ALBERT

Description: 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.