Occurrence and potential prediction of the giant jellyfish Nemopilema nomurai off Hyogo Prefecture, southwestern Sea of Japan, during 2006–2015

We monitored the occurrences of giant jellyfish Nemopilema nomurai off Hyogo Prefecture, southwestern Sea of Japan, from 2006 to 2015. Occurrence data were collected from daily by-catch number of medusae at three large-scale set-nets operated in Hyogo Prefecture. In addition, we examined parameters to base predictions of outbreaks of N. nomurai using environmental factors measured at Amarube set-net and by-catch data of a preliminary offshore trawl survey. Annual by-catch number of N. nomurai medusae exceeding ten thousand in the three set-nets was observed in 2006, 2007 and 2009. On the other hand, no medusa was observed in 2014 and 2015, and the annual values were 2–147 in the other five years (2008 and 2010–2013). Salinity measured at the Amarube set-net decreased from June to August or September, and then increased in every year. The first detection time of N. nomurai medusa occurred within one week from the day when the salinity reached a minimum value for the year. It is thought that prediction of the occurrence timing of N. nomurai may become possible in the future by monitoring salinity data. The present study revealed that in the years when large amounts of medusae were caught in the offshore trawl survey, many medusae were also collected in the set-nets. For predicting the amount of N. nomurai medusae impacting coastal areas of Hyogo Prefecture, the use of catch data from offshore trawl surveys will enable early assessment of potential damage to fisheries.