Acoustic observations of jellyfish in the Namibian Benguela

Andrew Stuart Brierley, BE Axelsen, E Buecher, CAJ Sparks, H Boyer, MJ Gibbons

Research output: Contribution to journalArticlepeer-review

Abstract

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 m(3), 168 A. aequorea per 100 m(3)). 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.

Original languageEnglish
Pages (from-to)55-66
Number of pages12
JournalMarine Ecology Progress Series
Volume210
Publication statusPublished - Jan 2001

Keywords

  • acoustics
  • Chrysaora hysoscella
  • Aequorea aequorea
  • multi-frequency
  • target strength
  • Benguela
  • jellyfish
  • Namibia
  • TARGET-STRENGTH
  • AEQUOREA-VICTORIA
  • CHRYSAORA-QUINQUECIRRHA
  • PREDATION MORTALITY
  • UPWELLING SYSTEM
  • SOUND-SCATTERING
  • CHESAPEAKE BAY
  • FISH LARVAE
  • ZOOPLANKTON
  • ABUNDANCE

Fingerprint

Dive into the research topics of 'Acoustic observations of jellyfish in the Namibian Benguela'. Together they form a unique fingerprint.

Cite this