Acoustic observations of jellyfish in the Namibian Benguela

dc.contributor.authorBrierley, Andrew S.
dc.contributor.authorAxelson, Bjorn Erik
dc.contributor.authorBuecher, Emmanuelle
dc.contributor.authorSparks, Conrad A.J.
dc.contributor.authorBoyer, Helen
dc.contributor.authorGibbons, Mark J.
dc.date.accessioned2011-09-08T07:35:49Z
dc.date.available2011-09-08T07:35:49Z
dc.date.issued2001
dc.description.abstractMulti-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.en_US
dc.identifier.citationBrierley, A.S., et al. (2001). Acoustic observations of jellyfish in the Namibian Benguela. Marine Ecology Progress Series, 210: 55-66en_US
dc.identifier.issn0171-8630
dc.identifier.urihttp://hdl.handle.net/10566/246
dc.inquiriesmgibbons@uwc.ac.za
dc.language.isoenen_US
dc.privacy.showsubmittertrue
dc.publisherInter-Researchen_US
dc.relation.ispartofseriesMarine Ecology Progress Series;
dc.rightsCopyright Inter-Research. The 4 year embargo on displaying the full text of this article has expired.
dc.source.urihttp://dx.doi.org/10.3354/meps210055
dc.status.ispeerreviewedtrue
dc.subjectNamibiaen_US
dc.subjectJellyfishen_US
dc.subjectMulti-frequency hydroacousticsen_US
dc.subjectBiomassen_US
dc.subjectTarget strengthen_US
dc.subjectNorthern Benguelaen_US
dc.titleAcoustic observations of jellyfish in the Namibian Benguelaen_US
dc.typeArticleen_US

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