Comparing internal and external drivers in the southern Benguela and the southern and northern Humboldt upwelling ecosystems

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

“…7). Notably, modeled impacts on jellyfish [presented here and generated by a slightly modified version of the Shannon et al (2008) model; see Appendix 1] are well within the magnitude of impacts on other ecosystem components.…”

“…Food-web models using the Ecopath with Ecosim approach were also used in the region for describing within-system structural changes over time (Heymans et al 2004, Watermeyer et al 2008a or for comparisons with other ecosystems (Jarre-Teichmann et al 1998, Shannon et al 2008, 2009). These modeling studies (e.g., Shannon et al 2009) have suggested that the energy flows within ecosystems can be disrupted by changes in the abundance of small pelagic fishes.…”

“…When the Shannon et al (2008) trophic model for the SB was employed with a recent version of Ecopath with Ecosim (see Appendix 1 for details), some exceptions to the expected patterns were noted, particularly with respect to sardines. Model sardine biomass declined when anchovy or Whitehead's round herring biomass was reduced by fishing, whereas other groups of forage fish competing with anchovies or herring for zooplankton prey benefited.…”

“…For purposes of our study, model experiments on depletion of the various small pelagic fishes species in the southern Benguela were run with a slightly modified version of the Shannon et al (2008) Ecopath with Ecosim model that was fitted to time series of abundance and catch for the period 1978-2003. The aim of including these simulations is primarily to demonstrate the vital role played by small pelagics in sustaining ecosystem function, which is eroded when these stocks collapse, as appears to be the case in the northern Benguela, where jellyfishes and goby are now strong ecosystem components.…”

“…The aim of including these simulations is primarily to demonstrate the vital role played by small pelagics in sustaining ecosystem function, which is eroded when these stocks collapse, as appears to be the case in the northern Benguela, where jellyfishes and goby are now strong ecosystem components. The original trophic model fitted to time series and used to explore ecosystem drivers (Shannon et al 2008), was reconfigured in Ecopath with Ecosim version 6 so that the dynamics of small and large horse mackerel and shallow-and deepwater Cape hake were represented as multistanza groups (one group growing into another) as opposed to simple linked model groups. Total mortality rates for the juvenile stanzas of the three species were estimated to retain biomass levels per model group as close as possible to those in the previous model version.…”