A donor-driven approach to modelling anchovy-sardine dominance shifts in the southern Benguela ecosystem

Abstract

Abstract The change in species dominance between anchovy and sardine that took place during the 1980s in the southern Benguela has been investigated previously using an Ecopath with Ecosim (EwE) model. The results from that model indicated that fishing pressure was unlikely to have been the driver of change. Instead, the findings indicated that changes within low trophic level groups such as the zooplankton affected prey availability for anchovy and sardine, leading to the shift in dominance. This paper applies the donor-driven Ecotran model to these same data for the southern Benguela ecosystem in the 1980s. The consumption matrix from the Ecopath model was converted to a production matrix, apportioning the production of prey groups in a constant proportion to their predators, rather than assuming the consumption of predator groups is taken in fixed proportions from their prey. This configuration allows donor-control of model ecosystem dynamics, appropriate when low trophic level groups affect the outcomes. The trophic structure for the Ecotran model was further amended to exclude “cannibalism”, and dissolved nutrient groups were added to allow for recycling. To test whether the donor-driven modelling affected model results, the same set of scenarios was applied using Ecotran models as were tested using the EwE model in previous research. Qualitative methods were used to compare the results of the two model techniques, as Ecotran scenarios represented static, steady-state solutions for the ecosystem state, whereas EwE scenarios represented dynamic solutions for the ecosystem state. These results indicate that for the majority of groups and scenarios the Ecotran model predicts the same direction of change as the EwE model when the 1980s ecosystem is used as a starting point. However there is more uncertainty in the direction of change predicted by the Ecotran model when it is applied to the 1990s state in an attempt to recreate the 1980s state, and thus more disagreements between the models. The results of the Ecotran model supported the conclusion from prior EwE analysis that the driver of change in anchovy and sardine dominance was likely changes in prey availability. The underlying cause of this shift in prey availability remains a question for further research.