Browsing by Author "Buecher, Emmanuelle"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Acoustic observations of jellyfish in the Namibian Benguela(Inter-Research, 2001) Brierley, Andrew S.; Axelson, Bjorn Erik; Buecher, Emmanuelle; Sparks, Conrad A.J.; Boyer, Helen; Gibbons, Mark J.Multi-frequency acoustic data (18, 38 and 120 kHz) were collected in conjunction with pelagic trawl sampling for gelatinous macrozooplankton during a cruise to the Namibian Benguela in September 1999. Sampling focused specifically on the scyphozoan Chrysaora hysoscella and the hydrozoan Aequorea aequorea, both of which occur in large numbers, are probably of major ecological importance, and physically hamper pelagic fishing and diamond extraction activities. C. hysoscella was detected predominantly at an inshore station and A. aequorea was found in greatest abundance further offshore in deeper water. Echo-sounder observations were linked directly to net catches, and relationships between catch density (number of individuals m–3) and nautical area scattering coefficients (sA) at each frequency were determined for both species in order to estimate target strength (TS) using the comparison method. TS for C. hysoscella (mean umbrella diameter 26.8 cm) was –51.5 dB at 18 kHz, –46.6 dB at 38 kHz and –50.1 dB at 120 kHz; for A. aequorea (mean central umbrella diameter 7.4 cm) TS was –68.1 dB at 18 kHz, –66.3 dB at 38 kHz and –68.5 dB at 120 kHz. These TS values compared favourably with previously published estimates for related species. Jellyfish were caught at high numerical densities (maxima 3 C. hysoscella per 100 m3, 168 A. aequorea per 100 m3). These high densities, combined with the not unsubstantial TS at frequencies used for fisheries surveys, imply that jellyfish could potentially bias acoustic estimates of fish abundance. We suggest a simple multifrequency approach that could be used to discriminate between echoes from jellyfish and some commercially important pelagic fish in the northern Benguela ecosystem.Item New observations on the diel vertical distribution of Hydromedusae in the southern Benguela ecosystem(NISC (Pty) Ltd and Taylor& Francis, 2003) Buecher, Emmanuelle; Gibbons, Mark J.Diel vertical migration (DVM) of six common species of hydromedusae was investigated during two drogue studies conducted in St Helena Bay on the west coast of South Africa in February 1995. Clytia spp., Obelia spp. and Bougainvillia macloviana, were largely confined to surface waters and did not appear to display any DVM. By contrast, Leuckartiara octona and Euphysa aurata displayed clear DVM and, like Proboscidactyla menoni, were found at greater depths than the other species. The depth distribution and amplitude of migration varied between surveys for most species, so definitive interpretations of patterns could not be made.Item Observations on the ecology of Pleurobrachia pileus (Ctenophora) in the southern Benguela ecosystem(NISC (Pty) Ltd and Taylor & Francis, 2003) Buecher, Emmanuelle; Thibault-Botha, D.; Gibbons, Mark J.The vertical distribution, abundance and size composition of a population of Pleurobrachia pileus was studied during a six-day drogue study conducted off the west coast of South Africa in February 1995. The population was centred in deep water, possibly because of the presence of a potential predator, Beroe sp., at shallower depths. The population of P. pileus failed to display clear patterns of migration, although nocturnal recruitment from deeper water resulted in a deepening of the depth of occupation then. The population was dominated by individuals <4 mm polar diameter, and large animals were confined to the upper water layers. These results are discussed with reference to possible spawning activities. Using published equations relating size to clearance rates, it was estimated that the population could remove up to 27% of integrated mesozooplankton standing stocks, and in excess of 100% (293%) at some depths.