© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Single-target echo detections of jellyfish
a Gatty Marine Laboratory, University of St Andrews Fife KY16 8LB, Scotland, UK
b Institute of Marine Research PO Box 1870 Nordnes, 5817 Bergen, Norway
c National Information and Marine Research Center PO Box 912, Swakopmund, Namibia
d Department of Nature Conservation and Oceanography, School of Life Science Cape Technikon, PO Box 652, Cape Town, South Africa
e Shannon Point Marine Center 1900 Shannon Point Rd, Anacortes, WA 98221, USA
f Zoology Department, University of the Western Cape Private Bag X17, Bellville 7535, South Africa
*Correspondence to A. S. Brierley: tel: +44 (0) 1334 463458; fax: +44 (0) 1334 463443. e-mail: asb4{at}st-and.ac.uk.
Acoustic target-strength (TS) measurements are presented for tethered and free-swimming individual Chrysaora hysoscella (Scyphozoa) and Aequorea aequorea (Hydrozoa) medusae in Namibian waters. Tethered individual C. hysoscella (17–54 cm total umbrella diameter) and A. aequorea (19–28 cm total umbrella diameter) were ensonified at 38 kHz using a portable echosounder. Mean TS values for individual medusae at this frequency ranged from –67.3 to –52.8 dB for C. hysoscella and from –65.4 to –50.1 dB for A. aequorea. There was a positive relationship between medusa diameter and TS for both species. TS of individual medusae varied cyclically over time by about 15 dB, probably because of the periodic contraction of the medusae whilst swimming. C. hysoscella was parasitized by hyperid amphipods (maximum infestation >1800 parasites per medusa). A fluid-cylinder scattering model was used to determine the expected backscatter from the parasites, and it suggested that even at the highest observed level of infestation the jellyfish itself remained the major contributor to total backscatter at 38 kHz. Single-target echoes from targets identified by trawling as medusae were obtained from vessel-mounted echosounders at 18, 38, 120, and 200 kHz. Triangulation between echosounder beams to identify targets detected simultaneously at all four frequencies increased confidence that echoes were in fact from single targets. The 38-kHz TS values from free-swimming medusae corresponded with values obtained from tethered animals at the same frequency, providing strong evidence that the TS estimates were robust. TS values at all four frequencies (Chrysaora hysoscella mean umbrella diameter 41 cm, TS at 18 kHz = –60.0 dB, 38 kHz = –65.5 dB, 120 kHz = –68.0 dB, and 200 kHz = –70.5 dB. Aequorea aequorea mean inner-umbrella diameter 6.5 cm, TS at 18 kHz = –66.0 dB, 38 kHz = –65.5 dB, 120 kHz = –68.0 dB, and 200 kHz = –73 dB) were consistent with previously published data. Given these robust TS estimates, the possibility may now exist for multi-frequency identification and evaluation of these jellyfish species in some circumstances, and for the use of acoustic-survey techniques to estimate jellyfish abundance.
Keywords: acoustic, amphipod, Bayes, Cnidaria, inference, medusa, Scyphozoa, target strength
Received 21 July 2002; accepted 19 September 2003.
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