The giant jellyfish Nemopilema nomurai is one of the largest of all jellyfish species, attaining a bell diameter of ca. 2 m and a wet weight of ca. 200 kg. This species is endemic in the East Asian Marginal Seas, residing in the Bohai, Yellow and East-China Seas and being expatriated to the Japan Sea by the Tsushima Current. The massive blooms of this species were extremely rare (e.g. in 1920, 1958 and 1995) in the Japan Sea. The last 5 years (i.e. 2002-2006), however, have seen mass occurrence that have caused severe damage to local fisheries both in Japan and Korea. During the peak period, several thousands of jellyfish are entrapped in a set-net per day, resulting in bursting nets, fin-fish catch decrease and spoiling, stung fishermen, increase in fishermen’s labor to remove jellyfish, etc. Hence, it is urgent to clarify the reasons why the population outbreak of N. nomurai has continued in recent years and to find proper means to alleviate the nuisance to fisheries.
We started the investigation on N. nomurai in 2003, following the mass occurrence in 2002, when our knowledge about this jellyfish species was minimal. Our prime target was to elucidate the life cycle. We succeeded at producing polyps by artificial fertilization, reproducing them asexually to metamorphose into strobila and ephyra, and rear them to larval medusa stage under laboratory conditions. We also demonstrated the morphological and physio-ecological characteristics of each life stage.
Our field survey confirmed that the medusae are liberated from benthic popyls in late spring-early summer in Chinese coastal waters, transported by the Changjian River Low Salinity Water Mass offshore the East-China Sea, and then carried northward to the Japan Sea by the Tsushima Current. In spite of their large body biomass, N. nomurai can eat only small-sized zooplankton, primarily copepods. Their daily growth rate is as high as 15% of their own weight during larval stage, and decreases with growth to 3-1% during summer and fall. Based on the food demand to meet the metabolism and observed growth rate, a medusa weighing ca. 80 kg may clear 1440 m3 seawater, almost equivalent to the water volume of a 50-m swimming pool, per day, indicating a significant impact to the marine food chain structure. Expatriated medusae die off near Japanese waters in winter.
The causes for the recent outbreak of N. nomurai population are still under investigation, but may be related to environmental changes in Chinese coastal waters, where the following changes are in progress: 1) warming, which accelerates the growth rate of polyps, 2) eutropication, which enhances the food supply to medusae, 3) over-fishing, or decrease of competitors, which result into open niche for jellyfish to invade, 4) water front and harbor constructions, which provide more substrate area for polyps to attach. The East Asian Marginal Seas is used to be a world-top level productive fishing ground and now facing the deterioration. My ultimate goal is to recover the fin-fish dominating ecosystem in the East Asian Seas through international collaboration and understanding for the coastal environment and fishery managements.
Jellyfish Invasion premieres April 20 at 9P et/pt on Nat Geo Wild.