Despite high primary productivity. the yield of pelagic fish in the southern Benguela is relatively low compared to that in the Humboldt system. Part of the constraint may be the ability of pelagic fish to reproduce successfully in a strongly pulsed upwelling environment, where enrichment, retention and concentration mechanisms are less compatible than in Peru-Chile. Anchovy Engraulis capensis spawn upstream of the main upwelling centres on the food-poor, thermally stratified western Agulhas Bank, over a protracted summer season (October-February) when high wind speeds of 7 -8 m·s-I are prevalent. Eggs spawned farther east, on the central or eastern Agulhas Bank, may be subject to increased cannibalism and advective losses, whereas those spawned farther west could be susceptible to heavy advective losses offshore during periods of strong southerly winds. Copepod concentrations are negatively correlated with spawner biomass on the western Bank and are inversely linked to high rates of gonad atresia in anchovy and reduced subsequent recruitment. There is a restricted area of suitable spawning temperatures (l6-19°C) on the western Bank, and fish outside that range appear to have less successful spawning, affecting recruitment; a reduction of the duration of spawning appears to decrease the chances of good recruitment. Oil content of pelagic catches on the west coast of South Africa suggests that good feeding conditions for adult fish are also favourable for growth of young fish in the nursery area. An inverse correlation between the numbers of recruits estimated in June and the strength of upwelling-favourable winds over the previous summer indicates the importance of advective loss of eggs and larvae in the transport and nursery areas. The presence of Agulhas Rings close to the shelfedge, predators on juvenile anchovy, and the strength of upwelling on the West Coast could also be important factors affecting recruitment success. There seem to have been changes in the inshore-offshore distributions of spawning anchovy and/or their eggs over the past three decades, resulting in different recruitment responses to south-easterly winds.
Understanding long-term variability of pelagic fish populations is important in developing forecasting strategies for fisheries management and planning. However, many current fisheries models have only shortterm datasets available, whereas those of suitable duration often lack reliability. As resources are placed under increasing pressure, all available information should be used to assist management. Two simple rulebased deterministic modelling approaches are described, which use semi-quantitative and qualitative rules to relate recruitment success of South African anchovy Engraulis capensis to physical and biological indices. The first model relates recruitment success to indices of wind and sea surface temperature by way of a rulebased decision support system. In the second model, significant environmental and biological factors were identified and related to anchovy recruitment by way of an expert system approach. These two approaches are evaluated and compared. It is suggested that these types of models, when satisfactorily validated, have great potential in supporting the future management of the South African anchovy fishery in the dynamic environment of the Benguela Current.
Previous studies of deep-sea sediment cores have found evidence for Milankovitch cycles, climatic cyclicity due to the periodicity of the Earth's orbital parameters. Many of the cores recovered on Leg 114 of the Ocean Drilling Program showed outward signs of cyclicity, especially at Site 704. We have analyzed the GRAPE density, carbonate content, and magnetic susceptibility using both standard and nonstandard spectral analysis techniques. One of the nonstandard techniques used was the Lomb-Scargle spectral estimation method, which is designed for unequally spaced data and which yields as part of the process the statistical significance of any observed spectral peaks. Pairs of spectra were compared for statistical similarity using the Kolmogorov-Smirnov method. All of the data sets contain some spectral peaks, including both the expected Milankovitch cycles as well as other peaks. Upon further investigation, we have found that the other peaks could be explained as the nonlinear climate system response to the Milankovitch orbital forcing functions, because the extra peaks appear to be simple linear combinations, harmonics and subharmonics of the Milankovitch peaks. At Site 704, there is a marked change in the response at the Brunhes/Matuyama boundary (0.73 Ma B.P.) from strong long-period cyclicity in the Brunhes to more prevalent shorter period cyclicity in the Matuyama.
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