JELLYFISH BLOOMS Acoustic survey of a jellyfish-dominated ecosystem (Mljet Island, Croatia)

Acoustic techniques have been proposed as a new tool to assess jellyfish populations. However, the presence of mixed echoes from jellyfish and other organisms that share their distribution often prevent accurate estimates of their abundance and distribution being obtained. The isolated population of Aurelia inhabiting the Veliko Jezero (Big Lake-BL) of Mljet Island, in the South Adriatic Sea, offered a good opportunity to employ acoustic techniques to assess an entire jellyfish population. During October 2–5, 2006, combined video and acoustic methods were used in BL to determine the vertical distribution of medusae. Two synoptic acoustic surveys were performed during the day and night. In the daylight echograms, medusae were clearly discernible from the acoustic data, and their presence verified by video camera images, as forming a layer of varying density at and below the thermocline (15–30 m). The depth of the jellyfish layer also coincided with the depth of maximum dissolved oxygen concentration. The echointegration of these daylight data enabled quantification the Aurelia population, at a frequency of 120 kHz. In the night echograms, the acoustic signals of Aurelia were at least partially masked by pelagic and demersal fish, which disaggregated from schools and formed a layer associated with a strong thermocline at 15 m. An average target strength (TS) of -76.4 dB was obtained in situ corresponding to a mean length of 10.8 cm and a mean wet weight of 134 g measured from sampled medusae. These results were combined with echo-integration values to provide an estimate of 4,238,602 individuals and a biomass of 568 tons of Aurelia in BL. This study provided a synoptic view of Mljet Lake and illustrated the potential of acoustic surveys of jellyfish populations to contribute to ecosystems studies.