Quantification of the abundance and distribution of the common jellyfish Aurelia aurita s.l. with a Dual-frequency IDentification SONar (DIDSON)

A Dual-frequency IDentification SONar (DIDSON) provided a powerful tool to determine the numerical abundance and spatial distribution of medusae of the common jellyfish Aurelia aurita s.l. in shallow coastal waters. The sonar image obtained in high frequency (1.8 MHz) mode enabled us to identify and count individual medusae of 4.1-19.6 cm (mean: 13.1 cm) bell diameter. Deployment of the DIDSON along three similar to 4-km-long transects in a shallow brackish-water lake (average depth: 5.1 m) revealed that A. aurita aggregated (e.g. > 8.0 medusae m(-3)) near the lake center. The medusae occurred throughout the water column, but tended to avoid low salinity surface and deoxygenated bottom layers. The overall average density of medusae estimated by the DIDSON was 3.3 times higher than that estimated by net sampling. Use of a DIDSON can facilitate quantitative determination of jellyfish populations that cause problem blooms worldwide in order to better understand their ecological importance.