Modelling regime shifts in the southern Benguela: a frame-based approach

Abstract

This study explores the usefulness of a frame-based modelling approach in the southern Benguela upwelling ecosystem, with four frames describing observed small pelagic fish dominance patterns. We modelled the dynamics of sardine Sardinops sagax and anchovy Engraulis encrasicolus populations under the influence of the small pelagic fishery and climate variability, performed extensive sensitivity analyses and exercised the model prototype in scenarios based on realistic management objectives. Model outputs were seen to roughly parallel observed dynamics. Frame dynamics were reasonably sensitive to frame switching rules. Reduced sardine recruitment in the ‘low’ frames had a strong negative impact on stock recovery, and bycatch of juvenile sardine from the modelled anchovy fishery could also greatly impair sardine recovery. Specific tests included exploring the robustness of fisheries management strategies under highly stochastic recruitment and uncertainty around long-term environmental forcing. The respective frame of the model is suggested as a composite indicator of ecosystem state. The model approach shows promise for evaluating small pelagic fisheries management strategies in an ecosystem context incorporating long-term environmental variability.