New approach to the understanding of a marine ecosystem using acoustic survey of jellyfish aggregation.

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

Gold Coast, Australia

Simposio; 22nd International Jellyfish Blooms Symposium; 2007

Jellyfish aggregations may play important roles in enclosed marine systems such as marine lakes. A dense, permanent population of Aurelia sp. in Mljet Lakes (Adriatic Sea) exhibited vertical migration patterns that sometimes reached the surface. The potentially important trophic impact of Aurelia on the pelagic food chain of the Mljet Lakes motivated our quantitative investigations of the swarms’ horizontal and vertical distributions. Two surveys (day and night) were performed covering all of Lake Veliko (0.35 nm2), using a calibrated scientific echosounder working at 120 kHz. A fixed station at the deepest part of the lake (46 m) was sampled four times in 24 hours by means of acoustics and underwater video. Additional sampling with nets, scuba diving and CTD were performed. Aurelia medusae were found throughout the lake. However, they were mainly aggregated along the south-southeast coast at depths between 4-28 m. Maximum densities occurred below the thermocline (18-26m) and there was vertical migration. Umbrella diameter range was 6.3-18.5 cm (148 g average wet weight). Acoustic target strength measured in situ was -73.80 dB. Average density and total biomass of medusae were estimated to be 0.12 ind/m3 and 662 t. Abundant schools of pelagic and demersal fishes were observed during the daytime. Dispersion of fish near the thermocline depth during the night masked the medusan acoustic signature. This study provided a synoptic view of Mjlet Lakes and illustrated the potential for acoustic surveys of jellyfish populations to contribute to ecosystems studies.