Acoustic observations of jellyfish in the Namibian Benguela

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 ecolog- ical importance, and physically hamper pelagic fishing and diamond extraction activities. C. hyso- scella was detected predominantly at an inshore station and A. aequorea was found in greatest abun- dance 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 scat- tering 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. Jelly- fish 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 fish- eries surveys, imply that jellyfish could potentially bias acoustic estimates of fish abundance. We sug- gest a simple multifrequency approach that could be used to discriminate between echoes from jelly- fish and some commercially important pelagic fish in the northern Benguela ecosystem.