Browsing by Author "Stuart, V."

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    Feeding and vertical migration of the chaetognath Sagitta friderici (Ritter Zahony, 1911) in the southern Benguela during spring 1987, with notes on seasonal variability of feeding ecology
    (NISC and Taylor & Francis, 1994) Gibbons, Mark J.; Stuart, V.
    The feeding biology and the vertical migration of Sagitta friderici were examined over 24 h at two stations in the southern Benguela during spring (October) 1987. Together with studies conducted during summer (February 1991) and winter (May 1984), they serve to allow valuable generalizations of the biology and ecology of this abundant chaetognath. Populations migrate vertically and feed nocturnally, although the timing and the extent of migration vary between studies. S.friderici exhibits ontogenetic layering and the cross-shelf distribution of maturity stages differs, suggesting that it is able to take advantage of cross-shelf water movement in order to maintain populations in the nearshore waters of the West Coast. S.friderici prey almost exclusively on copepodg (cannibalism is rare), and there is a positive relationship between the lengths of predator and prey that is influenced by the size structure of the prey environment. This casts doubt on the validity of a chaetognath species-specific relationship between predator and prey size. S. friderici selects its prey on the basis of size, and not species. Daily ration is related linearly to prey density, so reflecting the low density of prey and providing support for theoretical predictions regarding ingestion rates under oceanic conditions. The impact of S. friderici predation on the copepod assemblage is generally less than 3% of the standing stock, although it could be much higher under conditions of low copepod biomass and poor secondary production.
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    Trophic ecology of carnivorous zooplankton in the Benguela
    (NISC and Taylor & Francis, 1992) Gibbons, Mark J.; Stuart, V.; Verheye, H.M.
    Carnivorous zooplankton in the Benguela system have tended to be ignored by all but early taxonomists and a handful of recent researchers. An attempt is made here to address the importance of carnivores in this system but, because the database is poor, it was necessary to apply material from outside the region. The taxa covered include ctenophores, cnidarians (Scyphozoa and Hydrozoa, including Siphonophora), chaetognaths and hyperiid amphipods as well as such minor groups as pelagic gastropods and decapods. Most groups of carnivores within the Benguela are represented by only one or two "common" species. These tend to be epiplanktonic and generally concentrated inshore of the shelf-break, where their distributions are influenced by changes in local hydrography. Their densities frequently exceed those of herbivores. The dominant carnivores share a number of attributes: wide-ranging diets, high assimilation and growth efficiencies and mostly a Type I functional response to food concentration. They fall into two categories. The first is the gelatinous species, which have short lifespans and exhibit rapid growth and often massive reproductive response to high concentrations of food. They are patchily distributed and some appear to have behavioural traits that enable them to stay within localized patches of prey on which they may be dependent because of high metabolic costs. In the second category are crustacean predators and chaetognaths which have slower response times to the food environment but may have longer lifespans, lower daily maintenance needs and an ability to store lipids. They are less dependent on high densities of food organisms and can survive throughout winter, when gelatinous predators are rare. The effect of predation on abundance of copepods is estimated on the basis of both published and unpublished data. Should swarms of predators coincide with a critical gateway of zooplankton or larvae, then there may be total depletion. This applies especially to the nearshore zone of the West Coast and to semi-enclosed bays. The role of carnivorous zooplankton in the diets of commercially important fish species is stressed, but their importance in the regeneration of nutrients may be insignificant.