the north. There are large differences between the effects of altered fishing simulated under different assumptions oftop-down and bottom-up flow control. A new way of approaching fisheries management is proposed according to a 4-step plan using information from ecosystem modelling studies to address the call for "responsible fishing" and the need to take ecosystem effects into account in managing fisheries.

Grantor dc:publisher.institution

Department of Biological Sciences

Year dc:date.issued

2000

Author dc:creator

• Shannon, Lynne Jane

Advisors dc:contributor.advisor

• Field, John G
• Moloney, Coleen L
• Jarre, Astrid

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/18428

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/18428