Acoustic survey of a jellyfish-dominated ecosystem.

ALVAREZ COLOMBO, G.; BENOVIC, A.; MALEJ, A.; LUCIC, D.; MAKOVEC, T.; ONOFRI, V.; ACHA, E.M.; MADIROLAS, A.; MIANZAN, H.W.

HYDROBIOLOGIA

Año: 2008 vol. 616 p. 99 - 111

0018-8158

Increases in jellyfish abundance worldwide have been related to major changes in marine ecosystems. Regardless of the ecological importance of such increases, accurate estimates of their abundance and distribution are scarce because of methodological constraints. In spite of recent advances on their acoustic detectability, the reliability of field acoustic estimates of jellyfish abundance still entails a major challenge. The isolated population of Aurelia inhabiting the Veliko Jezero (Big Lake -BL) of Mljet Island, in the South Adriatic Sea, offers a unique opportunity to employ acoustic techniques for the assessment of an entire jellyfish population. In this study, combined video and acoustic methods were used, allowing to understand the horizontal and vertical distribution patterns of medusae, and to quantify the Aurelia population with a synoptic acoustic survey, at a frequency of 120 kHz. During the night, Aurelia acoustic pattern was at least partially masked by pelagic and demersal fish, which showed a disaggregated distribution around the thermocline.  During daylight however, medusae were clearly discernible from the acoustic data and confirmed by the video camera images, forming a layer of varying density at and below the thermocline (15 – 30 m) and in coincidence with the layer with highest values of dissolved oxygen. A mean TS of -76.89 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 echointegration values obtained during a survey at daylight, resulting in a total number of 4,700,000 individuals, which represents a total biomass of 634 tons. This study provided a synoptic view of Mljet Lake and illustrated the potential of acoustic surveys of jellyfish populations to contribute to ecosystems studies.