Abstract
This paper presents a new, updated, temporally-dynamic Ecosim model for the Southern Benguela (1978-2015) by capturing recent advancements in our understanding of the dynamics and newly available data series from an additional decade of research in the region. The paper documents the model development and the thinking behind incorporating a newly available upwelling index, and observed fishing effort as important drivers of modelled ecosystem dynamics. The most sensitive predator-prey interactions were identified and vulnerabilities (of prey to predators) were estimated to improve model fit to field observations under each scenario. Sardine interactions with prey and predators were consistently found to be sensitive interactions in model fitting, accounting for at least 40% of the most sensitive trophic interactions in the southern Benguela modelled food web. Model fits to data were improved by a total of 85% reduction in sum of squares when upwelling effects on large phytoplankton availability to zooplankton and small pelagic fish was incorporated, geographic shifts in sardine distribution were captured by means of altered availability of sardine to predators, corresponding vulnerabilities of prey to predators were estimated, and an additional, small, hypothetical forcing function was fitted to small phytoplankton production. African penguin and Cape gannet fits to data series were improved by incorporating a recently published bird Food Availability Index, although model fits of several fish groups then deteriorated, emphasising the need for additional empirical species-specific functional response studies. Attempts at fitting the full area model to either west coast or south coast time series did not improve overall model fits, highlighting limitations of the current non-spatialized model in describing coast-specific dynamics. This study shows promise in unravelling the observed dynamics of the Benguela upwelling ecosystem, and points to the importance of exploring spatially disaggregated approaches, in particular the new foraging habitat capacity of the Ecospace approach, to improve our understanding of processes whereby variability in upwelling influences dynamics of the Southern Benguela ecosystem. This is important in reconciling knowledge needed to manage fisheries and to protect marine biodiversity by means of ecosystem-based management in South Africa, and to advance management advice under future scenarios of climate change.
Highlights
The Benguela upwelling system is one of four major eastern boundary current systems in the world
Mesopelagic fish were disaggregated in the revised model since relative abundances of lanternfish (Lampanyctodes hectoris) and lightfish (Maurolicus muelleri) have been changing over the past decade (DAFF, unpublished data; Janet Coetzee, personal Communication), and since there is a move to target lanternfish using midwater trawling on the West Coast (ANON, 2016)
The dual, interacting contributions of both fishing and environmental variability in driving observed dynamics in the Southern Benguela ecosystem have been highlighted by means of ecosystem modeling (Shannon et al, 2004b; TraversTrolet et al, 2014; Ortega-Cisneros et al, 2018b) and using survey-based (Shannon et al, 2010) and modeled-derived (Fu et al, 2018) indicators
Summary
The Benguela upwelling system is one of four major eastern boundary current systems in the world. The southern Benguela is a well-studied upwelling ecosystem bounded by the warm Agulhas current to the south and east, and the northern Benguela upwelling area off Namibia to the north. It is influenced by strong, wind-driven upwelling, which results in high biological productivity and supports several important commercial fisheries including the small pelagics, hake and squid fisheries and an important small-scale sector. In the last two decades, substantial changes in environmental variables have been observed in this system, including changes in sea surface temperature, upwelling intensity, incidence of harmful algal blooms and extreme events (Rouault et al, 2010; van der Lingen et al, 2016; Lamont et al, 2018) among others. The total cumulative upwelling (TCU) and the number of upwelling days in the Agulhas Bank significantly increased between 1979 and 2014, while a linear, albeit non-significant trend in the TCU has been identified for the southern Benguela (Lamont et al, 2018)
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