Ecopath with Ecosim is a modeling software that allows the creation of mass-balanced models of the trophic flows of an ecosystem to explore the past and present impacts of fishing and environmental change on the trophic flows of a given food web. Currently, Algoa Bay supports the world’s largest breeding colony of endangered African penguins (Spheniscus demersus). The breeding success of African penguins is largely dependent on the availability of their food, mainly anchovy (Engraulis encrasicolus) and sardine (Sardinops sagax). African penguins breed year-round however, peak breeding season begins during the summer months (January to March) in Algoa Bay, when penguins begin building up fat reserves and laying eggs. In order to investigate the interaction between the small pelagic fish and penguin population, mass-balanced trophic models of the Algoa Bay ecosystem were constructed using the software Ecopath with Ecosim. Input parameters were derived from data compiled from published literature and survey data collected by DAFF (Department of Agriculture, Forestry, and Fisheries; formerly Marine and Coastal Management, MCM) and SAEON (the South African Environmental Observation Network). Two seasonal models were created to depict the summer (January to March) and winter (June to August) seasons from 2010-2014. Additionally, two seasonal models were created to represent a marine protected area where all fishery catch was set to zero. These static marine protected area models were created to investigate how the removal of fishery predation impacts the trophic structure of the Algoa Bay ecosystem by comparing ecotrophic efficiency values to those of the fished system. Two Ecosim simulations were used in fishery sensitivity analyses. The first, by setting all fishery catches to zero over a period of 30 years for each season to investigate the sensitivity of trophic groups to the removal of fishing pressure. The second, by setting fishery mortalities of anchovy and sardine to levels reported in the west coast (also over a period of 30 years) to investigate the sensitivity of trophic groups to an increase in small pelagic fishing pressure. Seasonal differences were observed with an increase in primary production and zooplankton biomass in the summer season compared to the winter season. The increase in plankton biomass resulted in an approximately 3x greater summer total system throughput, and total net primary production. Transfer efficiencies were higher than the average of 10% for aquatic ecosystems at trophic levels II and III with the seasonal averages being 17% and 19% respectively. The greatest seasonal change in modeled biomass occurred at trophic level III with summer biomass being 72% larger, indicating that small pelagic fishes benefit the most from the increase in summer plankton biomass. Linefish ecotrophic efficiency values were smaller in the marine protected area models in the summer and winter respectively, reflecting the reduced mortality on linefish under MPA conditions. Although the removal of fisheries resulted in an overall drop in predation pressure of 27% in the static MPA models, predation by other predatory trophic groups in the bay increased by 13% as a result of their subsequent biomass increases after the removal of fishery predation. Ecosim results showed the greatest change in biomass occurred in linefish which increased in both seasons when fishery catch was set to zero. The increase in linefish biomass can be attributed to the removal of fishery pressure on themselves and their prey, and may be an indication that linefish are overexploited in the bay. As a result of the biomass increase of some predator groups, some lower trophic level fish groups decreased despite the removal of fishery pressure. The results of the marine protected area analyses indicate that fisheries do not necessarily only have direct impacts on a target species as food-webs can have moderating effects. When fishing mortalities for sardine and anchovy were increased to west coast values, African penguins had the largest decrease in modeled biomass followed by other seabirds. Although, the observed decrease in modeled biomass of African penguins over the 30-year period does not fall within IUCN criteria for major concern, African penguins are already listed as endangered. An added decline of 10% on the largest breeding colony could have major implications on the future of the species. This study was a preliminary attempt at constructing mass-balanced trophic models of the Algoa Bay ecosystem, highlighting seasonal differences while investigating the possible impact of implementing a marine protected area in the bay and the sensitivity of trophic groups to fishing. Future research is needed to improve the more uncertain model parameters; however, these models are a good base for future work and the application of spatialized modeling of the bay using EcoSpace
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/31308 |
| Date | 25 February 2020 |
| Creators | Weigum, Emily |
| Contributors | Shannon, Lynne, Jarre, Astrid |
| Publisher | Faculty of Science, Department of Biological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Masters Thesis, Masters, MSc |
| Format | application/pdf |
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