The aim of this study is to find out the behavior and acoustic backscattering of the large jellyfish Nemopliema nomurai using hydroacoustics in situ. N. nomurai was distributed at depths ranging from 10~15 m during the day. Regarding the behavior of N. nomurai, there was no significant change in depth, and 3D tortuosity was not high. The vertical direction was ±10° from the horizontal, and moving speed was 0.9~1.5 m s–1.With regard to hydro–acoustical characteristics, the mean TS of N. nomurai ranged from –69.6~–56.0 dB at 38 kHz and –69.4~–54.5 dB at 120 kHz. TS variation (Max TS–Min TS) at 38 and 120 kHz was 0~10.2 dB and 0.2~16.0 dB, respectively. Mean TS and TS variation (Max TS–Min TS) of N. nomurai were higher at 120 kHz than at 38 kHz. The results showed that the use of hydroacoustics was effective in estimating the distribution depth, behavior, and acoustic characteristics of the target.

According to the catch condition of Scomberomorus niphonius in autumn season affected greatly, the catch price for the set net fishery. Catch production and the selling price were relatively even except 2009 showing a great big blooming jellyfish of Nemopliema nomurai in 2008~2011. The fishing cost of the set net fishery in Yeosu has increased gradually by the decrease of catch production and unexpected environmental change like as jelly fish blooming. The increase of fishing cost diminished net income and caused a negative impact in profitability. The lowest Fisheries ratio of gross profit to gross costs the set net fishery were appeared 60.2% in 2010, respectively.Bycatch was highest in 2008 and lowest in 2009. In general, the bycatch was occurred from May to July every year and when Scomber japonicus was most dominant in the catch price by bycatch had a advantage in the profit side. However, the catch increase of immature small fishes by the bycatch, which will bring about the decrease of fisheries resources. Finally, the present state in set net fisheries will act as a defect on the long-term management of fisheries resources.