Density Estimates of Moon Jellyfish (Aurelia coerulea) in the Yeongsan Estuary using Nets and Hydroacoustics

Abstract

Using nets and acoustic frequencies of 38 and 120 kHz, we estimated the density distribution of the moon jellyfish, Aurelia coerulea, in the Yeongsan Estuary, where a large bloom was detected. Aurelia aurita captured in nets had bell diameters of 3.2–22.0 cm and wet weights of 2.0–265.0 g, and the relationship between wet weight (W) and bell diameter (BD) was W=0.1284BD25263 (R2=0.92). We examined the target strength (TS) difference (120–38 kHz) using a theoretical acoustic scatter model and found that the TS difference ranged from −1.5 to 2.2 dB when the bell diameter was the smallest during underwater pulsation, and from −1.4 to 1.7 dB when the bell diameter was the largest. Based on frequency differences, we found that the mean nautical area scattering coefficient (NASC) of A. coerulea for transects was in the range of 16.4 to 167.0 m2/nm2. In nets near the Yeongsan River Barrier and to its south, jellyfish were caught at a high density of 5.5 ind./m2. The mean density of A. coerulea measured using acoustic methods and nets was 0.16 ind./m2 and 2.02 ind./m3, respectively.