Bibliography: leaves 190-216. / A simple population dynamics model is constructed to simulate temporal variability in the biomass of a dominant copepod Calanoides carinatus (Copepoda: Calanoida) along the West Coast region of South Africa. C. carinatus is extensively preyed upon by the commercially important anchovy Engraulis capensis and variability in zooplankton production may serve as an useful predictor of variability in anchovy recruitment levels. The model developed here circumvents the need to include a large number of parameters because it uses satellite-derived estimates of chlorophyll a concentration and sea surface temperature as primary inputs. Abundance estimates necessary to initialise the model are readily obtainable from biannual research cruises. The model successfully simulates observed features of a copepod population's response to pulses of upwelling and results obtained are consistent with data from field studies. The model is robust with respect to most of its parameters because minor changes in their values result in predictable changes in model output. The effect on model predictions of errors in field estimates is quantified. The model showed greatest sensitivity to parameters which are difficult to determine empirically, such as predator-induced mortality rates. Gaps in our present understanding of the nature and scale of processes affecting copepod egg abundance, survival and viability in the Southern Benguela system, were identified as the dominant impediment to attempts to simulate copepod population dynamics in the region. The Southern Benguela system is patchy on a range of different space and time scales. The effect of fine-scale distributional heterogeneity on mesoscale patterns of copepod productivity was investigated by assuming that spatial patchiness affected the degree of overlap between zooplankton and phytoplankton populations. The effect of spatial patchiness is particularly prevalent under poor feeding conditions, and may result in predictions based on average feeding conditions underestimating zooplankton production by as much as 30% in some circumstances. Estimates of zooplankton production are sensitive to both the spatial arrangement and intensity of food patches in a heterogeneous environment. There is a need to isolate the essential mechanisms causing distributional heterogeneity and to quantify the effect of spatial patchiness on model predictions to permit the correct averaging of model results over broad horizontal areas. Because of the model's sensitivity to the predator-induced mortality rate, a temporally and spatially integrated system is used to quantify this parameter as a function of varying patterns of predator and prey abundance. Shoals of anchovy recruits are explicitly modelled feeding on patches of C. carinatus prey, and the fish's performance is quantified through temporal and spatial integration of periods and patches of prey abundance and shortage. Constant high fish densities dampen the spatial variability in copepod abundance, whereas a pulsed predation pressure permits locally depleted copepod populations a short respite in which to recover some growth, thereby allowing the persistence of a few good prey patches which offer favourable energy returns for foraging fish. The model suggested that at high densities of anchovy recruits, predicted growth rates are strongly density-dependent and predation rates may exceed copepod production rates. Absolute measures of prey availability are sometimes unable to predict anchovy feeding success as mechanisms permitting temporal and spatial segregation play a vital role in synchronizing the relationship between fish predation pressure and prey turnover rates. The model emulates observed variability in anchovy growth rates and analysis of the output indicates that the availability of high sustained abundances of food along the West Coast may be a critical "bottleneck" contributing to the strength of recruitment to the pelagic puseseine fishery in South African waters. Observed chlorophyll a concentration and sea surface temperature data in 1971 and 1972 were used as inputs into an annual version of the basic model, and model-predicted patterns of copepod biomass were compared with observed patterns of zooplankton biomass in the two years. The ability of the model to simulate major differences in the general features observed in the two years supports its use as a tool to describe net patterns of zooplankton productivity over large horizontal areas. The model · identified the need to quantify the role of major size-class groups, such as the microzooplankton and macrozooplankton, in mediating the flow of energy from phytoplankton to fish.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/14969 |
| Date | January 1995 |
| Creators | Plagányi, Éva Elizabeth |
| Contributors | Field, John G, Hutchings, Larry |
| Publisher | University of Cape Town, Faculty of Science, Department of Oceanography |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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