Browsing by Author "Shannon, Lynne Jane"

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    Modelling the oceanographic transport of young Cape anchovy Engraulis capensis by advective processes off South Africa
    (1995) Shannon, Lynne Jane; Field, John G; Nelson, G; Crawford, Robert J M
    A Monte-Carlo type model has been developed to investigate the importance of passive transport by currents above the thermocline for anchovy recruitment off South Africa. Simulation studies indicate that mean year-class strength of Cape anchovy is relatively robust to altered advective processes off South Africa. This occurs despite the fact that changed flow alters the likelihood of offshore advection and hence losses of anchovy from the system. Two different approaches have been taken to address the effects of altered advection, and the applicability of each is discussed. One approach involves altering westward advection in proportion to the mean current field (derived from Acoustic Doppler Current Profiler measurements), and the other, altering westward and northward advection by the addition of fixed offshore current velocities. The proportional approach did not affect year-class strength significantly, whereas the other approach, which incorporated large changes in the flow field, yielded statistically significant differences in predicted year-class strengths between advection scenarios. Reduced flow in the latter approach led to a mean year-class strength 2.7 times stronger than a proposed base flow scenario (which incorporated westward and northward drift in addition to the ADCP currents), whereas enhanced flow resulted in a mean year-class strength of similar magnitude to that of the base flow scenario. Changed flow may alter the geographic distribution of eggs and larvae, which might in turn influence recruitment of young-of-the-year anchovy to the South African purse-seine fishery. The north-flowing shelf-edge jet current plays an important role in transporting anchovy eggs and larvae from spawning grounds in the south to nursery areas frn1her north along the west coast of South Africa. Enhanced model advection westward and norward from the spawning grounds in the south serves to transport anchovy into the region of the jet current. However, advection into unproductive waters offshore is also enhanced and prevents good recruitment under these flow conditions. On the other hand, reduced westward and northward advection in the model, shown through wind records to characterise El-Nino years in coastal areas of South Africa, serves to retain anchovy reproductive products and often transports young anchovy into coastal areas, preventing offshore loss. Therefore the advection model suggests that good year-class strengths (in terms of numbers) are likely to be supported in years when westward and northward advection are reduced. A further reduction in westward advection may be less favourable by causing advective losses offshore along the south coast of South Africa. This may be viewed in terms of an "optimal environmental window" hypothesis, where reduced westward advection is favourable for anchovy survival off South Africa, but further reduction of westward advection as well as enhanced westward advection appear unfavourable. It is concluded that although passive transport, of anchovy in South African waters is relatively robust, it may account for a substantial proportion of recruitment variability.
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    Trophic models of the Benguela upwelling system : towards an ecosystem approach to fisheries management
    (2000) Shannon, Lynne Jane; Field, John G; Moloney, Coleen L; Jarre, Astrid
    Periods of anchovy and sardine abundance alternate in many upwelling areas. In the southern Benguela ecosystem, anchovy were dominant in the 1980s whereas sardine abundance increased in the 1990s. Mass-balanced models were constructed of the trophic flows in the southern Benguela ecosystem for 1980-1989 and 1990-1997 and in the northern Benguela ecosystem for the 1980s, based on the best available data and estimates. Comparing biomass per trophic level, transfer efficiencies, mixed trophic impacts and several other ecosystem-level attributes suggests that trophic functioning of the southern Benguela ecosystem was similar in the 1980s and 1990s. However, because stocks and catches of small pelagic fish and model zooplankton biomass were larger in the 1990s, the ecosystem was less tightly constrained by predators and food availability than in the 1980s. An extension ofOdum's development theory also suggests that the ecosystem was more mature in the 1990s. Using the trophic models, indices quantifying interactions between species and trophic groups were developed to provide useful measures for the comparison of marine ecosystem structure and function. A new trophic similarity index for anchovy and sardine explains the strong similarities in trophic functioning of the southern Benguela ecosystem in the 1980s and 1990s. Modelling experiments using the dynamic ecosystem simulation tool Ecosim, suggest that observed changes in pelagic fish catches between the 1980s and 1990s are unlikely to have played a large role in driving changes in abundance of anchovy and sardine in the southern Benguela ecosystem. It is hypothesized that changed environmental conditions may have influenced the size composition of planktonic food of anchovy and sardine, thus changing their relative abundance, with some signs of a "regime shift' from anchovy to sardine dominance. Steady-state models show that there are structural and functional differences between the northern and southern Benguela sub-systems. Dynamic simulations of altered fishing strategies show that in the south, heavy fishing on model hake and small pelagic fish has larger negative effects on horse mackerel and large pelagic fish respectively than in the north. There are large differences between the effects of altered fishing simulated under different assumptions oftop-down and bottom-up flow control. A new way of approaching fisheries management is proposed according to a 4-step plan using information from ecosystem modelling studies to address the call for "responsible fishing" and the need to take ecosystem effects into account in managing fisheries.