Acoustic observations of jellyfish in the Namibian Benguela

Multi-frequency acoustic data (18, 38 and 120 kHz) were collected in conjun ction with pelagic trawl sampling for gelatinous macrozooplankton during a cruise to the Namibian Benguela in September 1999. Sampling focused specifi cally on the scyphozoan Chrysaora hysoscella and the hydrozoan Aequorea aeq uorea, both of which occur in large numbers, are probably of major ecologic al 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 nauti cal area scattering coefficients (SA) at each frequency were determined for both species in order to estimate target strength (TS) using the compariso n method. TS for C, hysoscella (mean umbrella diameter 26.8 cm) was -51.5 d B at 18 kHz, -46.6 dB at 38 kHz and -50.1 dB at 120 kHz; for A. aequorea (m ean central umbrella diameter 7.4 cm) TS was -68.1 dB at 18 kHz, -66.3 dB a t 38 kHz and -68.5 dB at 120 kHz. These TS values compared favourably with previously published estimates for related species. Jellyfish were caught a t high numerical densities (maxima 3 C. hysoscella per 100 m(3), 168 A. aeq uorea per 100 m(3)). These high densities, combined with the not unsubstant ial TS at frequencies used for fisheries surveys, imply that jellyfish coul d potentially bias acoustic estimates of fish abundance. We suggest a simpl e multifrequency approach that could be used to discriminate between echoes from jellyfish and some commercially important pelagic fish in the norther n Benguela ecosystem.